[Remediation of Cd/Ni Contaminated Soil by Biochar and Oxalic Acid Activated Phosphate Rock]

Taking soil contaminated with a combination of Cd and Ni as the research objective, biochar, and oxalic acid activated phosphate rock (APR) were applied both together and separately for the remediation of this contaminated soil. The effects of different ratios of amendments on the remediation of Cd and Ni heavy metals in the soil and on inorganic nitrogen and microbial biomass nitrogen (MBN) in the soil were compared. The results show that an increasing amount of biochar and APR, increases the soil pH gradually and acid-extractable Cd and Ni are gradually transformed into reducible, oxidable and residual Cd and Ni, resulting in a reduction in Cd and Ni bioavailability. After 40 days incubation, the acid extractable Ni decreased by 37.04% with a 14.8% increase in residual Ni, and acid extractable Cd decreased 40.28% with a 35.20% increase in residual Cd with the amendment of C50P3 (Applying 50 g·kg^-1 biochar and 3 g·kg^-1 APR) when compared to C0P0 treatment (Applying nothing). Furthermore, the MBN content for C50P0 (Applying 50 g·kg^-1 biochar only) and C0P3 (Applying 3 g·kg^-1 APR only) increased by 1.5 and 1 times, respectively, while the content of ammonium nitrogen decreased by 12.5% and 6.4%, respectively and the content of nitrate nitrogen decreased by 11.6% and 10.2%, respectively. This comparison shows that the combined effect of the application of biochar and APR is superior to each respective separate treatment. A mixture of 50 g·kg^-1 of biochar and 3g·kg^-1 of APR (C50P3) demonstrates the best effect on the remediation of the Cd and Ni in soil. Furthermore, the application of amendments promoted the transformation of inorganic nitrogen into organic nitrogen.