Change in the binding form of copper and zinc in sewage field soil by addition of biochar after eight years of field exposure

The former sewage field south of Berlin are contaminated with heavy metals due to decades of wastewater irrigation. Since the abandonment of irrigation, the pH values have decreased and the accumulated organic substance mineralized, so that the mobility of the heavy metals increased and the plant-available zinc and copper contents reach concentrations critical for plant growth.

In 2011, biochar was added to immobilize the heavy metals as part of a field trial. On samples aged in the field with 0 and 5% biochar, we examined the change in the binding form of copper and zinc using the sequential extraction procedure according to Zeien and Brümmer (slightly changed) after eight years of field exposure.

As to copper, the plots with 5% biochar addition tended to have lower concentrations as a percentages of the total contents in fractions I-III (I mobile, II easy to supply, III bound to Mn oxides) compared to the plots without biochar addition. In fractions IV and V (IV organically bound, V bound to poorly crystalline Fe oxides) no clear differences could be found. In fractions VI and VII (VI bound to crystalline Fe oxides and VII residual), the plots with 5% biochar showed significantly higher percentages of copper. In samples with biochar addition from 2011, changes in the copper binding form were not clearly ascertainable. For zinc, only fractions I and II decreased significantly after 8 years of field exposure in the plots exposed to biochar. No clear changes could be seen in all other fractions. These effects were also present directly after biochar addition. Therefore, biochar appears to be a suitable additive to immobilize zinc and, in the long run, especially copper.

Further studies on the remobilization of copper and zinc on biochar particles separated from the aged field samples are currently running and can probably also be presented.