Summary 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.

As natural resources become increasingly limited, the value of restoring contaminated sites, both terrestrial and aquatic, becomes increasingly apparent. Traditionally, goals for remediation have been set before any consideration of goals for ecological restoration. The goals for remediation have focused on removing or limiting contamination whereas restoration goals have targeted the ultimate end use. Here, we present a framework for developing a comprehensive set of achievable goals for ecological restoration of contaminated sites to be used in concert with determining goals for remediation. This framework was developed during a Society of Environmental Toxicology and Chemistry (SETAC) and Society of Ecological Restoration (SER) cosponsored workshop that brought together experts from multiple countries. Although most members were from North America, this framework is designed for use internationally. We discuss the integration of establishing goals for both contaminant remediation and overall restoration, and the need to include both the restoration of ecological and socio‐cultural‐economic value in the context of contaminated sites. Although recognizing that in some countries there may be regulatory issues associated with contaminants and clean up, landscape setting and social drivers can inform the restoration goals. We provide a decision tree support tool to guide the establishment of restoration goals for contaminated ecosystems. The overall intent of this decision tree is to provide a framework for goal setting and to identify outcomes achievable given the contamination present at a site. Integr Environ Assess Manag 2016;12:264–272. 2015 The Authors. Integrated Environmental Assessment and Management published by Wiley Periodicals, Inc. on behalf of Society of Environmental Toxicology & Chemistry (SETAC)

The mobility and bioavailability of heavy metals in soils contaminated by irrigation with wastewater increase with increasing mineralization of accumulated organic substance and decreasing pH. In laboratory experiments addition of biochar reduced heavy metal availability to plants and enhanced plant growth. However, literature from field trials is scarce. Therefore, we conducted a 2-year field experiment with orchard grass (Dactylis glomerata L.) to study the effects of miscanthus-derived biochar applied to sewage field soil on biomass production and concentrations of zinc (Zn), copper (Cu), cadmium (Cd), lead (Pb) and various nutrients in plants and in the soil solution. Biochar was mixed into the contaminated topsoil (30 cm) with addition rates of 0, 1, 2.5 and 5% on a dry-mass basis (g 100 g −1 ). The soil solution was collected with suction plates installed at a depth of 30 cm. Addition of biochar increased biomass production and reduced Zn and Cd concentrations in the soil solution. Zinc concentrations were also reduced in plants. This effect seems to be attributable to an increase in pH caused by biochar addition. In contrast, Pb and Cu concentrations in the soil solution generally increased and were related to the concentrations of dissolved organic carbon (DOC). Copper concentrations also increased in the plants; however, only at the beginning of the measurement period. Our data indicate that increased concentrations of DOC, Cu and Pb in the soil solution might be a transient effect. Therefore, further research is needed to determine the long-term effect of biochar amendment on element immobilization and leaching into groundwater.

The leaching of trace metals from anthropogenically contaminated sites poses the risk of groundwater pollution. Biochar has recently been proposed as a soil additive to reduce trace-metal concentrations in the soil solution and to increase water retention, thus reducing drainage. However, field studies on the effects of biochar addition on trace-metal leaching are scarce. Therefore, we added 0, 1, 2.5, and 5 g 100 g -1 of biochar derived from giant miscanthus (Miscanthus × giganteus J.M. Greef & Deuter ex Hodk. & Renvoize) to soil contaminated by former wastewater irrigation and examined water retention and cumulative leaching of Zn, Cd, Cu, and Pb in a 2-yr field study. Cumulative trace-metal leaching was determined by selfintegrating accumulators (SIAs) based on ion-exchange resins and compared with data calculated from mean concentrations in the soil solution collected with tension lysimeter plates and groundwater recharge rate. The highest rate of biochar addition increased water retention and thus reduced the amount of drainage water. Mean cumulative Zn and Cd fluxes decreased due to both reduced concentrations in the soil solution and reduced drainage. Although Cu and Pb concentrations in the soil solution increased with biochar addition, the reduced drainage resulted in similar fluxes in the biochar and the control treatment. The cumulative Zn, Cd, and Cu fluxes determined with SIAs were in the same range as the calculated values, while SIA-based Pb fluxes were much higher than those calculated. Since the suction plates excluded colloids, the high SIA-based Pb fluxes indicate colloidal transport and reveal the importance to elucidate the colloidal pathway for risk assessment.

Biochar is proposed as a soil amendment for metal immobilization. Studies on former sewage field soils have indicated that biochar addition immobilized Zn, probably by the precipitation of Zn phosphates. Direct evidence of the presence of these mineral phases was still lacking. We investigated the colloidal precipitate obtained after ultracentrifugation of the soil-water extract a biochar particle and a corresponding bulk soil sample from a sewage field soil that had been amended with 5% (w/w) biochar by P K-edge XANES spectroscopy. The P K-edge XANES spectrum of a colloidal precipitate visually resembled spectra of a synthetic Zn phosphate [Zn 3 (PO 4 ) 2 ] and of hopeite (Zn [6] Zn 2[4] PO 4 ×4H 2 O). Spectra evaluation by principal component analysis (PCA) and linear combination fitting (LCF) confirmed the presence of Zn-P-phases in the colloidal precipitate and, to a lesser extent, in the biochar and bulk soil sample. The P speciations varying in differently sampled soil compartments point to a small-scale heterogeneity of the biochar-induced P transformation processes.