Reduction of heavy metal uptake by lettuce (Lactuca sativa) under synthetic wastewater irrigation using adsorbents for soil amendment

Many developing countries across the globe have adopted the use of wastewater for irrigation due to the scarcity of fresh water resources coupled with high demand for food for the growing population. Agricultural soils are contaminated with toxic metals such as lead (Pb) and cadmium (Cd) by wastewater irrigation leading to the uptake of the metals by plants. In this study, shea nut shell biochar, groundnut shell biochar, raw shea nut shell, and raw groundnut shell adsorbents were amended with soil to reduce the uptake of Pb and Cd by lettuce grown under wastewater irrigation. Two particle sizes of each adsorbent; < 5 mm and > 5 mm each was added to the soil at ratios of 1:2 and 1:5. The plants were grown for 52 days and irrigated with synthetic wastewater for 38 days before harvesting. There was a significant difference (p < 0.001) in the concentration of Pb and Cd in the tissues of lettuce and soil among treatments. Treatment with a 1:2 (biochar to soil) recorded the lowest concentrations of Pb and Cd in the soil and lettuce. The concentration of Pb and Cd in the soil ranged from 0.64 ± 0.0025 mg/kg to 1.99 ± 0.0025 mg/kg and 0.12 ± 0.001 mg/kg to 0.27 ± 0.0185 mg/kg respectively. < 5 mm shea nut shell biochar at a ratio of 1:2 treated soils recorded the lowest concentrations of Pb and Cd, whereas the highest concentrations were recorded in the control. The concentration of Pb and Cd accumulated in the lettuce ranged from 2.25 ± 0.023 mg/kg to 3.58 ± 0.005 mg/kg and 0.14 ± 0.002 mg/kg to 0.26 ± 0.003 mg/kg respectively. Generally, the reduction of both metals in the soil and lettuce was in the order of shea nut shell biochar > groundnut shell biochar > raw groundnut shell > raw shea nut shell > Control. The health risk assessment indicated that the daily intake of metals for both Pb and Cd for both adults and children was below the recommended limits. The values of health risk index and target hazard quotient were < 1. It is thus recommended that, to reduce Pb and Cd contamination in soils, < 5 mm shea nut shell biochar should be used at a ratio of 1:2 to reduce heavy metal uptake by plants. • Pb and Cd levels differ significantly (p < 0.001) in lettuce and soil across treatments. • A 1:2 biochar-to-soil ratio shows the lowest Pb and Cd in soil and lettuce. • Metal reduction follows: shea nut shell biochar > groundnut shell biochar > raw shells >> control. • Pb and Cd intake for adults and children is below recommended limits. • Using <5 mm shea nut shell biochar at 1:2 ratio reduces heavy metal uptake in plants