Leachability, bioaccessibility and plant availability of trace elements in contaminated soils treated with industrial by-products and subjected to oxidative/reductive conditions

Two industrial by products, red mud (RM) and aluminium-rich water treatment residue (Al-WTR) were used (2% w/w) for the immobilization of As, Cu and Pb in two polluted soils. Soil-1 was a mine contaminated soil (2428 As, 234 Cu and 559 Pb mg·kg-1); Soil-2 was a chromated copper arsenate contaminated soil (860 and 1500 mg·kg-1 of As and Cu respectively). Two different redox conditions (aerobic and 3 repeated reduction/oxidation cycles) were also applied. The immobilization of trace elements and their bioaccessibility were evaluated through a leaching test and simplified bioaccessibility extraction test (SBRC) respectively, whereas phytoavailability was assessed in pot trials with Lolium perenne. The RMs addition increased the solubility of As and Pb, under both redox conditions, in Soil-1 with respect to untreated soil (i.e. +40 and 38% respectively in RMsw-Soil-1 in aerobic condition). In Soil-2 Al-WTR was the amendment most efficient at decreasing As mobility (< 50% with respect to untreated soil). The high correlations detected between trace element mobility and dissolved organic carbon (DOC) suggests that As, Cu and Pb formed soluble organic complexes. The SBRC results suggested that this test was mainly affected by physical and chemical characteristics of soils, whereas trial pot showed that the treatments with RMsw and Al-WTR in aerobic condition were able to decrease the plant uptake of As by 34 and 57% in Soil-1, and by 88 and 87% in Soil-2. These treatments also reduced uptake of Cu from Soil-2 (by 68 and 86%) and of Pb from Soil-1 (by 52 and 34%). The results of this study show that the efficacy of the soil amendments is a complex function of soil conditions, source of contamination, and perhaps most importantly the environmental endpoint considered.