Effect of dried olive pomace – derived biochar on the mobility of cadmium and nickel in soil

• Soil pH and organic matter content increased after biochar addition to soil.
• Metal sorption increases as the biochar amendment rate of soil is higher.
• Biochar addition affects metal adsorption on soil surfaces and metal precipitation.
• Using the biochar produced at 700 °C leads to higher metal retention on soil.
• Both tested biochars improve the capacity of soil to immobilize cadmium and nickel.

This study aims to evaluate dried olive pomace derived biochar as a potential soil amendment for cadmium and nickel immobilization. Biochar was produced through pyrolysis under oxygen-limited conditions, at 400 °C (BC400) and 700 °C (BC700). Batch sorption/desorption experiments were conducted, investigating different agitation times (30–1440 min), initial metal concentration in the solution (100–3000 μM) and desorption pH (2–7). Results showed that by amending soil with biochar at increasing rates, enhanced sorption and decreased desorption of both metals were observed. In fact, biochar addition resulted in increased soil pH, possibly enhancing not only metal adsorption on soil surfaces (minerals or oxides), but also metal precipitation. Modeling results concerning sorption equilibrium corroborated this statement. Amended samples showed higher metal retention even at low pH values. Both biochars enhanced the ability of soil to immobilize cadmium and nickel, however in most cases BC700 was proven to be more efficient.