Influence of biochar on trace metals mobility and leaching in contaminated mining soils

Sardinia has been one of the most important Italian regions for mining. The principal minerals extracted were sphalerite (zinc, iron sulfide) and galena (lead sulfide). After mining stopped, and due to the absence of effective recovery, trace metals (TM) concentrations in these mining sites and the neighbouring areas often largely exceeded the legal limits. Reclamation of such TM-contaminated soils should be therefore considered as a priority objective. The use of organic amendments can represent one of the most sustainable methods to mitigate the effect of trace metals in soil, due to their potential ability to reduce the mobility and bioavailability of TM and to promote the recovery of soil functioning. The aim of this study was therefore to evaluate the influence of a biochar (added at 5% rate) on the mobility and leaching of TM in differently polluted soils. Soil samples (S1, S2 and S3) were collected at different distance from the Montevecchio dismissed mining site (Southwestern Sardinia) where argentiferous galena and sphalerite were the main ores extracted. Such soils were characterized by increasing levels of contamination (i.e. total concentrations of Pb, Cd, and Zn were 55, 318 and 1899 mg•kg−1; 0.38, 4.75 and 74.03 mg•kg−1; 115, 622 and 3803 mg•kg−1 in S1, S2 and S3 respectively). Generally, biochar addition to soil (S+Bio) determined an improvement of parameters related to soil fertility (total C and N and available P in particular), and this could be of substantial help in the recovery of marginal or degraded lands. Moreover, the addition of biochar significantly decreased the TM labile fractions, i.e. those extracted using a 0.5 M Ca(NO3)2 solution (e.g. < 79 and 14% for Pb; 29, and 23% for Cd; 55% and 16% for Zn in S2+Bio and S3+Bio respectively). Overall, the residual fractions of Pb, Cd and Zn, i.e. those poorly bioavailable for plants and microorganisms, increased in treated soils (>25 and 18% for Pb; >21 and 10% for Cd and >4 and 7% for Zn in S2+Bio and S3+Bio respectively). Leaching experiments highlighted a similar trend, with biochar addition determining a significant reduction of leached TM (< 80 and 60 % for Zn; < 32 and 31% for Cd; < 68 and 35% for Pb in S2+Bio and S3+Bio, respectively). Overall, the results obtained suggested that biochar addition to TM-polluted soils could be useful to reduce the labile TM pools and to increase the ability of such soils to promote plant growth.