Potential of rice straw biochar, sulfur and ryegrass (Lolium perenne L.) in remediating soil contaminated with nickel through irrigation with untreated wastewater

BACKGROUND: Untreated wastewater carries substantial amount of heavy metals and causes potential ecological risks to the environment, food quality, soil health and sustainable agriculture. METHODOLOGY: In order to reduce the incidence of nickel (Ni(2+)) contamination in soils, two separate experiments (incubation and greenhouse) were conducted to investigate the potentials of rice straw biochar and elemental sulfur in remediating Ni(2+) polluted soil due to the irrigation with wastewater. Five incubation periods (1, 7, 14, 28 and 56 days), three biochar doses (0, 10 and 20 g kg(−1) of soil) and two doses of sulfur (0 and 5 g kg(−1) of soil) were used in the incubation experiment then the Ni(2+) was extracted from the soil and analyzed, while ryegrass seeds Lolium perenne L. (Poales: Poaceae) and the same doses of biochar and sulfur were used in the greenhouse experiment then the plants Ni(2+)-uptake was determined. RESULTS: The results of the incubation experiment revealed a dose-dependent reduction of DTPA-extractable Ni(2+) in soils treated with biochar. Increasing the biochar dose from 0 g kg(−1) (control) to 10 or 20 g kg(−1) (treatments) decreased the DTPA-extractable Ni(2+) from the soil by 24.6% and 39.4%, respectively. The application of sulfur increased the Ni(2+)-uptake by ryegrass plant which was used as hyper-accumulator of heavy metals in the green house experiment. However, the biochar decreased the Ni(2+)-uptake by the plant therefore it can be used as animal feed. CONCLUSIONS: These results indicate that the biochar and sulfur could be applied separately to remediate the Ni(2+)-contaminated soils either through adsorbing the Ni(2+) by biochar or increasing the Ni(2+) availability by sulfur to be easily uptaken by the hyper-accumulator plant, and hence promote a sustainable agriculture.