The stabilization mechanism of high-efficiency magnesium phosphate cement for arsenic remediation in lollingite polluted environments

Industrial non-ferrous metal smelting and processing activities are a major contributor to the migration of arsenic-containing minerals throughout the environment, such as lollingite, which poses a severe threat to human health due to its rapid oxidation and subsequent release of arsenic (As). Herein, two phosphate mineral systems (potassium phosphate and calcium phosphate) were used to explore the potential for As solidification and establish the As immobilization mechanisms. Experimental results show that the compressive strength of the calcium phosphate system was higher than that of the potassium phosphate system due to the higher bond energy of the Ca-O-Mg-O-P-O system (31109 kJ/mol), as compared to the K-O-Mg-O-P-O system (16189.2 kJ/mol). Furthermore, the coordination capability of arsenic oxyanions with Ca or Fe ions was stronger than that of phosphorus oxyanions with Ca or Fe ions. The main mechanism of inhibition of As migration by magnesium phosphate cement (MPC) binders, was the formation of FeAsO4 center dot H2O, Ca-3(AsO4)(2)center dot H2O, MgHAsO4 center dot H2O, CaMgAsO4(OH) and KMgAsO4 center dot 6H(2)O coprecipitates, with physical encapsulation by MgKPO4 center dot 6H(2)O (K-struvite), Ca2Mg(PO4)(2)center dot 2H(2)O (Ca-struvite) and Mg2PO4(OH) phosphate-gel. Using Fe2O3-modified MPC binders with a Fe/As molar ratio of 3, the As immobilization efficiency reached 98.73%. The results of toxicity characteristic leaching procedure (TCLP) analysis confirmed that the maximum leached As concentrations from MPC binders cured for 7 days and 28 days were 0.31 mg/L and 0.06 mg/L, respectively, both of which were below the maximum permitted value for safe disposal (5 mg/L). These results verify the potential of MPC binders to be utilized as low-environmental impact and high-efficiency cementitious matrices for As pollution control and environmental remediation.