Modified chicken manure biochar enhanced the adsorption for Cd2+ in aqueous and immobilization of Cd in contaminated agricultural soil

Biochar is thought to be good sorbent for heavy metal and exploring ways to increase the efficiency of heavy metal adsorption by biochar is of great importance. Chicken manure biochar was modified with sulfur, hydroxyapatite and MnFe₂O₄ respectively. The properties and composition of the pristine and modified biochar was characterized. The pH and ash content of biochar was significantly increased after modification. Energy dispersive spectroscopy results showed that biochar modified with sulfur, hydroxyapatite and MnFe₂O₄ was successfully loaded on S, Ca/P and Fe/Mn respectively. The adsorption kinetic of Cd²⁺ absorption by pristine and modified biochar was better fitted by the pseudo second-order kinetic model, suggesting that the adsorption of Cd²⁺ on biochar followed the process of chemisorption. The Cd²⁺ adsorption isotherms of sulfur modified chicken manure biochar (SCMB), hydroxyapatite modified chicken manure biochar (HCMB) and MnFe₂O₄ modified chicken manure biochar (FMCMB) was better fitted by Freundlich model, while the Cd²⁺ adsorption by pristine chicken manure biochar (CMB) was well fitted by Langmuir model. The maximum Cd²⁺ adsorption capacity of SCMB, HCMB, FMCMB and CMB was 188.20, 111.53, 109.94 and 19.65 mg·g⁻¹ respectively. Quantitative analysis of Cd²⁺ adsorption mechanism by biochar showed that the contribution of ion exchange for Cd²⁺ adsorption of CMB accounted for 58 %, while SCMB, HCMB and FMCMB decreased to only 12 %, 8 % and 4 % respectively. Meanwhile, the contribution of precipitation, complexion and metal-Cπ coordination for Cd²⁺ adsorption increased after modification. Pot experiment showed that application of SCMB significantly increased soil pH value, decreased the bioavailable Cd in soil and Cd uptake by brassica chinensis shoots, suggesting that SCMB can be a potential material for the safety use of Cd contaminated agricultural soil.