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

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 char-acteristics, 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 re-quirements. 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 & sdot;6H2O, MgHPO4 & sdot;3H2O, Mg6Al2CO3(OH)16 & sdot;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.

Automated summary

This study developed a sustainable binder, MPS, for efficiently stabilizing soils contaminated with high concentrations of lead and cadmium. The use of oxalic acid-activated bone meal as a substitute for KH2PO4, and the introduction of sodium carbonate and carbide slag as activators in MPC systems with high-volume fly ash were explored. The formulated MPS showed cost-effectiveness and outstanding performance in stabilizing Pb and Cd contaminated soils.