Application of Waste Lemon Extract to Toxic Metal Removal through Gravitational Soil Flushing and Composting Stabilization

The present study aims to investigate the treatment efficiency of soil flushing using waste lemon extract for samples collected from contaminated farmland, in which the copper concentration was measured as 2487 ± 139 mg/kg. The flushing solution, containing 9.9 g/L citric acid, was prepared from the waste lemon extraction process. The soil-flushing treatment using a solution containing commercial citric acids of 10 g/L was also conducted for comparison. Additionally, the collected soil was mixed with crushed waste lemons and the mixture was subjected to a composting process for subsequent stabilization study. After 120-min batch experiments, the desorbed copper concentration for waste lemon-extract experiment was 36.9 mg/L, which was higher than that (28.6 mg/L) for commercial citric solution experiment. The reduction in soil copper concentration (1504 mg/kg) treated by waste lemon-extract flushing was more than that treated by commercial citric solution (1256 mg/kg) at the comparable citric acid concentration. More metals were removed by waste lemon-extract flushing. This is because the waste lemon-extract solution contains additional co-dissolved organic substances with a longer flushing time, which allows more exchange reactions between adsorbed metals and flushing solution. For the treatment with waste lemon extract, the soil pH values were 4.56, 5.70 and 6.29 before, after flushing and after compost treatment, respectively. The observed variation in soil pH also showed that waste lemon extract might be a better flushing agent, while flushing with commercial citric solution decreased the pH in the soil environment. The plant copper availability dropped from 677 mg/kg to 156 mg/kg after waste lemon-extract flushing and stabilization with composted waste lemon. Therefore, the use of waste lemon extract for soil flushing not only removed toxic metals from the soil but also prevented the occurrence of soil acidification, an often-observed phenomenon using an acidic solution in conventional soil flushing. After soil flushing, the application of composted waste lemon could stabilize the toxic metals and increase the pH to a range suitable for plant growth.