Iron Powders as a Potential Material for Arsenic Removal in Aqueous Systems

Kameswara Srikar Sista; Deepak Kumar; Gourav Ranjan Sinha; Abhijeet Premkumar Moon; Srinivas Dwarapudi

doi:10.2355/isijinternational.ISIJINT-2021-258

Abstract

Iron powders due to their wide spectrum of applications is one of the potential materials of all time. Water remediation is one of the prominent and widely explored applications of iron. Arsenic is one of the most common and life-threatening pollutants present in the globe. Ingestion of arsenic contaminated products, especially water containing arsenic above 10 ppb results in acute health disorders. Due to its existence in various oxidation states such as As(V), As(III), As (0), As(-III), arsenic removal from aqueous systems is not a straightforward problem to solve. Among various materials which assist arsenic removal, iron powders due to their low cost, high reactivity, commercial availability, multiple mechanisms of removal, reusability and on-site usage, is one of the prominent and lucrative treatment medias. Iron powder employs one or combination of oxidation, reduction, adsorption, precipitation and co-precipitation mechanisms for the arsenic removal. Physico-chemical properties (purity, size, etc.) of powders along with aqueous system properties (pH, oxygen, contaminants, etc.) play a significant role in steering the arsenic removal. Iron powders from electrolytic and reduction routes are largely preferred for water remediation. This review is first of its kind work highlighting the potential of micron and macro scale iron powders in water remediation, especially arsenic removal. Special emphasis is given on the different routes of synthesis, mechanisms of removal, research evolution and commercial presence of iron powders for water remediation.

1. Introduction

Powdered form of iron with particle sizes ranging between few microns to milli meter scales is commercially termed as “iron powder”.^1,2) With its core metallurgical property combined with physical and chemical characteristics like purity, size, surface area, shape, and morphology, it attracts large spectrum of applications such as friction materials, powder metallurgy, soft magnetics, coatings, additive manufacturing, metal injection molding, diamond cutting tools, chemicals, oxygen absorbers, food fortification, metal fuels and environmental remediation.^3,4,5) Apart from the physico-chemical properties of iron powders (IP), functional properties also judge its scope of applicability for a particular application. One such application in which IP are being historically used and is of prime importance for today’s world is water remediation. Removal of contaminants from water by use of zerovalent metals like iron, Zinc, Nickel and Tin even though has its inception before 1920’s, more detailed studies and defined process understanding begun only after 1980’s. Zerovalent iron (ZVI) is one of the prime media used as iron walls and permeable reactive barriers to remove inorganics, chloro organic compounds, nitro compounds, azo compounds, etc., from water bodies.⁶⁾ Powdered form of elemental iron or ZVI is one of the widely used reactive media in permeable reactive barriers.^7,8,9) Reactive barriers with ZVI as media is one of the most promising technology for ground water treatment.^10,11) ZVI in the form of chips, flakes, granules, lumps, fillings, turnings, etc is under continuous exploration in both laboratory and field applications. Majority of permeable reactive barriers in the world are iron based.^12,13) More than 150 works relevant to usage of ZVI in permeable reactive barriers for multi-contaminant removal including lab scale, column, bench scale and field applications are reported.¹⁴⁾ Application of ZVI to remove various contaminants from aqueous systems is shown in the Fig. 1.^15,16,17)

Fig. 1.

Application of ZVI for various contaminant removal in aqueous systems. (Online version in color.)

Among various contaminants showcased in Fig. 1, one such prioritized and alarming pollutant which needs high attention is Arsenic (As).¹⁸⁾ Arsenic is a crystalline metalloid prevalent in the environment and is one of the abundantly available trace elements in earth’s crust (20th rank), sea water (14th rank) and human body (12th rank).¹⁹⁾ Arsenic mainly exists in four different oxidation states: Arsenate (As (V)), Arsenite (As (III)), Arsenic (As (0)) and Arsine (As (-III)). The presence and dominance of arsenic in a particular oxidation state is greatly influenced by physiochemical conditions of the environment. More than 200 mineral forms of Arsenic are naturally found.^20,21,22) Inorganic and organic forms of arsenic are subsistent with varying chemical, bio-chemical and toxic effects on environment.^19,20,22,23)

Natural and anthropogenic sources contribute a lot to release of Arsenic into environment.^24,25) Among various sources of Arsenic intake, ingestion through drinking contaminated ground water (As > 10 ppb)²⁶⁾ is major and over 200 million people globally are exposed to this threat.²⁷⁾ Among many global locations (USA, England, China, India, Bangladesh, Nepal, Pakistan, Turkey, New Zealand, Iran, Italy, Spain, France, Germany, Saudi Arabia, Sweden, Argentina, etc.), India, Bangladesh and Nepal are majorly suffering from Arsenic contaminated ground waters.^28,29,30) Uptake of Arsenic contaminated water above the WHO permissible limit of 10 ppb for a short duration results in acute health effects such as vomiting, gastro-intestinal discomfort, dizziness, loss of appetite, muscle weakness, nausea, miscarriages, diarrhea, etc., and for a long duration attracts several health disorders such as respiratory, cardiovascular, digestive, endocrine, renal, neurological and reproductive systems leading to chronic health issues such as, arsenicosis, bronchitis, melanosis, leuco-melanosis, keratosis, hyper-keratosis, blackfoot disease, edema, gangrene, lung cancer, kidney cancer, skin lesions, skin cancer, etc.^{24,31,32,33,34,35)}

Therefore, it is very essential to control arsenic consumption beyond the permissible limits. Selection of appropriate method for real time application relies on several factors addressing its technological feasibility and socio-economic complications. Two strategies are widely used for handling the arsenic menace, i) routing an alternate arsenic free source of water, and ii) removal of arsenic from existing water source.³⁶⁾ First strategy seems, simple and best, but is not feasible at every location, due to lack of alternate water resources. Second strategy brings into picture various methods by which arsenic is either contained or removed to provide water with Arsenic levels below 10 ppb. Till date, many arsenic removal methods like, oxidation, adsorption, chemical coagulation-flocculation, chemical precipitation, ion exchange, electro kinetics, bio-remediation and membrane technologies are in wide practice and each technique has its own advantage and limitations.^{19,36,37,38,39,40,41,42,43,44,45,46)} Many natural and synthetic materials having arsenic uptake capability are employed in one or the other methods mentioned above and many factors like, pH, dissolved oxygen (DO), water chemistry, presence of other anions, microbial conditions, etc., influences the removal of Arsenic from contaminated water. Overview on various materials used for the arsenic removal from aqueous systems is presented in Fig. 2.

Fig. 2.

Various materials used for arsenic removal from aqueous systems.

Among various materials being used for arsenic removal such as carbon products, zeolites, clays, biological extracts, metals, oxides, metal organic frameworks, etc., one such material which is of paramount interest in the global research fraternity is “Iron”.^21,47,48,49) Iron in the form of powders, rods, wires, and other aggregates forms is being widely used for the environmental remediation applications, especially for ground water treatment.^{4,50,51,52,53,54,55)} Advantages like wide availability, low cost, high purity, high reactivity, non-hazardous and chemical free treatment approach makes IP widely acceptable and suitable material for water remediation.⁵⁶⁾

Till date, very few review articles focusing on the usage of IP for water remediation have been reported, yet none of them specific to Arsenic removal. Most of those works are inclined towards usage of nano IP and are restricted to laboratory scale both in terms of media synthesis as well as application testing.^{15,16,57,58,59,60,61,62,63,64)} Nano scale IP is also less preferred for field applications due to difficulties in handling and end usage.^16,61,65) Thus, there stands an immense need for a detailed work elucidating the usage of iron powders, especially micron and macro sized IP. In the present review, a detailed overview on the synthesis of micro and macro scale IP and their potential usage for arsenic removal is presented. Importance of iron powders for contaminant removal and possible mechanism is also discussed in detail. Special focus is laid on elucidating several powder and aqueous parameters influencing the arsenic removal. Further, research and developmental chronology of usage of IP for arsenic removal along with their commercial presence is also presented. This is first of its kind review highlighting arsenic removal from the perspective of IP, where in an overall understanding comprising powder synthesis, removal mechanism, influencing properties, research evolution and commercial presence is presented.

2. Iron Powders: Synthesis, Mechanism and Multi-Contaminant Removal

2.1. Synthesis

Based on the approach used for the synthesis of IP, they can be categorized into mechanical, physical and chemical methods. Mechanical methods involve usage of mechanical means like milling and pulverization to bring down iron lumps into powders. Whereas, physical and chemical methods involve various physical and chemical reactions respectively in combination with sequence of operations to convert iron bearing materials and compounds into iron powders. Atomization, electrolytic deposition, carbonyl and reduction processes are widely known at commercial scale and are the major routes of manufacturing iron powders.⁶⁶⁾ Figure 3 shows schematic illustration of various steps involved in the IP manufacturing through different process along with their corresponding morphology. Iron powder synthesis using atomization route is a physical method in which molten iron obtain by melting solid iron or scrap is subjected to atomization by means of pressurized jets of air or water where in the molten metal breaks up into tiny droplets and further solidifies to form pure, spherical/irregular iron powders. One another physical method used for iron powder synthesis is electrolytic deposition. In this route, iron powders of high purity and flaky or dendritic shapes are obtained as a cathodic deposition resulting from electrolytic cell reactions comprising suitable aqueous electrolyte and iron bearing anode like iron ingot or low carbon steel. Carbonyl method of iron powder synthesis is a chemical method in which iron scrap is treated with carbon monoxide gas at controlled processing conditions to obtain iron pentacarbonyl (Fe(CO)₅) followed by distillation, evaporation and decomposition to finally obtain a high purity, spherical iron powders. On the other hand, IP synthesis from chemical method like reduction employs use of solid or gaseous reducing agents like carbon, carbon monoxide, hydrogen, cracked ammonia, etc., to convert iron bearing raw materials like iron oxide, mill scale, etc to a pure, spongy and irregular powders of iron. Obtained powders or chunks in each of the processes are subjected to annealing or down streaming or combination of them based on the required powder size, property and special features of the targeted application. Information showcasing basic powder characteristic and application favourability of the powders based on the synthesis route is shown in Table 1.^2,4)

Fig. 3.

Commercial iron powder manufacturing processes and product morphology.

Table 1. Commercial iron powder manufacturing- properties and applications.

S. No	Production method	Purity	Size	Shape	Prime applications	Indicative synthesis cost
1	Gas Atomization	Better (Fe(T) > 99%)	Fine	Spherical	Additive Manufacturing, magnetic materials	High
2	Water Atomization	Better (Fe(T) > 99%)	Coarse to fine	Irregular	Powder metallurgy	High
3	Electrolytic	Best (Fe(T) > 99.5%)	Coarse to fine	Flaky or dendritic	Chemical reactions and catalysts	Moderate
4	Carbonyl	Best (Fe(T) > 99.5%)	Ultra-fine	Spherical	Metal Injection molding, Wave absorbers, Magnetic materials	High
5	Reduction by solids	Good (Fe(T) > 98%)	Coarse to fine	Irregular	Powder metallurgy and chemical reactions	Low
6	Reduction by gases	Good (Fe(T) > 98%)	Coarse to fine	Irregular	Powder metallurgy and chemical reactions	Moderate

Note: Average particle size: Coarse > 75 microns; Ultra-fine < 10 microns; Fine 10–75 microns.

IP feasible for usage in chemical applications are the one best suited for water remediation applications as well. Among the various methods of synthesis, iron powders obtained from electrolytic and reduction routes are mostly used for water purification/remediation application due to their porous structure and higher surface areas which promotes their reactivity when compared to other iron powders.^67,68,69,70) Besides, the water purification market being cost sensitive, it is also important to reduce the cost of the media being used for purification.^71,72) Iron powders synthesized from iron bearing industrial by-products or from iron bearing ores will stand promising to cater water remediation applications.^4,73,74)

Chemical characteristics of IP like purity, composition, etc., are measured by means of wet chemical analysis, X-ray diffraction, X-ray fluorescence, Inductive coupled plasma, Energy dispersive X-ray analysis, electron probe micro analysis, Raman spectroscopy, etc. Whereas, physical attributes of powders like size, shape, particle size distribution, density, porosity, surface area, etc., can be revealed by using Scanning electron microscopy, Transmission electron microscopy, Optical microscopy, static laser scattering, dynamic image analysis, true density pycnometer, bulk density pycnometer, mercury porosimeter, surface area analyser, etc. Other functional testing with respect to application are performed based on the usage of iron powder.

2.2. Mechanism for Contaminant Removal

Mode of contaminant removal varies with respect to the oxidation state of iron present, for example, hematite having +3 oxidation state facilitates contaminant removal by adsorption, whereas magnetite and wustite having nearly +2 oxidation states facilitates contaminant removal by adsorption, precipitation and reduction. Zerovalent form of iron having zero state of oxidation implements one or the combination of adsorption, reduction, oxidation, precipitation and co-precipitation principles to remove contaminants from water.^75,76) This behavior of IP is attributed to the formation of various combinations of corrosion products on the surface and their subsequent interaction with contaminant for removal.

IP being unstable under aqueous environment undergoes series of reactions to develop corrosion products like Fe(OH)₂, Fe(OH)₃, FeOOH, Fe₃O₄, Fe₂O₃ and green rusts. Spontaneous chemical oxidation of IP in aqueous systems results in formation of ferrous and ferric intermediates which are further transformed into stable oxides.⁷⁷⁾ Possibility of the formation of different types of corrosion products is not random but depends on the ambient conditions of aqueous media like oxic or anoxic and acidic or neutral. Due to presence of water, oxidation (corrosion) of iron is feasible in both aerobic and anaerobic conditions. Reactions promoting corrosion of iron powders to give various reactive and absorptive species are shown in the Table 2.^{17,77,78,79,80,81)}

Table 2. Corrosion reactions of iron in aqueous systems.

Corrosion:	Equations
F e 0 +2 H + =F e +2 + H 2 (g)	(1)
2F e 0 + O 2 +2 H 2 O=2F e +2 +4O H -	(2)
4F e +2 + O 2 +2 H 2 O=4F e +3 +4O H -	(3)
F e +2 +2 H 2 O=Fe (OH) 2 +2 H +	(4)
F e +3 +3 H 2 O=Fe (OH) 3 +3 H +	(5)
4F e +2 + O 2 +10 H 2 O=4Fe (OH) 3 +8 H +	(6)
4F e +2 + O 2 +6 H 2 O=4FeOOH+8 H +	(7)
3F e +2 + O 2 +2 H 2 O=F e 3 O 4 +4 H +	(8)
F e 0 + O 2 +2 H + =F e +2 + H 2 O 2	(9)
F e +2 + H 2 O 2 =F e +3 +O H - +OH(radical)	(10)

Few research works emphasizing the reaction of IP with aqueous system and subsequent mechanism of contaminant removal are reported. Leading insights were provided by Noubactep^78,82) in his works and later many other research works in agreement with the same are also reported.^{77,79,80,83,84)} A combined understanding of the reported works reveals that, under acidic aqueous conditions, where the availability of H⁺ ions is abundant, corrosion of iron proceeds by hydrogen evolution mechanism and follows the reaction as shown in the Eq. (1). The corrosion of Fe⁰ to Fe⁺² proceeds rapidly and based on the availability of oxygen, two different reactive products on iron can be formed. Under anerobic conditions with restricted oxygen availability, Fe⁺² formed on the surface of iron cannot form further stabilized oxides and hydroxides and gets rapidly dissolved in the highly acidic aqueous system, restricting the mechanism at Eq. (1). On the other hand, if high availability of oxygen is present as in aerobic systems, reactive species like H₂O₂ and OH• radicals are generated along with corrosion products of Fe⁺² and Fe⁺³ as shown in the Eqs. (9) and (10). Similarly, under neutral to slightly basic pH conditions, as in the case with natural waters, corrosion of iron proceeds by oxygen reduction as shown in the Eq. (2) resulting in formation of Fe⁺². Later, based on the availability of oxygen, Fe⁺² will oxidize and hydrolyze to from Fe⁺³ and Fe(OH)₂ which further hydrolyzes to form Fe(OH)₃, FeOOH and Fe₃O₄ as shown in the Eqs. (3) to (8). Under aerobic conditions, continuous formation of Fe⁺² from Fe⁰ and further oxidation of Fe⁺² to from oxide and hydroxide scales is more feasible, whereas in anaerobic conditions, due to restricted oxygen availability, rapid formation of Fe⁺² as well as further oxidation of Fe⁺² to from oxide and hydroxide scales is largely restricted. Pictorial illustration of corrosion behaviour of iron under given ambience and formation of possible corrosion products in aqueous systems is shown in the Fig. 4.

Fig. 4.

Possible outcomes of Iron powder corrosion in Aqueous System. (Online version in color.)

Iron powder facilitates multi-mode removal of contaminants due to formation of various corrosion products as shown above. Different corrosion products steer the removal process through different modes. Various reactions representing multimode contaminant removal of iron powders is shown in the Table 3.^{17,78,82,83,84)} It is highlighted that, species like Fe⁰, Fe⁺² and H₂ promotes reduction of contaminants by active donation of electrons as shown in the Eqs. (11) to (13), whereas species like H₂O₂ and OH• radical - promotes oxidation as shown in the Eqs. (14) and (15). On the other hand, adsorption, precipitation and co-precipitation is promoted by species having active sorption sites like Fe(OH)₂, Fe(OH)₃, FeOOH, Fe₂O₃, Fe₃O₄ and many other forms (Fe(OH)_nⁿ⁻², Fe(OH)_nⁿ⁻³, Fe(H₂O)_x²⁺, Fe(H₂O)_x³⁺: n ≤ 3, x ≤ 6) as shown by the Eqs. (16) to (18). Aged iron oxy-hydroxides being present for longer duration of time promotes entrapment of contaminant onto surface leading to adsorption, while newly formed iron-oxy hydroxides along with adsorption also promotes structural entrapment of the contaminants in the process of aging leading to co-precipitation.^81,82) Adsorption and co-precipitation are considered as one of the fundamental contaminant removal mechanism of Fe/H₂O system, which is followed by reduction and oxidation of contaminants based on ambient conditions.⁷⁹⁾

Table 3. Reactions of iron facilitating multi-mode removal.

Reduction of contaminant:	Equations
F e 0 + C + =F e +2 +C	(11)
F e +2 + C + =F e +3 +C	(12)
H 2 + C + = H + +C	(13)
Oxidation of Contaminant:
C - + H 2 O 2 =C	(14)
C - +OH(radical)=C	(15)
Precipitation of Contaminant:
C+nO H - =C (OH) n	(16)
Adsorption of Contaminant:
Soprtion site+C=C-Soprtion site	(17)
Co-precipitation of Contaminant:
C+n F e x (OH) y (3x-y) =C- [ F e x (OH) y (3x-y) ] n	(18)

Note: C⁺- containment in oxidized from, C⁻ - contaminant in reduced form, C-contaminant.

Even though theory explicitly describes the differential role of adsorption, precipitation and co-precipitation, in real time application due to complex interplay between the three processes, it remains difficult to differentiate them and confirmative synergy between them facilitates the removal process. Iron powder due to its high reduction potential (−440 mV), acts as good reductant.⁹⁾ Multi-contaminant removal efficacy of iron powder for removal of chloro, phenolic, dye, nitrate, nitro and other compounds is reported.^{69,70,85,86,87,88,89,90,91,92)} Moradi and team reported the reductive dehalogenation of chlorinated aliphatic hydrocarbons by ZVI.⁸⁵⁾ Similar works on reductive behaviour of ZVI benefitting the removal of contaminants like 4-phenyl azo phenol⁶⁹⁾ and technetium-99⁷⁰⁾ are also reported. Wang and co-workers revealed that, ZVI facilitated the Cr (VI) removal through adsorption-reduction pathway.⁸⁹⁾ Similar results on usage of ZVI along with activated carbon promoting adsorption and reduction of contaminants is also observed.^86,87) Luo and team reported the removal of phosphate by adsorption and precipitation when ZVI is implemented as media for removal.⁹⁰⁾ ZVI enabling removal of aniline by combination of oxidation, adsorption, co-precipitation and removal of antimony, chromium by combination of adsorption, co-precipitation is reported by Xue and team.⁹³⁾ ZVI promoting removal of phenol in presence of H₂O₂ through fentons reaction is reported by Segura and team.⁸⁴⁾ Thus, ZVI promotes multi-contaminant removal through multi-mode removal approach. Besides various modes of contaminant removal directing the removal process, mass transport of contaminant towards ZVI as well as the mass transport of active species from ZVI also influence the contaminant removal, where in the thickness of oxide scale formed on the surface and its properties like, porosity, surface area, density, etc., also comes into play.⁷⁸⁾ Works related to arsenic removal are not mentioned in this section as they are discussed in detail in the next section.

When a fresh ZVI is used for contaminant removal in aqueous systems, it reacts with the available oxygen to form various combination of iron oxides based on the prevailing ambient conditions. Later the formed oxides along with the fresh ZVI combinedly promotes the contaminant removal. Once the reaction proceeds to saturation, surface of the ZVI is covered by thick layer of oxides thus promoting no further reaction. Pictographic illustration of possible behaviour of ZVI in aqueous solutions enabling removal of contaminants is shown in the Fig. 5.^{15,16,63,81,88,93)}

Fig. 5.

Life cycle of iron powder subjected to contaminant removal in aqueous system. (Online version in color.)

Spent ZVI media post usage for remediation applications can be attempted for reuse by subjecting to proper regeneration. While most of the research is inclined on the usage of adsorptive media, very few works are focused on the subject of spent media regeneration and reuse.⁹⁴⁾ This process of regeneration is completely media and contaminant specific. In case of arsenic removal using ZVI, caustic stripping (NaOH, pH>13) followed by acid neutralization (HCl/HNO₃, pH<2) is used.⁹⁵⁾ As iron-oxy hydroxides present on the ZVI surface promotes arsenic adsorption and complexation, methods like leaching of arsenic followed by re-adsorption onto MgO under highly alkaline ambience to regenerate spent iron-oxyhydroxides are used.⁹⁶⁾ In a similar way arsenic adsorbed onto ferric hydroxide adsorbents is regenerated by use of NaOH and NaCl regenerants.⁹⁷⁾ In the case of heavy metal removal, Alkalis act as very good regenerants for chemical adsorbents, whereas, acid regenerants stands effective for bio-based adsorbents.⁹⁸⁾ In case of natural organic matter removal using ZVI, periodic regeneration with H₂SO₄ is a good solution.⁹⁹⁾ Controlled, periodic washing of ZVI with HCl to remove oxide shell formed and regenerating the surface for nitrate reduction is also reported.¹⁰⁰⁾ Thus, alkali or acid treatment is widely used for iron media regeneration. As the spent iron media post washing or treatment for desorption, will be in the form of oxides, a possible reduction of iron oxides using hydrogen to re-generate pure iron powder can also be explored.^73,101) With the future of hydrogen generation turning out from renewable sources, economics of the process will also stand feasible.¹⁰²⁾ However, this process cycles of regeneration or re-use of iron remains finite. Overall, the process of regeneration is quite important as it would attract a saving of about 80% of cost in comparison to the media replacement approach.⁹⁵⁾

Thus, IP due to their commercial synthesis feasibility, tailored to specification benefit, multi- contaminant removal capability, easy reusability and wide suitability for field applications is being actively applied in the water remediation segment. A detailed study illustrating usage of iron powders for arsenic removal, influence of powder characteristics and water properties is illustrated in section below.

3. Iron Powders for Arsenic Removal

Beside various advantages iron powders hold in terms of availability, reactivity, feasibility and reusability as mentioned above, larger affinity of arsenic towards iron and its oxide also makes IP suitable for arsenic removal.⁵⁴⁾ Removal of Arsenic using IP involves one or combination of oxidation, reduction, adsorption, precipitation and co-precipitation mechanisms.^15,63) Corrosion products formed on the surface of iron such as mixed oxides and hydroxides of Fe (II and III), drives the removal of arsenic from aqueous solutions.¹⁰³⁾ Among various other forms, powdered form of iron stands significant due to its porous structure and large surface area which enhances arsenic removal. Products like, pure iron, iron oxides, iron oxy-hydroxides are commonly used for arsenic removal.^48,104) Among them pure powdered iron (or) iron powder remains outstanding due to its good reactivity, multi-mode removal ability and reusability as mentioned in the previous sections. Powder properties such as size, surface area, porosity, etc., influences the removal effectiveness and for a given characteristic of IP employed, properties of water such as aeration, dissolved oxygen, pH, presence of other anions influence the removal effectiveness. Details on the aforementioned parameters are explicitly discussed in this section.

3.1. Powder Properties

Major IP properties that plays vital role in the arsenic removal are particle size, purity and surface area. Details of various types of IP used by researchers globally for arsenic removal are shown in the Table 4.

Table 4. Various works on usage of iron powder for arsenic removal.

S. No	Author and year	Removal	Iron powder properties				Study	Ref
S. No	Author and year	Removal	Type	Size (mm)	Surface area (m²/g)	Fe content (%)	Study	Ref
1	Lackovic et al., 2000	As (III), As (V)	Chips	1–1.6	0.1	>90	Column	¹⁰⁵⁾
2	Su and Plus, 2001	As (III), As (V)	Powder	<0.15	0.091	99	Batch	¹⁰⁶⁾
			Chips	>1	2.53	–
			Chips	>1	2.33	–
			Powder	<0.045	0.192	97
3	Manning et al., 2002	As (III), As (V)	Powder	<0.15	0.1	99	Batch	¹⁰⁷⁾
			Filings	<0.4	3.5	–
4	Nikolaidis et al., 2003	As (III)	Filings	<4.75	–	89.8	Field column	¹⁰⁸⁾
5	Bang et al., 2005	As (III), As (V)	Filings	<0.15	0.55	95	Batch and Column	¹⁰⁹⁾
				<0.4	0.169	95
6	Leupin and Hug, 2005	As (III)	Filings	~0.2	–	>98	Column	¹¹⁰⁾
7	Lien and Wilkin, 2005	As (III)	Powder	0.3–2.3	2.53	–	Batch and Column	¹¹¹⁾
8	Bang et al., 2005	As (III), As (V)	Filings	0.12–0.17	–	–	Batch	¹¹²⁾
9	Sun et al., 2006	As (III), As (V)	Powders	<0.075	–	>99	Batch and Column	¹¹³⁾
			Chips	<0.85	–	>95
10	Tyrovola et al., 2006	As (III), As (V)	Filings	<4.75	–	89.8	Batch and column	¹¹⁴⁾
11	Yu et al., 2006	As (III)	Powder	0.5–0.7	0.6	–	Batch	¹¹⁵⁾
12	Biterna et al., 2007	As (V)	Powder	<0.045		97	Batch and Column	¹¹⁶⁾
			Filings	–	–	>97
13	Katsoianniss et al., 2008	As (III)	Powder	<0.25	0.1	>95	Batch	¹¹⁷⁾
14	Sasaki et al., 2009	As (III)	Filings	0.25–2	1.8	89.8	Batch and Column	¹¹⁸⁾
15	Liu et al., 2009	As (V)	Powder	0.5–1	1.8	96.28	Batch	¹¹⁹⁾
16	Mak et al., 2009	As (V)	Powder	0.5–1	1.8	96.28	Batch	¹²⁰⁾
17	Rao et al., 2009	As (V)	Powder	0.5–1	1.8	96.28	Batch	¹²¹⁾
18	Wilkin et al., 2009	As (III), As (V)	Powder	0.3–2.3	–	–	Field	¹²²⁾
19	Wan et al., 2010	As (III)	Powder	<0.075	0.01	>99	Batch and Column	¹²³⁾
20	Abedin et al., 2010	As (V)	Powder	<0.18	–	93.5	Batch and Column	¹²⁴⁾
				<0.25	–	98.1
				<0.18	–	94.8
21	Biterna et al., 2010	As (III), As (V)	Powder	<0.045	–	97	Batch and Column	¹²⁵⁾
22	Sun et al., 2011	As (V)	Powder	~0.3	–	99.2	Batch	¹²⁶⁾
23	Mamindy-Pajany et al., 2011	As (V)	powder	~0.017	0.2	–	Batch	¹²⁷⁾
24	Mak et al., 2011	As (V)	Filings	0.5–1	1.8	96.28	Batch	¹²⁸⁾
25	Mak et al., 2011	As (V)	Filings	0.5–1	1.8	96.28	Column	¹²⁹⁾
26	Rahamani et al., 2011	As (III)	Powder	<0.15	–	–	Batch	¹³⁰⁾
27	Mak and Lo, 2011	As (V)	Filings	0.5–1	1.8	96.28	Batch	¹³¹⁾
28	Shafiquzzam et al., 2013	As (III), As (V)	Powder	<0.075	–	–	Batch	¹³²⁾
29	Katsoyiannis et al., 2015	As (III)	Powder	~0.043	0.1	97	Batch	¹³³⁾
30	Kumar et al., 2016	As (V)	Powder	0.25–2	–	–	Column	¹³⁴⁾
31	Casentini et al., 2016	As (V)	filings	<4.75	–	87–93	Field	¹³⁵⁾
32	Liang et al., 2017	As sludge	Powder	0.15–0.8	–	>99	Batch	¹³⁶⁾
33	Hussain et al., 2017	As (III)	Powder	0.21	0.14	>99	Batch	¹³⁷⁾
34	Lopez-Munoz et al., 2017	As (III)	Powder	0.01–0.05	10	97	Batch	¹³⁸⁾
35	Sun et al., 2017	As (V)	Filings	0.25–2	–	96.28	Batch	¹³⁹⁾
36	Yang et al., 2017	As (III), As (V)	Powder	0.037–0.045	–	98	Batch	¹⁴⁰⁾
37	Deng et al., 2018	As (III)	powder	<0.01	–	>98	Batch	¹⁴¹⁾
38	Xu and Huang, 2019	As (V)	Powder	0.005	–	98	Batch	¹⁴²⁾
39	Song et al., 2019	As (III), As (V)	Powder	0.04	4.64	99	Batch	¹⁴³⁾
40	Xi et al., 2019	As (III)	Powder	0.01–0.05	0.58	>98	Batch	¹⁴⁴⁾
41	Du et al., 2020	As (III)	Powder	<0.015	–	–	Batch	¹⁴⁵⁾
42	Zacarias-estarda et al., 2020	As (III), As (V)	Powder	<0.045	–	–	Batch	¹⁴⁶⁾

From the tabulated information it is vivid that, most of the researchers make use of iron in the form of loose powders and few reports on the usage of iron in the form of filings and chips is also observed. Practically, fillings and chips are also considered as coarse form of iron powders. Usage of iron powders in the size range of from 5 mm to 15 microns is observed. Most of the researchers preferred the usage of iron powders with purity > 95%, with only few reports showcasing usage of less pure powders. Surface area of the powders also largely varied from 0.01 to 10 m²/g. Even though literature is available on the usage of iron powders for arsenic removal, very few studies are focused on comparing their performance of arsenic removal with respect to powder properties. First study on usage of several iron powders for arsenic removal and their comparative performance is done by Lackovic and team.¹⁰⁵⁾ In their study four different iron powders (1-Connelly GPM (CGPM), 2-Master Builders (MB), 3-J.T Baker treated (BK), 4-J.T Baker untreated) with surface areas 1.9, 1, 0.1, 0.03 m²/g respectively are studied. Comparative studies in laboratory columns between MB and BK resulted that MB (1 m²/g; 927 μg-As/g-Fe) with higher surface area removed arsenic more effectively than BK (0.1 m²/g; 220 μg-As/g-Fe) with lower surface area. Similarly, a higher arsenic adsorption by CGPM (1.9 m²/g; 1150 μg-As/g-Fe) compared to BK (0.1 m²/g; 470 μg-As/g-Fe) is also reported in field column studies. In another study done by Su and Plus, four different iron powders procured from Fisher (0.091 m²/g), Peerless (2.53 m²/g), Master Builders (2.33 m²/g) and Aldrich (0.19 m²/g) are studied for arsenic removal. Obtained results shows that As (V), As (III) and As (III + V) removal rate constants are of the order: Fisher > Peerless = Master Builders > Aldrich. Here, Fisher iron powder despite of having lower surface area showcased superior removal rates. Authors commented that the arsenic removal is not proportionally related to surface area of powders but also depends on other factors like purity of iron and presence of other impurities as well.¹⁰⁶⁾ In another study done by Manning and team, obtained results revealed that iron powder with higher surface area, Fe-40 (3.5 m²/g), showcased greater corrosion tendency and subsequent adsorption capability compared to iron powder with lower surface area, Fe-100 (0.1 m²/g).¹⁰⁷⁾ In a similar study, difference in removal efficiencies of arsenic with two different iron powders is attributed to the difference in their surface areas with higher surface area product resulting in higher removal.¹¹⁶⁾

Thus, above results indicates the importance of initial surface area of iron powders. However, it must also be noted that removal efficiencies are not uniquely correlated with surface area alone but also takes into consideration the composition, surface properties, mechanical abrasion and nature of oxide coating being formed.⁷⁷⁾ Surface area results are also dependent on particle size and particle size distribution of powders. Generally, finer size powders exhibit greater arsenic removal characteristics than coarser powders due to higher surface areas.¹⁰⁹⁾ In a study performed by Abedin and team, it is observed that iron powders with lower particle sizes, KB-90 (Size <0.18 mm) and K-100 T (Size <0.18 mm), removed higher mass of arsenic compared to coarse powders, TK-H (Size < 0.25 mm).¹²⁴⁾ Thus, influence of size of powders on arsenic removal can be reflected through surface area. However, size of particles might also stand important dictating filling and packing requirements in laboratory and field column studies. The surface area of the initial iron powders drastically varies after being subjected to corrosion. A 40-fold increase in surface area from 1 m²/g before treatment to 37.8 m²/g post treatment is reported by Nikolaidis and team.¹⁰⁸⁾ Thus, the role of surface area of raw powders on the removal properties is an indicative feature to predict the powder performance but not a conclusive one. Similarly, purity of iron powders indicates the amount of iron in Fe⁰ form available for the reaction. So, most of the research reports presented in the Table 4 are in agreement that, higher purity of iron powders fetch superior performance. However, to achieve very high purity, cost of production of iron powder will also increase. So, it is wise to choose the optimum purity required for the operation process. Besides, it is also reported that, presence of impurities like Mn will also enhance removal of arsenic by supporting them in oxidation.^106,114) On the other hand, if the micron size powders are brought down to nano size their reactivity, surface area and subsequent arsenic removal will be greatly improved.¹³⁰⁾ However, micron sized iron powders find more practical significance in usage and application compared to the nano ones.^{65,133,145,147)} Very few research works are reported on the powder property analysis and a clear white space in this domain co-relating arsenic removal performance of IP with variation in their size, shape, morphology, composition and surface area is available.

3.2. Aqueous Properties

Apart from the powder properties which influence the iron corrosion behaviour and subsequent arsenic removal, water properties are of prime importance in controlling iron corrosion as well as arsenic removal. Properties such as water chemistry, aeration, pH and presence of other compounds greatly influence the arsenic removal behaviour and detailed discussion on this subject is presented in this section.

3.2.1. Aerobic/Anaerobic Conditions

In aerobic waters, due to sufficient oxidation, Arsenic is generally present in As (V) form and in anaerobic waters it is present in As (III) form. However, in most of the natural waters, they may co-exist as well. In aerobic waters, due to presence of higher dissolved oxygen content, iron corrosion occurs rapidly with time. This allows the arsenic removal more rapidly due to formation of sufficient oxides and hydroxides. As time lapses, very thick iron oxide layer is formed on the surface thus retarding the Arsenic removal. On the other hand, in anaerobic waters, the oxygen content in the water is low and the formation of oxides and hydroxides of iron is very slow. Thus, the Arsenic removal rates are also relatively low. The oxide formed on the surface of iron is slightly porous. It is reported that the, corrosion rate of iron in aerobic conditions is nearly 40 time higher than that in anaerobic conditions.¹⁴⁸⁾ However, depending on the pH condition of water other than adsorption-precipitation-co-precipitation, reduction of As (V) to As (III) and both to As (0) might also support Arsenic removal in anaerobic conditions. Even though aerobic and anaerobic conditions give some hint about the presence of arsenic species and corrosion behaviour of iron, it is very important to parallelly understand the influence pH on removal as well to have better understanding on the overall removal.

3.2.2. pH

pH of the aqueous system plays a significant role in arsenic removal by IP. Speciation changes of arsenic with change in pH is the important factor influencing the arsenic removal. In aerobic waters, arsenic present in As (V) form gets removed by adsorption and precipitation by iron oxides and oxy-hydroxides. The extent of Arsenic adsorption gradually decreases with increase in pH from 3 to 10. Three major reasons responsible for this behaviour are 1) Arsenic speciation, 2) surface charge of adsorbents and 3) iron corrosion. As (V) remains in neutral form till pH 2.2, and then occurs in H₂AsO₄⁻ anion till pH 6.9 and HAsO₄⁻² from 6.9 to 11.4. Besides, with increase in pH the surface charge on the iron adsorbent changes as the pH exceeds the zero point charge (ZPC) of the formed oxides. For example, the ZPC of hematite is 8.1, goethite is 6.9 and of magnetite is 6.4. At pH<ZPC, the surface exhibits positive charge and at pH>ZPC it remains negative.¹²⁷⁾ Combining the speciation and surface charge results, at pH>8 adsorption of negatively charged As (V) oxy anions onto negative charged iron surface remains difficult and thus As (V) uptake decreases. On the other hand, the rate of electrochemical corrosion of iron is reported to be high at lower pH values (pH <6) and decrease with increase in pH from 6–9.^77,109,112) This results in decrease in hydroxide formation which also decreases the Arsenic removal potential. Few studies also reported a greater removal of As (V) at pH-7, rather than pH-4 or less.^116,124) This decrease in arsenic removal at pH < 5 is attributed to the rapid dissolution of iron and less formation of oxides and thickening of oxide scale, thus discouraging the adsorption, precipitation and subsequent removal of arsenic. Thus, for As (V) removal in aerobic waters, pH range of 5 to 7 is more suitable. However, if As (III) species is present in aerated waters, the removal is not direct but undergoes a two-step mechanism, wherein the As (III) initially disappears and later undergoes removal by adsorption and precipitation.¹²⁵⁾ The initial disappearance of As (III) is due to the oxidation of As (III) through intermediate As (IV) to form As (V) species.¹¹⁷⁾ It is reported that, at lower pH values (pH <5), the dominating species that promotes oxidation of As (III) is •OH radicles formed by fenton’s reagents Fe⁺² and H₂O₂. At this low pH, As (III) is oxidized swiftly but its removal as As (V) is not possible due to less formation oxy-hydroxides. At high pH values (pH>5), intermediate oxidant species formed by iron corrosion promotes the oxidation of As (III). The Fe (II) and As (III) species are oxidized simultaneously, otherwise said, the oxidation of Fe (II) products to Fe (III) will promote the oxidation of As (III) as well.^107,113) In the pH range of 5–9, the As (III) oxidation as well as its removal in the form of As (V) are both rapid, however at very high pH values (pH>10) both oxidation of As (III) as well of removal of As (V) stands negligible.¹²⁵⁾ Apart from oxidation, few amounts of As (III) is also adsorbed onto the oxy-hydroxides formed. Reported studies reveal that, adsorption of As (III) species is predominant in the pH range of 7–9, with a maximum at pH-9.⁷⁷⁾ Below pH-7, the adsorption of As (V) dominates and above pH-9, the speciation of As (III) changes from neutral to negative discouraging adsorption.¹⁰⁶⁾ Due to this complicated behaviour of As (III), it is always preferred to oxidize As (III) to As (V) via external aid, encouraging easy As (III) removal in the form of As (V) and also provide large operating pH range for removal.

In anaerobic waters, the removal of arsenic turns yet complicated because, other than adsorption, removal is dominated by precipitation and is sometime supported by reduction as well.¹¹³⁾ Influence of pH on corrosion of iron is slow but is similar to the one as reported in aerobic conditions. The major difference lies in the formation of porous hematite and magnetite scales due to lack of oxygen.¹⁴⁸⁾ Arsenic present in the form of As (III) has three different options for removal now, 1) to undergo adsorption-precipitation, 2) to undergo reduction and 3) to undergo oxidation. pH influence on adsorption and precipitation of As (III) is similar to the one explained in aerobic conditions and occurs best in the pH range of 7–9.^113,114,115) On the other hand, oxidative removal of As (III) is also observed by few researchers.^111,118) Sasaki and team reported that the oxidation of As (III) is performed by intermediate oxidative species formed from Fe (II) oxidation. Whereas, Lien and team reported that the As (III) is sorbed onto the iron powder surface and thus gets oxidized during surface oxidation process. In both the cases the oxidation of arsenic is supported by iron oxidation. Arsenic present in the form of As (V) also undergoes removal by either adsorption-precipitation or by reduction. Removal of As (V) by adsorption and surface complexation with oxides like magnetite is widely reported.^{77,105,106,109,112,113,114,149)} It is observed that acidic pH range (4–7) is suitable for As (V) adsorption and a decrease in adsorption with increase in pH, similar to aerobic conditions is reported.

Reduction of Arsenic species, As (V) and As (III) is quite debatable topic in the research fraternity. Thermodynamic feasibility of Arsenic reduction with iron is well established.^118,126) However, in practical grounds it is not so possible. Even though most of the researchers denied the reduction of Arsenic, very few studies on the reduction of Arsenic species is reported.^106,109,126) It is also observed that the reducing species like, Fe⁺², e⁻, H₂ or H• produced from iron corrosion promote the reduction. Obtained results reveal that Arsenic reduction take place at the pH values <8.5 and the formation of porous oxide layers facilitate the mass transport of Arsenic to the iron surface thus promoting reduction.

Overall, pH plays very important role in the arsenic removal from aqueous systems. Adsorption and precipitation are definitely the prominent mechanisms for arsenic removal by which Arsenic species forms inner sphere bidentate complexes.^{106,107,116,149)} All the above explanation of influence of pH on Arsenic removal is independent of influence of other anions or foreign particles present. However, in practical situations, presence of foreign particles is inevitable and their interference on the Arsenic removal performance is unavoidable. Effect of presence of other anions and conditions on arsenic removal is discussed in the next section.

3.2.3. Other Substances

Substances like phosphates, silicates, sulfates, chlorides, nitrates, carbonates, borates, bicarbonates, molybdates, chromates in the form of anions and other natural organic matter (NOM) in the form of humic and fluvic acids greatly influence the removal of arsenic by iron powders. Their influence is not always linear but also depends on the anion concentration, presence of other anions and physio-chemical condition of the aqueous systems. Su and puls in their work reported the influence of various anions on arsenic removal.¹⁵⁰⁾ Obtained results shows that, presence of phosphate and silicates decrease the removal kinetics of both As (V) and As (III) due to their competitive sorption onto the iron surface forming inner sphere complexes. Similar observation of phosphates and silicates is also reported by few researchers.^110,125,148) On the other hand, borates and sulfates hinders As (III) removal more rapidly compared to As (V) and the influence is also pH dependent. For example, the influence of sulfates remains, negligible at alkaline pH. This result is supported by biterna and team as well.¹²⁵⁾ In a similar way, chromate, molybdate and nitrates negatively influence the arsenic removal kinetics due to their reduction reactions at iron surface. Reduction of nitrate to form ammonia and nitrates in presence of iron powder is observed by tyrovola and team.¹¹⁴⁾ Their study reveals that, nitrate reduction is not influenced by speciation of arsenic, but is influenced by pH and temperature of aqueous system. In another study done by Sun and team, other than the effect of anions (phosphate, nitrate, sulfate), effect of natural organic matter such as humic materials influencing removal is specially reported.¹¹³⁾ Obtained results reveal that, presence of humic material strongly diminished the arsenic removal. The reason is attributed to 1) metal-bridge formations between humic acid and arsenic compounds, 2) competitive sorption between arsenic anions and electronegative humate. Similar effect of presence of humic acid on arsenic removal is also observed by other researchers.^119,120,121) Controlled effect of humic acid on arsenic removal by usage of iron oxide coated sand along with iron powder where in the coated sand trapped the humic acid to restrict its interference in arsenic removal is also reported.¹²⁹⁾ In another study, effect of fluvic acid on arsenic removal is also reported¹³¹⁾ and obtained results reveal that, blockading tendency of active site on iron powder is higher for fluvic acid compared to humic acid.

Besides the reported negative effects of most of the anions on arsenic removal, few interesting studies on positive influence of anions is also reported. In a study done by Mak and team, influence of hardness (calcium and bicarbonates) on arsenic removal is studied.¹²⁰⁾ Interesting results of increase in arsenic removal with presence of HCO₃⁻ and/or Ca⁺² is reported. This is attributed to the formation of CaCO₃, which nucleates the growth of iron-hydroxide particle to promote arsenic removal. Further, in the study done by sun and team¹¹³⁾ similar interesting observation of increase in arsenic removal at higher concentration of sulfates due to formation of precipitated compounds of FeAsS is also reported. Similarly, positive influence of sulfate on arsenic removal in anaerobic waters due to sulfide is observed.¹²⁵⁾ Further interesting results on positive effect of nitrates, chlorides and sulfates is also reported by few researchers.^77,116,151)

Overall, it can be observed that anions like phosphates, silicates, borates, chromates, molybdates along with NOM like fluvic acid and humic acid strongly diminish the arsenic removal performance of IP, whereas anions like sulfates, nitrates, chlorides show negative, neutral as well as positive effects depending on concentration and physico-chemical conditions. On the other hand, hardness of water (bicarbonates and calcium) promotes arsenic removal. So, even though the effect of additional substances seems clear, complex interactions between the targeted arsenic form for removal and unnecessary compounds present at the site of removal will always create un-expected real-time challenges in arsenic removal by iron.

4. Evolution of Arsenic Removal Using Iron Powders: Past, Present and Future

First reports on the usage of Zero valent iron powders for arsenic removal falls back to the year 1998, when Lackovic and Nikolaidis proposed an innovative arsenic remediation technology (AsRT) using iron filings and sand.¹⁵²⁾ Later many researchers begun the usage of iron powders for arsenic remediation and till date many new aspects in its usage, as well as usage of supplementary materials and process to improve the removal of arsenic is being practiced. Pictographic representation of focus of researchers and significant improvements that took place in the past, current research focus and future perspectives are shown in the Fig. 6.

Fig. 6.

Chronological developments in usage of iron powders for arsenic removal.

In the 1st decade of 21st century, most of the research is focused on usage of iron powders obtained either commercially or from industrial by-products. Studies to find the mechanism of arsenic removal using iron powders in aerobic and anaerobic conditions at laboratory batch and pilot field are performed. Effect of pH, presence of anions and other elements on arsenic removal is widely explored. Usage of nano iron powders also gained much attention in the same decade. Later in the years 2010–2020, most of the research is focused on improvement of micro and nano IP as effective media for arsenic removal. Modifying or compounding IP with other material like, sand, iron oxide, bio-char, clay, activated carbon, CuSO₄, MnO₂, TiO₂, microbes, etc., benefitting the arsenic removal process, either by enhancing the IP performance or by modifying the ambience assisting and promoting IP performance is observed. Mak and team made use of IP in combination with iron oxide coated sand for removal of chromate and arsenate.^128,129,131) Reported results revealed that usage of IP in combination with iron oxide coated sand resulted in 70% increase in As (V) removal capacity and 40% increase in Cr (VI) removal capacities in a column. It is also observed that the synergistic effect of duo stands significant for adsorption of aqueous system comprising Cr (VI) and As (V) at the same time as well. Similar study on addition of activated carbon to assist and enhance the adsorption efficacy of IP towards arsenic removal is reported by Chen and team.¹⁵³⁾ Similar but interesting approach to assist and enhance the adsorption of IP media by means of bacteria inoculated IP columns is reported by Azhdarpoor and team.¹⁵⁴⁾ Apart from assisting IP by partnering with it, few interesting works on enhancing the IP performance by either improving reactivity or enhancing adsorption feasibility are also reported. Liang et al observed that mechano-chemical modification of IP with MnO₂ showcased formation Fe-Mn binary oxides which resulted in intensive corrosion thus enhancing the arsenic removal by 15% and also promoted arsenic sludge stabilization.¹³⁶⁾ Similar study on usage of mechanochemically sulfidated micro scale IP benefitting arsenic (As (V)) removal in oxic and anoxic conditions via enhanced iron corrosion, formation of As₄S₄ and FeAsS precipitates is reported by Zhao and team.¹⁵⁵⁾ IP combined with KMnO4 and Fe⁺² ions achieving higher removal of arsenite compared to pristine IP is also observed.¹⁴¹⁾ In another study, addition of CuSO₄ to IP, enhanced the corrosion rate of IP due to formation of Fe–Cu galvanic system, thus improving rate of arsenic removal.¹⁴⁴⁾ In another study, Sun and team treated IP with acid (HCl) to improve its performance of arsenic removal.¹²⁶⁾ Reported results reveal that, acid treated IP showcased similar reactivity as of nano ZVI but at a comparatively lower cost of usage.

Further interesting works to assist arsenic removal by IP via converting difficult to remove arsenic forms such as As (III) into other forms is also reported. In a study done by Katsoyiannis et al, enhanced arsenite removal by usage of iron powder combined with H₂O₂ is presented.¹³³⁾ Similar work on usage of IP in combination with TiO₂ particles to achieve enhanced oxidation of arsenite to arsenate, which is further adsorbed by IP is reported.¹³⁸⁾ Usage of immobilized TiO₂ in combination with IP for photocatalytic oxidation of As (V) to As (III), thus assisting arsenic removal by IP is also reported.¹⁵⁶⁾ In line with above works, few reports on usage of persulfates and ligands enhancing arsenic removal by providing necessary oxidation assistance is also observed.^137,143) Besides chemical assistance works on biological assistance such as addition of arsenite oxidizing bacteria and sulphate reducing bacteria to enhance arsenic removal by IP is also reported.^123,134,146) An interesting study is reported by Zhang and co-workers where in, IP is mechanically activated with CaCO₃ to achieve superior arsenic removal. Obtained results reveal that, addition of CaCO₃ not only improved the reactivity of IP by enhancing its corrosion behaviour and facilitating controlled exposure of IP to arsenic during reaction, but also promoted oxidation of the arsenite to arsenate assisting IP for arsenic removal.⁶⁵⁾ A dual way benefit assisting IP for contaminant removal is realized in this study.

In addition to aforementioned works, usage of physical aspects like air injection,¹⁴²⁾ microwave energy,⁸¹⁾ mechanical milling¹⁴⁵⁾ and magnetic field¹⁵⁷⁾ to improve IP reactivity assisting arsenic removal and integration of few technologies such as microbial fuel cell, membrane distillation, etc., to improve the arsenic removal by IP are also studied.^158,159) Even though many modifications and assistance are reported for arsenic removal the core behaviour of IP with respect to given ambient conditions remains as the soul promoting the arsenic removal. All other modifications will only act as suitable addons and supplements to gear up the removal process.

Thus, current research is focused on usage of modified IP, IP combined with other materials, physical and energy aids, technological integrations, etc., to benefit the arsenic removal. Also, huge leap in the synthesis and utilization of nano iron powders also took place in this decade. Considering the recent interests of the research fraternity, future research may attract inclination towards advanced compounding and modification of iron powders using novel chemical and biological agents, usage of iron based alloys, integration of IP media treatment with other arsenic removal technologies and mobile/movable arsenic removal strategies favouring field implementations.

5. Commercial Presence of Iron Powders in Water Remediation Applications

Scalable production and tailored to specification feasibility of IP also lead to huge market pull and attracted lot manufactures into the domain. Among various commercial technologies available, IP produced from cost effective methods like carbothermic reduction, electrolytic and water atomization are largely preferred for water remediation applications. Reason behind emphasis on the cost is directly related to the techno-economic feasibility of water purification in most of rural world. Major IP producers in the world targeting water purification applications are, Hoganas, Hepure, CRES, IMP, etc. Details on various IP commercially available in the market focusing on environmental, especially water remediation applications are presented in the Table 5.

Table 5. Commercially available iron powders for water remediation.

S. No	Manufacturer	Product	Application/usage	Specifications	Ref
1	Hoganas, Sweden	Cleanit-POU	Drinking water treatment (As, Cr–VI, Pb, Se removal)	Fe-based media	⁷²⁾
		Cleanit LC and LC+	Drinking water (Se, As, Cr–VI, Pb, U removal) and waste water treatment	Fe-based media
		Cleanit EC	Industrial wastewater treatment	Fe–Ag–Cu–Al-mixed electrodes
		CleanER	Insitu Environmental Remediation	Electrolytic + Atomized + reduced iron powders
2	Hepure, USA	FeroxFLow	VOC, heavy metal, PCB removal	Fe (%)-95.48, Avg Size: ~120 μm	⁷¹⁾
		FeroxPRB	Ground water and soil treatment	Fe (%)-95.48, Avg Size: ~325 μm
		FeroxTarget	Environmental remediation	Fe (%)-95.48, Avg Size: ~44 μm
		FeroxPlus	Halogenated hydrocarbons removal	Fe (%)-10–40, oil: 30–40%, Avg Size: ~45 μm
3	CERES, USA	PRB Aggregate	Permeable reactive barriers (PRBs)	Fe (%)-94–99, Size: ~0.3–2 mm	¹⁶⁰⁾
		MICRO₃₂₅	Soil and ground water treatment	Fe (%)-94–99, Avg Size: ~44 μm
		MICRO₁₀₀		Fe (%)-94–99, Size: 150–45 μm
4	Regenesis, USA	AuqaZVI	In situ-Chemical Reduction of Ground water	Fe + Fe (II) sulfide	¹⁶¹⁾
		s-MicroZVI		(Fe + Fe (II) sulfide)- 40% wt + (Glycerol)-60% wt	¹⁶²⁾
5	BASF, Germany	ZVI Microspheres 200	Groundwater treatment	Fe (%)- 97.5, Avg Size: 5 μm	¹⁶³⁾
6	Industrial Metal Powders, India	EC10	Water remediation	Fe (%) >98, Avg Size: < 10 μm	¹⁶⁴⁾
7	Compass Remediation Chemicals	8/50 ZVI	Soil mix and PRB’s.	Fe (%)- >96, Size: 150 μm– 4 mm.	¹⁶⁵⁾
		50D ZVI	Push Injection, Pneumatic and Hydraulic Fracturing	Fe (%)- >96, Size: < 300 μm.
		140D ZVI		Fe (%)- >96, Size: < 100 μm.
8	EOS remediation LLC, USA	EOSZVI	Biotic/abiotic reduction	Fe (%)- 50%, Oil- 41%.	¹⁶⁶⁾
		EOSZVI^XL	Soil mixing, injection	-under launch-
9	Peerless, USA	14D	Push Injection	Size: < 1.4 mm.	¹⁶⁷⁾
		50D	Push Injection, Pneumatic and Hydraulic Fracturing	Size: < 300 μm.
		8/50 ZVI	Soil mix and PRB’s.	Size: 150 μm- 4 mm.
10	Pal Trockner Pvt Ltd	AdsorpAs	Arsenic removal	Fe(OH)₃- 52–57%, Size: 0.2–2 mm	¹⁶⁸⁾
11	Kobe Steel, Japan	Ecomel 53NJ	Heavy metal removal	Fe based alloy, Avg Size: ~50 μm.	¹⁶⁹⁾
12	JFE Mineral Company, Japan	UFPI	VOC and TCE removal	Fe (%) > 98, Size: < 0.1 μm	¹⁷⁰⁾
		MSI-X	Heavy metal removal	Fe (%) > 90, Avg size: 130 μm	¹⁷¹⁾
		MSI-N901T	VOC removal	Fe coated with catalyst	¹⁷²⁾

It can be observed from the tabulated information that, besides pristine IP, addition of other metal powders like Silver, Copper and aluminum is also done to enhance the overall remediation process. Most of the iron powders produced are of 95+% purity with an average particle size ranging from as low as 0.1 microns to as high as 4000 microns. Thus, iron powders, its alloys and mixed products are being commercially produced and are being actively used for water contaminants abatement applications. This commercial presence is one of the attributes which benefits IP for arsenic removal as well.

Overall, it can be noted that, iron powder is one of the widely used treatment media for water remediation applications due to its multi fold advantages in terms of availability, synthesis, mechanism, and usage. Present review not only elucidated various research works on arsenic removal using IP, but also emphasized various aspects related to IP such as its synthesis, mechanism, and characteristics. Special focus is also laid to elucidate the research evolution taking place in the usage IP for arsenic removal. Apart from the research, commercial presence of various form of IP available for water remediation applications is also presented. Among the literature referred in the manuscript one interesting study is performed by Boglaienko and team,⁵²⁾ where in 11 different types of IP from various origins is compared for evaluation of reductive separation of 99 Tc (VII). Studies of similar intention with respect to arsenic removal exists, but are very few.^124,173) A clear white space is available for investigating the viability of various kinds of IP, especially industrially or commercially available grades, for arsenic removal. Special focus on physico-chemical properties of powders influencing the removal at real time applications needs attention. With a magic material like iron powder in hand, which can be commercially produced, which can uptake arsenic by means of multiple mode of removal and which can be a cost effective solution for real time field applications, addressing the problem of arsenic is quite viable.

6. Conclusions

A focused review on usage of micron and macro scale iron powders for arsenic removal, their synthesis, various parameters influencing the removal process along with the details of the removal mechanism is presented. Evolution of arsenic removal using iron powders and its commercial presence is also discussed. It is observed that, corrosion products formed on the surface of iron in aqueous systems are the key drivers dictating the removal of contaminants. Powder properties such as purity, composition, particle size, particle size distribution, surface area and aqueous media properties such as aerobic/anerobic conditions, pH and presence of other anions/compounds greatly influence the arsenic removal. It is highlighted that water properties stand more important as changes in aqueous media properties drastically influences the IP corrosion behavior as well as arsenic speciation behavior greatly affecting the entire removal process. Iron powders due to their availability, synthesis feasibility, scalability, reactivity, multi-mode removal capability and re-usability qualifies to be one of the prioritized medias being used for arsenic removal. Among various commercial synthesis processes available, powders from electrolytic and reduction routes are largely preferred due to their low cost and porous morphology which benefits the remediation applications. Industrial players like Hoganas, Hepure, CRES, JFE and IMP majorly cater iron powders and its relevant products for environmental remediation, especially water remediation applications.

Beginning of 20th century paved path for research works digging towards proper reasoning and understanding of arsenic removal mechanisms by IP. Later on more focus of the research fraternity shifted towards improving the arsenic removal performance of IP by compounding and modifying IP with potential chemicals such as sand, iron oxide, copper, bio-char, clay, activated carbon, H₂O₂, MnO₂, TiO₂, microbes, etc., and by usage of physical energy aids such as air injection, mechanical milling, microwave energy, magnetic field, etc. Even though the chemical and physical aids supports the arsenic removal process either by enhancing the IP corrosion/adsorption or by modifying the Arsenic into easy uptake forms, it is observed that it’s the core iron powder which is driving the removal process potentially. Clear whitespace is available for research in the aspects of utilizing commercially available iron powders for arsenic removal, by exploring the significance of their physical and chemical attributes, process economics, media regeneration/reusability and onsite feasibility.

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