The addition of biochar as a fertilizer supplement for the attenuation of potentially toxic elements in phosphogypsum-amended soil

Abstracts Phosphogypsum (PG) is an acid by-product generated during the breakdown of phosphate ore in the presence of sulfuric acid and has been used as a soil amendment, i.e., fertilizer, in most parts of the world. PG application can improve some soil properties and increase crop yields. However, PG application in agriculture threatens agroecological systems and food safety because of the high amounts of environmentally and biologically available potentially toxic elements (PTEs) in PG. Recently, biochar has been found to strongly immobilize and adsorb PTEs as well as suppress PTE phytoavailability. Hence, in this context, bench-scale batch and column experiments were performed to test the availability of PTEs in PG under different pyrolysis temperatures and different percentages of rice husk biochar (RHB). Subsequently, pot experiments were performed to test the effect of RHB addition on the PTE concentrations in crops when PG was added to different types of soil, and the effect of the RHB and PG combination on crop yield was determined. The batch and column experimental results showed that the pyrolysis temperature and biochar dose significantly reduced the PTE concentrations in PG because the biochar physicochemical properties, components, and types of functional groups, which were analysed by XRF, XRD and FTIR, were mainly controlled by temperature. The preparation temperature (600°C) and applied dose (60%, w/w) of RHB were optimized through a mixed-effect model. Furthermore, the mechanism underlying P, F and heavy metals in PG leachates adsorbed onto biochar was determined by XPS analysis, which showed that surface ions and different functional groups played a dominant role through precipitation and complexation. Compared to the control, application of the combination to loam soil or sand significantly increased (56.04% and 604.66%, respectively) crop yield. The combination significantly decreased PTE concentrations (As, F, Cd, and Pb by 39, 41, 75, and 86%, respectively, in water spinach and by 100%, 64%, 75%, and 88%, respectively, in asparagus lettuce) in edible parts of crops compared to PG treatment only. Moreover, RHB+PG reduced pollution to the surrounding environment by decreasing the PTE contents in the leachate. These findings suggest that the improved effectiveness of the combination was greater in sand because of its weak buffering ability, and the RHB and PG combination can be a useful and safe soil amendment to provide good guidance for waste management and comprehensive utilization of waste.