Effect of biochar on the mobility and bioavailability of potential toxic metals in a former mine technosol

Soil contamination by metal(loid)s from industry and mining activities is one of the most important environmental problems. Indeed, metal(loid)s are not degradable and can accumulate in soils with high concentrations may cause harmful effects on the ecosystem (e.g. loss of biodiversity). In addition, their contact and accidental consumption by animals and humans can cause health problems: by changing enzyme constituents they may cause loss of function, and some metal(loid)s are considered carcinogenic. Consequently clean-up techniques such as physical or chemical methods have been developed to reduce the impact of pollution in the soil. However these techniques are expensive and disturb the normal biological soil activities. One low-cost alternative and environmental friendly remediation technique is phytoremediation, which uses specific plants and the associated biota to control soil metal(loid)s availability in situ. Morever, plant establishment can be difficult due to the soil physico-chemical properties such as acidic pH and environmental phytotoxicity. Thus, amendments need to be applied to improve the soil conditions and phytoremediation applications. Among possible amendments, biochar, the product of biomass pyrolysis, is gathering attention in recent years because of its beneficial effects on soil fertility but also for its ability as metal(loid)s sorbent. In this research, we aimed to investigate the effects of biochar on the technosol collected from a former mine (Pontgibaud/France) which was polluted by high levels of As (539 mg kg-1) and Pb (11453 mg kg-1). For this purpose, we evaluated three main factors: (i) soil physico-chemical properties; (ii) metal(loid)s mobility and bioavailability in the soil; (iii) phytoremediation ability of tolerant Ailanthus altissima and Robinia pseudoacacia plants in the context of amended contaminated soil. Three different mesocosm experiments were conducted by using technosol (P) amended with (i) 5% three different biochar derived from lightwood (LW), hardwood (HW) and pinewood (PW) and 5% compost (C) separately or in combination, (ii) 5% biochar hardwood (B) and 5% compost separately or in combination, (iii) 5% biochar hardwood in combination with 5% compost + with osmocote (osm+) or without osmocote (osm-). The tested species were Ailanthus altissima in the first and third mesocosm and Robinia pseudoacacia for the second mesocosm. Metal(loid) concentrations and the main chemical parameters were measured in soil pore water. Moreover, analysis of soil microorganisms were carried out in the same soil samples. Additionally, plant growth, dry weight of the organs and metal(loid)s concentration and distribution were determined for the two plant species. Our results showed that, 5% biochar and 5% compost additions to the Technosol have improved soil conditions by increasing soil pH (1-3 unit), electrical conductivity, water holding capacity and also reduced mainly Pb mobility (by 89 to 99%) and availability in the soil pore water; but As concentration was not diminished by the amendments used. Osmocote addition has also increased soil bacterial flora with a significant increase for microorganisms belong to Actinomycetes, Mycobacteria, Enterobacter and Azotobacter groups, and induced a slight increase and decrease of Pb and As concentrations in soil pore water, respectively (mesocosm 3). Hardwood biochar associated to compost (HWC), allowed the best Ailanthus altissima growth (mesocosm 1) and became better when osmocote was applied (mesocosm 3). But Robinia pseudoacacia plant biomass production was not affected by these amendments (mesocosm 2). The highest concentration and distribution of Pb or As was determined in the root system following the stems and leaves. However, the accumulation of Pb by plants was significantly more important than As. Usage of reagents (CaCl2, EDTA and HCl) to wash plant root systems have shown that the Pb adsorbed on the roots was very leachable by EDTA than CaCl2 and HCl; whereas these three solutions had the same effect on As. It was obtained that the application of biochar and compost on the contaminated soil improved soil condition by decreasing its acidity and can be used successfully to immobilize Pb by reducing its bioavailability for plants. Lastly, application of osmocote solution is highly effective for the production of plant biomass, which is an essential criteria for the good candidate species in phytoremediation.