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Suitability of biochars (pyro‐ and hydrochars) for metal immobilization on former sewage‐field soils

Mineralization of organic material and decreasing pH are expected to increase bioavailability and leaching of metals in soils formerly contaminated by irrigation with wastewater. Biochar has recently been proposed as a sorbent for metals, although the effects described in the literature are still inconsistent. Therefore, we conducted pot experiments to study the effects of the application of two different biochars to a sewage‐field soil on growth and nutrition of oat (Avena sativa L.) as well as on the mobility of zinc (Zn), copper (Cu), cadmium (Cd) and lead (Pb) in the soil solution. Poplar‐derived hydrochar (obtained by hydrothermal carbonization) and maize‐derived pyrochar (obtained by pyrolysis) were mixed with the soil at concentrations of 0, 1, 2.5 and 5% (g 100 g−1). Soils were leached after 2, 3 and 5 weeks with deionized water. Hydrochar reduced biomass production and had almost no effect on metal concentrations in plants and leachates. The negative effect on biomass yield may be attributable to decreased nitrogen (N) availability. Pyrochar increased biomass production and reduced plant Zn and Cd concentrations. In contrast, metal concentrations in soil leachates generally increased. Comparison of ultra‐centrifuged and filtered samples, microprobe analysis of centrifugation residues and Visual Minteq calculations suggest colloidal transport of Zn precipitates, whereas Cu is transported in the dissolved fraction. We conclude that the tested hydrochar is not suitable for metal immobilization. In the case of pyrochar, further research on colloidal transport of metal precipitates is needed to assess its suitability for soil remediation, despite positive effects on plant growth and the reduced uptake of Zn and Cd.