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.