Research On Solidification And Stabilization Of Lead-Cadmium Contaminated Soils By A Novel Phosphorus-Containing Binder MPS

Conventional Portland cement-based binders have many disadvantages,such as high pH,environmental unfriendly,not suitable for high concentration of heavy metal pollution,relatively poor durability performance of treated soils and so on,which have limited their utility in solidification/stabilization（S/S）of heavy metals conamtinated soils.It is of great significance to develop novel environment-friendly binders and systematically study their effectiveness,heavy metal immobilization mechanisms,and long-term performancesunder actual field conditions in treating heavy metal contaminated soils.In this paper,a novel binder MPS was developed,and the mechanism and effect of stabilizing high concentration Pb and Cd contaminated soil were studied.The main research contents and results are as follows:（1）The activation method and the effect of oxalic acid on bone meal material（BM）were studied,and the remediation effect of bone meal material on Cd and Pb contaminated soil before and after activation was compared.The results show that the increase of the concentration of oxalic acid and the liquid-solid ratio of（ml:g）to BM can improve the solubility of activated bone meal material（HBM）and promote the release of available phosphorus.The optimal activation effect of the BM is 1 mol/L oxalic acid with a liquid-solid ratio of 2:1.The solidification effect of HBM on Pb contaminated soil is obviously better than that of the same amount of BM,and the long-term（90d or more）solidification effect of HBM on Cd contaminated soil is slightly better than that of the same content of BM.（2）By comparing the solidification effects of magnesium silicate and magnesium phosphate on Pb and Cd contaminated soil,it shows that the heavy metal leaching concentration of magnesium phosphate stabilized soil is lower.So,it is proposed to develop a novel binder MPS,which consists of magnesium phosphate and HBM.（3）The effects of MgO:KH₂PO₄ molar ratio,fly ash content,sodium carbonate,carbide slag and HBM content on the strength,leaching characteristics and p H value of MPS stabilized soil were studied.The results show that the increase of MgO:KH₂PO₄ molar ratio is beneficial to the increase of strength of MPS solidified soil and the decrease of leaching concentration of Cd and Pb.Fly ash can reduce the p H value of stabilized soil,but it is not conducive to strength growth and anti-toxic leaching ability.When the content of fly ash is more than 40%,the unconfined compressive strength of stabilized soil is too small,and the leaching concentration of Cd and Pb is significantly increased.Compared with carbide slag,sodium carbonate has a better effect on the strength of MPS stabilized soil,which can improve the strength by 20-38%.When the amount of HBM to the cementitious material is more than15%,the strength of the stabilized soil is greatly weakened,and when the amount of HBM to the cementitious material is more than 25%,the leaching concentration of the stabilized Cd and Pb contaminated soil increases.（4）By analyzing the effects of MgO:KH₂PO₄ molar ratio,fly ash amount,sodium carbonate and carbide slag and HBM amount on the solidification effect of the novel binder MPS,The best proportion of MPS is as follows:the molar ratio of MgO:KH₂PO₄ is 6:1;the content of borax is 5%of the mass of MgO,the content of fly ash is 40%of the mass of cementitious material,the content of sodium carbonate is 10%of the mass of fly ash,and the content of activated bone meal is 15%of the mass of cementitious material.Under this ratio,MPS stabilized soil has good strength and leaching characteristics,and the leaching concentrations of Cd and Pb can meet the requirements of the limit in GB 5085.3-2007,and the leaching concentration of Pb can be lower than the IV water limit in GB/T 14848-2017.（5）The hydration products of MPS and the solidification mechanism of Cd and Pb were analyzed by microscopic experiments and chemical forms of heavy metals.The results show that the main hydration products in MPS paste are magnesium hydrogen phosphate trihydrate and magnesium potassium phosphate hexahydrate,which exist as the form of rods and plates.Hydrotalcite and magnesium silicate hydrate gel mainly exist as the form of rose network structure.Pb is immobilized in the form of lead phosphate and lead hydroxyphosphate in MPS-Pb paste,while Cd is immobilized in cadmium hydroxyapatite and some complex or amorphous form in MPS-Cd paste.Meanwhile,there are many particles and clastic substances in MPS-Pb paste and MPS-Cd paste,and the hydration products are no longer complete due to the hindrance of hydration process.（6）The strength characteristics,leaching characteristics and environmental friendliness of the lead-cadmium composited contaminated soil stabilized by MPS and PO 42.5 cement were compared.The results show that MPS solidified soil has good characteristics of strength,and the 28d unconfined compressive strength of 12%MPS stabilized soil can reach 2727 KPa.The 28d unconfined compressive strength of 10%MPS is also close to that of PO 42.5 cement stabilized soil.The solidification effect of MPS on contaminated soil in Baiyin site is slightly weaker than that of PO 42.5 cement.The Cd leaching concentration of 12%MPS stabilized soil cured for 7 days and 10%PO 42.5 cement stabilized soil cured for 14 days can reach the limit.The risk of alkali pollution to the environment caused by MPS is less than that of PO 42.5cement.The p H value of 28d PO 42.5 cement stabilized soil is 10.16%-19.6%higher than that of MPS stabilized soil with the same amount.