Effect of hydrogel based soil amendments on heavy metal uptake by spinach grown with wastewater irrigation

Food crops irrigated with wastewater can uptake heavy metals, causing serious health ailments in humans. Use of a polyacrylamide superabsorbent polymer hydrogel and the same hydrogel mixed with pyrolyzed plantain peel biochar as soil amendments are proposed to reduce heavy metal uptake by wastewater-irrigated spinach (Spinacia oleracea L.) plants. A sorption test was carried out to establish the ability of these treatments to bind the heavy metals. In a lysimeter field experiment, the amendments were mixed in the top 0.10 m of soil (1% w/w) and spinach plants were grown using synthetic wastewater irrigation. After each irrigation, soil samples were obtained at different depths (0, 0.10, 0.30, and 0.60 m from the surface) for heavy metal analysis. Spinach leaves, root, and stem samples were obtained at the harvest for metal analysis. Sorption test results showed that the hydrogel-biochar amended soil adsorbed 0.80, 0.46, and 0.44 mg g⁻¹ of cadmium, copper, and zinc, respectively, from a 0.5 mM multi-metal solution; the hydrogel treatment adsorbed 0.59, 0.41, and 0.24 mg g⁻¹ of the metals, respectively. These amounts were at least 90% more than those adsorbed by the non-amended soil. In terms of the total metal uptake by spinach leaves, the hydrogel-biochar mix treatment performed better than the hydrogel treatment; it reduced the total uptake by 48%, whereas the hydrogel treatment was only able to reduce it by 15% when compared to the control that exhibited a total metal load of 1028 mg kg⁻¹. Both treatments were able to significantly (p < 0.05) reduce copper uptake in plant stems, and exhibited the potential to reduce chromium, copper, and iron uptake by spinach leaves.