Adsorption behavior of arsenate on hydroxides of Fe and Al with Fe-OH and Al-OH groups has been widely studied and also compared with that of phosphate. In this study, we for the first time compared the adsorption of arsenate and phosphate on natural allophane, until initial adsorbates concentration up to 70 μM with allophane/solution ratio of 50 mg/105 mL. For both arsenate and phosphate, the isotherms fitted well to Langmuir equation, and calculated maximum adsorption, Xm, decreased with increasing pH. A parameter related to adsorption strength, K, was greater for arsenate than for phosphate at pH from 5 to 8. Molecular orbital calculations with a model cluster of allophane at B3LYP/6-31+G(d,p) level showed that, for both monodentate and bidentate adsorptions, Al-OAs(P) bond length was shorter for arsenate than for phosphate, and ΔE value of the adsorption reaction was lower for arsenate than for phosphate. These supported the observed greater K value for arsenate than for phosphate. Because previous studies on gibbsite showed stronger adsorption for phosphate than for arsenate, the inverse results on allophane was attributed to the effect of Si tetrahedra attached to gibbsite sheet which may altered the nature of the Al-OH group of allophane. The strong adsorption and high removal ratio of arsenate shown in this study, especially at lower concentrations, suggested the applicability of aluminum silicate materials such as allophane as an excellent adsorbate for arsenate in water environments.

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The mineralogical, chemical, and physical properties of bentonites vary to a great extent according to their parent rock and the location of mines. In this study, characteristics of lead sorption have been assessed for 16 bentonites from 5 localities in Japan and U.S.A in terms of their mineralogy and chemistry to gain more substantive information about the differences in their lead sorption capacities. In batch sorption tests, most of the Na-bentonites from Japan sorbed higher amounts of lead than the ones from Wyoming in US, and the Na-bentonites from Gunma exhibited the highest levels of lead sorption. Calcium carbonate minerals in the Na-bentonites from Yamagata and Gunma function effectively as buffers against pH reduction caused by the addition of acid solution containing lead and the protons released by surface precipitation of lead on clay minerals and hydroxides in the samples, resulting in the enhancement of lead sorption. Multiple regression analyses showed that CaCO_3 content in all the sampled bentonites contributes the most to maximum lead sorption, followed by exchangeable sodium percentage and cation exchange capacity. Selective sequential extraction for lead-added bentonites indicated that lead is sorbed through carbonate and exchangeable phases. Cerussite (PbCO_3) was identified by X-ray diffraction in the Na-bentonites from Yamagata and Gunma after sorption of lead, indicating that lead is mainly sorbed in the form of cerussite.

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A 2:1 regular interstratified illite-smectite (ISI) has been found in the hydrothermal alteration zone of the Toyoha mine accompanying a 1:1 regular interstratified (IS) and nonregular interstratified I/S. These coexisting regular ISI and IS constituents are examined by XRD through refinement and simulation processing of their complicated XRD patterns. The basal reflections of the ISI constituent are very close to an integral series, yielding mean d(001)=35.36±0.13Å with CV value of 0.53% for the 12 reflections in air-dry and mean d(001)=36.98±0.11Å with CV value of 0.42% for the 12 reflections after glycolation. The CV values less than 0.75% are decisive of high degree of regulation, revealing a proper 2:1 regular interstratified illite-smectite with complete ordered ISI succession. The IS constituent is also equally a proper 1:1 regular interstratified illite-smectite with complete ordered IS succession, that is rectorite giving mean d(001)=25.13±0.10Å in air-dry and mean d(001)=26.98±0.12Å after glycolation. The existence of ISI regular intestratified is to be significant in terms of fundamental particles constructing illite-smectite interstratified system.

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