Effectiveness of biochar to reduce soluble Cd concentrations in a smelter impacted sandy soil

Soil in the vicinity of smelter factories are to some extent significantly contaminated by heavy metals. The incorporation of organic amendments to stabilize metal concentrations in such soils may provide a long-term and cost-effective remediation solution. In a three-year study, the potential of holm oak-derived biochar (pyrolysed at 650°C) for the immobilization of Cd was assessed in a smelter impacted sandy soil. Pot experiments were prepared with the soil-biochar mixture and monitored by means of rhizon soil moisture samplers (SMS) and diffusive gradient in thin films (DGT). Analyses of pore water samples obtained by rhizon SMS showed that biochar significantly reduced the concentration of soluble Cd to 10 µg/l, compared to 120 µg/l at the start of the experiment. The Cd immobilization effect was relatively stable throughout the three years of the study, suggesting that aging did not significantly affect the sorption capacity of biochar. This observation was attributed to the consistently alkaline pH and lower concentration of dissolved organic carbon (DOC) in the biochar treatments. The latter may have been caused by sorption of DOC onto the surface of biochar, thereby increasing its negatively charged functional groups that are able to sorb cations. For comparison, compost and peat significantly increased the Cd concentrations in the soil pore water. This was due, in part, to the high concentrations of DOC in the compost and peat treatments, which may have caused the formation of soluble organo-metallic complexes. DGT measurements were taken in the third year to gain insight into the kinetics of metal resupply from the solid phase to soil solution. The results revealed that the addition of biochar, compost, or peat had no significant effect on the inherently slow metal desorption kinetics of the contaminated soil. In conclusion, these results highlight the superiority of biochar over other organic amendments for metal immobilization, however, it also raises questions about influence of biochar on the solid phase buffering of metals in soil solution. Further studies are required to better understand the kinetics of metal resupply from the solid phase in biochar amended soils.