Cesium extraction from solutions by magnesium phosphates sorption

An investigation has been made of the possibility of selective separation of Cs+ ions from solutions with the aid of the following inorganic ion exchangers: Mg3(PO4)2x22H2O and NH4MgPO4x6H2O. The ion exchange properties have been studied under dynamic conditions. It has been found that the first exchanger has no sorbent power for Li+ and Na+, weakly absorbs K+, more efficiently Rb+ and selectively Cs+ ions (epsilonsub(Cs)=0.90 mg-equiv./mg-ion PO43- from 0.05 NCsCl solution). The absorption of Cs+ ions is accompanied by the formation in solution of an equivalent amount of Mg2+ ions. Simultaneously the structure of the phosphate changes into that of struvite. It has no absorption power for Li+, Na+, K+ (at concentrations of LiCl, NaCl and KCl up to 2.5N), has weak sorption for Rb+ (and only in the case for sufficiently concentrated solutions) and good sorption for Cs+ ions (epsilonsub(Cs)=0.85 mg-equiv/mg-ion PO43- from 0.05NCsCl solution). The exchange of ions Cs+ reversible NH4+ is completely reversible. The sorption process is considered as the function of the properties of solid and liquid phases of the system. It has been shown that the exchange ions form part of the structural elements in the crystalline lattice of the phosphate. They occupy positions inside distorted trigonal prisms with one O atom and five H2O molecules at the vertexes. Correspondence in the size of ions and voids in the phosphate lattice is one of the factors determining the selectivity of the sorption process. Another factor here is the state of exchange ions in liquid phase. Its role in sorption can be very important since Cs+ ions must upon entering the crystalline lattice of the phosphate undergo dehydration, and the energy effect of the sorption process on the whole is small