Heavy metal uptake by wastewater irrigated potato plants grown on contaminated soil treated with hydrogel based amendments

Abstract

Heavy metal uptake by food crops and the potential for groundwater contamination are of major concern in areas where untreated wastewater is used for irrigation. To minimize heavy metal uptake by wastewater irrigated food crops and to minimize its transport to deeper soil layers, the use of polyacrylamide super absorbent polymer (SAP) and SAP-gasified plantain peel biochar mix (SAP+GBC), as soil amendments, is proposed in this study. A sorption test was conducted to determine the ability of the treatments to adsorb heavy metals (Cd, Cr, Cu, Fe, Pb and Zn). Field experiment was conducted by growing synthetic wastewater irrigated potato plants ( Solanum tuberosum L.) in lysimeters packed with sandy soil, contaminated with heavy metals. Prior to conducting the study, heavy metals Cd, Cr, Cu, Fe, Pb and Zn were present in top 0.10 m of lysimeter soil within ranges of 15.6–23.9, 23.3–27.6, 30.3–62.2, 11538.6–10847.7, 64.9–131.3 and 38.7–46.8 mg kg −1 , respectively. Irrigation occurred eight times, at an interval of 10 days, based on the crop water requirement. After every irrigation, soil samples were collected from different depths (surface, 0.10, 0.30 and 0.60 m) for heavy metal analysis. Upon maturity, potato tubers, plant root, stems, leaf, tuber flesh and tuber peel tissues were sampled separately for heavy metal analysis. Soil samples, collected at the end of the experiment, were also subjected to pH and cation exchange capacity (CEC) analysis. Compared to the control, treatment SAP+GBC was able to retain significantly higher amounts of Cd, Cr and Fe in topsoil (0–0.10 m depth; p<0.05), whereas, the SAP treatment retained significantly higher amounts of Cd, Cu, Fe and Zn in topsoil (p<0.05). Both hydrogel based treatments (SAP and SAP+GBC) were able to significantly (p<0.10) reduce Cd uptake in tuber flesh (1.98 and 1.54 mg kg −1 , respectively) and peel tissue (20.75 and 23.89 mg kg −1 , respectively), as compared to the control (2.63 mg kg −1 in flesh and 43.87 mg kg −1 in peel). Concentrations of Zn in tuber flesh tissue for both, SAP (8.30 mg kg −1 ) and SAP+GBC (8.65 mg kg −1 ) treatments were found to be significantly (p<0.05) lower than that of the control (15.28 mg kg −1 ), indicating the effectiveness of the treatments in reducing Zn uptake by potato tubers. Treatment SAP+GBC was also able to significantly (p<0.05) reduce Pb uptake in tuber flesh (0.05 mg kg −1 ) and peel tissue (1.64 mg kg −1 ), as compared to control (0.09 mg kg −1 in flesh and 4.99 mg kg −1 in peel). Overall, it was found that tuber peels had significantly higher levels of heavy metals, as compared to tuber flesh. The present study indicates that the hydrogel based amendments have the potential to reduce the uptake of Cd, Pb and Zn metals, by wastewater irrigated potato tubers grown on sandy soils. • Irrigating potato with wastewater led to accumulation of heavy metals in soil. • Both treatments led to reduced Cd and Zn uptake by potato tubers. • Treatment SAP+GBC led to reduced Pb uptake in potato tubers. • SAP+GBC treated soil retained higher levels of Cd, Cr and Cr in topsoil. • SAP treated soil retained greater amounts of Cd, Cu, Fe and Zn in topsoil.