Contrasting effects of Corn cob and Cocoa pod husk biochars on Heavy metal Bioavailability, Speciation, and Uptake by Maize in a Mining-Contaminated soil

Abstract

Soil contamination by heavy metals (HMs) poses substantial risks to agricultural productivity and environmental health. Biochar, a carbon-rich material produced from organic waste, has been proposed as a soil amendment to lower HM bioavailability and enhance soil fertility. This study assessed the effects of cocoa pod husk biochar (CPHB) and corn cob biochar (CCB) on the bioavailability, speciation, and uptake of HMs (Cd, Cu, and Pb) by maize (Zea mays L.) in contaminated agricultural soil. A 60-day greenhouse experiment was conducted with biochar application rates of 1%, 2%, and 3% (w/w) at 70% field capacity. Soil properties such as pH, soil organic carbon (SOC), and cation exchange capacity (CEC) were analyzed before and after the experiment. HM bioavailability and speciation were determined using sequential extraction methods, while metal accumulation in maize roots and shoots was measured to calculate the bioconcentration factor (BCF) and translocation factor (TF). Biochar amendments raised soil pH, CEC, and SOC, with the most notable improvements observed in CPHB at 3%. The proportion of exchangeable HM fractions decreased relative to total content (Cd: by 5.12%, Cu: by 4.88%, Pb: by 3.89%), while residual fractions increased (Cd: by 58.14%, Cu: by 60.24%, Pb: by 52.11%), particularly at 2% and 3%. HM accumulation in maize roots and shoots significantly declined, with CPHB at 3% showing the greatest reduction. Correspondingly, the application of CPHB at 3% led to a decrease in BCF and TF values for Cd, Cu, and Pb, indicating diminished metal uptake by maize roots and limited translocation from roots to shoots. These findings suggest that CPHB, especially at a 3% rate, effectively reduces HM toxicity and bioavailability while promoting soil fertility, thus offering a promising strategy for remediating soils affected by mining activities.