Investigating the carcinogenic and non-carcinogenic health hazards of heavy metal ions in Spinacia oleracea grown in agricultural soil treated with biochar and humic acid

This study addressed the bioaccumulation and human health risk among consumption of Spinacia oleracea grown in agricultural soil treated with humic acid and biochars. The biochars came from two local feedstocks of rice husk (RH) and sugar beet pulp (SBP) pyrolyzed at temperatures 300 and 600°C. The effect of soil treatment with biomass/biochar and humic acid in the accumulation of Heavy metals (HMs) in spinach leaves was considered under the controlled laboratory conditions. Optimum conditions for minimum accumulation of HMs were evaluated according to a central composite design (CCD) under the response surface method (RSM) by Design Expert software. Total concentrations of Cu, Cd, and Ni in soil and biomass/biochar were beyond acceptable standards worldwide. The concentrations of Zn, Cd, Pb, and Ni in spinach exceeded the safety threshold limit, and its consumption is not recommended. Results of the desirability test suggested the minimum values of biomass/biochar application rate due to their metal-contaminated nature. The minimum concentration of humic acid also was suggested due to its role in promoting pollutant bioavailability. Results showed that the treatment of metal-contaminated soil with HMs polluted-biochars fails to stabilize pollution. Also, polluted biochars increased the hazard indices for non-carcinogenic and carcinogenic diseases beyond the permissible levels.