Evaluation of a sustainable magnesium phosphate-based binder modified with oxalic acid-activated bone meal, sodium carbonate, and fly ash for stabilizing Pb- and Cd-contaminated soils

Abstract

The ability of magnesium phosphate cement (MPC) is limited in stabilizing heavy metal-contaminated soils by its high costs and environmentally-unfriendly production. Therefore, reducing MgO and KH2PO4 dosages is essential. This study explored the use of oxalic acid-activated bone meal (ABM), a meat industry byproduct, as a potential substitute for KH2PO4, and introduced sodium carbonate (NC) and carbide slag (CS) as activators in MPC systems with high-volume fly ash (FA) to enhance strength. The aim was to develop a sustainable binder, MPS, for efficiently stabilizing the soils contaminated with high concentrations of lead (Pb) and cadmium (Cd). The effects of various ABM/KH2PO4 ratios, MgO/KH2PO4 ratios, FA dosages, and NC and CS additions on the stabilization performance were evaluated based on unconfined compressive strength (UCS) and leaching characteristics, and MPS composition was determined. Subsequently, the stabilization mechanisms of MPS on Pb and Cd were analyzed through microscopic tests. The results showed that increasing the MgO/KH2PO4 ratio enhanced UCS and reduced Cd and Pb leaching. The FA addition reduced UCS and increased the mobility of Cd and Pb. At a 40% FA dosage, NC optimally enhanced UCS by 20.8% − 37.9% without significantly influencing leachability. ABM can substitute KH2PO4 within an appropriate range, meeting strength and leaching requirements. The formulated MPS had an MgO/KH2PO4 ratio of 6, 40% FA dosage, 4% NC dosage, and an ABM/KH2PO4 ratio of 0.768, exhibiting cost-effectiveness and outstanding performance in stabilizing Pb and Cd contaminated soils. Microscopic tests revealed the presence of following MPS hydration products: MgKPO4·6H2O, MgHPO4·3H2O, Mg6Al2CO3(OH)16·4H2O, and magnesium silicate hydrate (MSH) gel. Formations of Pb3(PO4)2, Pb10(PO4)6(OH)2, Cd5(PO4)3OH, and CdCO3 are the main chemical mechanisms underlying the stabilization of Pb and Cd.