Abstract
We aim to establish a zero-emission-type phosphorus removal and recovery system using a Mg-Al-Cl form of hydrotalcite (HT), which is one of the inorganic ion exchanger compounds having a layered structure. In previous papers, it was confirmed that HT is a practical adsorbent having high selectivity and a large adsorbing capacity for phosphate ions. Phosphate ions adsorbed onto HT can be efficiently desorbed, thereby regenerating HT adsorption capacity, using a “two-step regeneration system”, which uses a NaOH-NaCl mixed aqueous solution as the desorbing solution and a MgCl2 aqueous solution as the regenerating solution. Moreover, phosphate ions can be recovered as magnesium ammonium phosphate (MAP) from the desorbing solution in the system. In this paper, the conditions, such as optimum pH and the mixing molar ratios of PO43-, Mg2+ and NH4+ ions, for recovering phosphate ions in high yields as MAP from the desorbing solution were investigated. In addition, the effects the number of times the desorbing and regenerating solutions were used on phosphate recovery ratio and what regeneration ratio were studied. As a result, more than 99% of the phosphate ions were recovered as MAP under the conditions in which the mixing molar ratios of PO43-, Mg2+ and NH4+ ions were 1:3:5 at an initial pH of 12 (convergence pH 8.7). The desorbing and regenerating solutions can be used repeatedly. When using the desorbing and regenerating solutions five times, the phosphate recovery ratio was higher than 99%. Further more, the phosphate adsorption capacity of HT was maintained at 80% of the initial value. These results suggest that the establishment of the zero-emission-type system that removes and recovers phosphate from wastewater using HT progressed greatly.
