Stability of calcium and magnesium phosphate sorbents in solution of calcium chloride and complexing agents

Abstract

The study of the stability and physicochemical transformations of calcium and magnesium phosphate sorbents in a solution of calcium chloride and complexing agents: disodium salt of ethylenediaminetetraacetic acid and oxyethylidenediphosphonic acid established that calcium phosphates undergo no signifi cant changes in aqueous systems under study, while magnesium phosphates (trimagnesium phosphate and magnesium hydrogenphosphate) are converted into calcium phosphates in solutions of CaCl 2 , therewith the introduction of ethylenediaminetetraacetic acid (EDTA) into the solution of CaCl 2 has no signifi cant effect on the magnesium phosphate conversion process, and introduction of oxyethylidene diphosphonic acid (OEDP) inhibits crystallization of calcium phosphate formed. Observed conversions should be considered when using magnesium and calcium phosphates as sorbents.Sorbents obtained earlier based on calcium and magnesium phosphates have shown high effi ciency in purifi cation of aqueous solutions from various metal ions (Zn 2+ , Cu 2+ , Pb 2+ , Cd 2+ , Ni 2+ , Co 2+ , etc.), including ions of stable and radioactive strontium [1,2]. An effi ciency of sorption purifi cation is largely determined by both the nature of the sorbent and sorption process conditions (composition of a solution, a ratio of the solution volume to weight of the sorbent, pH, etc.) [3−5]. Researching the sorption of ions of various metals was performed by means of model solutions in the absence of any other impurities, while the actual wastewater and liquid radioactive waste contain substantial quantities of dissolved calcium salts and organic compounds: surfactants and chelating agents [usually ethylenediaminetetraacetic acid (EDTA), oxyethylidenediphosphonic (OEDP) acid [6].Given that phosphate sorbents contain calcium and magnesium cations, which form stable complexes with EDTA and OEDP a possible interaction in systems chelating agent−sorbent can be propose, and in the solutions of CaCl 2 calcium and magnesium phosphates may undergo chemical and phase transformations. However, the reports on the interaction of calcium and magnesium phosphates with aqueous solutions of chelating agents are reduced to the study of the interaction of hydroxyapatite with EDTA [7,8].In the study researching the stability, chemical and physical processes occurring in the interaction of calcium and magnesium phosphate sorbents with calcium ions and chelating agents (EDTA and OEDP) was performed for effective practical use of calcium and magnesium phosphates in the sorption purifi cation of real wastewater and liquid radioactive waste. EXPERIMENTALСalcium and magnesium synthetic phosphates of different composition were used as objects of study: acidic CaNPO 4 (calcium hydrogen phosphate, CHP) and MgHPO 4 ·3H 2 O (magnesium hydrogen phosphate trihydrate, MHPTH); medium Ca 3 (PO 4 ) 2 ·2H 2 O (tricalcium phosphate dihydrate, TCPDH) and Mg 3 (PO 4 ) 2 ·10H 2 O (trimagnesium phosphate decahydrate, TMPDH) with a small admixture of MgNH 4 PO 4 ·H 2 O and Mg 2 PO 4 OH·4H ...