The effect of biochar on immobilization and phytoavailability of chromium, nickel and lead in soils amended with slag

Abstract

Application of industrial wastes in agricultural fields as sources of plant nutrients has become a common practice in agriculture. In other countries and recently South Africa, steel plant slag (SPS) (an industrial waste) is used in agricultural fields as liming material. Steel plant slag is a potential source of plant nutrients, especially in areas where plant iron chlorosis is a problem. The use of SPS as liming material on agricultural soils seems viable despite the detrimental effects due to heavy metals contained in SPS. The presence of heavy metals [chromium (Cr), nickel (Ni) and lead (Pb)] in higher concentrations in potential agricultural soils has negative effects to fauna and flora. Naturally soils possess traits that counter the accumulation of heavy metals, for example, texture and pH of a soil can play a role in the adsorption of heavy metals. An alternative strategy would be that capable of enhancing the metal-binding capacity of soil amendments such as biochar. In this study, the aim was to evaluate the effect the different types of texture, the efficacy of different types of biochar and rate of biochar on the concentration of chromium, nickel and lead. The greenhouse pot experiment was therefore conducted to investigate the effect of biochar on the immobilization and phytoavailability of chromium, nickel and lead in soils amended with slag. A 2 x 2 x 4 factorial experiment was conducted with three factors consisting of two soils (sandy and clay), two biochar and four biochar rates (0 t/ha, 5 t/ha, 10 t/ha, and 20 t/ha). All the soil and biochar were thoroughly mixed. 4 kg of soils was placed in a pot. Slag was then applied to each pot at an amount equivalent to 15 g/kg (75 g per pot). The treatments were replicated four (4) times to give a total of 64 pots and were arranged in a completely randomized design (CRD) in a greenhouse. The soils were analyzed for texture, exchangeable cations (Mg2+, Ca2+, K+), pH, electrical conductivity and soil aggregates. Slag was analyzed for potassium, phosphorus, pH, electrical conductivity, total nitrogen, chromium, nickel and lead, and biochar was analyzed for organic carbon content, pH, electrical conductivity, calcium, magnesium, potassium and phosphorus. The data showed that acacia and poultry litter biochar significantly (P< 0.01) immobilized and reduced the phytoavailability of Cr, Ni and Pd in the soil. For example, the application of 20 t/ha biochar in this study significantly decreased the concentrations of chromium, nickel and lead. The biochars immobilized the selected heavy metals from a range of 9.3% to 89.9% and reduced the phytoavailability from a range of 0.0248% to 8.1x10-8%. Acacia and poultry litter biochar significantly increased the shoot dry biomass of spinach plant by 1.2% to 85.1%. Following the application of biochar to the soil, electrical conductivity and pH increased significantly (P< 0.01) from 24% to 22% and 11.3% to 44.7% respectively. Application of acacia and poultry litter biochar significantly (P <0.01) increased soil stable aggregates by 14.3% to 43.5%. Application of acacia and poultry litter biochar reduced the toxicity of heavy metals (chromium, nickel and lead) in spinach by immobilizing and reducing phytoavailability of the heavy metals. Acacia and poultry litter biochar have positive effects on the soil pH, electrical conductivity, plant growth (shoot dry biomass of spinach) and soil stable aggregates.