Usage of biowashing to remove impurities and heavy metals in raw phosphogypsum and calcined phosphogypsum for cement paste preparation

In the previous studies, chemical, physical, and heat treatment methods are commonly used to treat waste phosphogypsum (PG). This study reports a new method that uses biowashing to remove phosphorus, fluorine impurities, and 12 kinds of heavy metals in PG. Raw phosphogypsum (RPG) and calcined phosphogypsum (CPG) were treated by biowashing, and the potential of treated PG used for cement paste preparation was investigated. The effects of biosolution on the stability of PG and the growth of bacteria in RPG and CPG were analyzed by zeta potential and optical density. A series of characterization tests were used to analyze the removal of impurity in PG after biowashing. Additionally, the removal ratio for the impurities, compressive strength, and leaching behavior of PG-incorporated cement paste (PGC) were discussed, and the mechanism was revealed according to the test results. The results showed that, compared with RPG, the growth of bacteria was better in CPG, and the phase of CPG after biowashing was more stable. Biomineralization could effectively remove 74–77% of phosphorus and fluorine impurities in PG, but the removal ratio in CPG was 45–55% higher than that in RPG. The PG after biowashing contained a large amount of calcium carbonate, which improved the early strength of the PGC by 15–20%. Compared with PGC, the leaching concentration of heavy metals in biowashed-PG incorporated cement (APGC) decreased by 50–90%. Biowahing removes the impurities mainly by turning the insoluble impurities into soluble H3PO4 and HF, which are filtered out with bacterial solutions. This study provides a new idea for the management of PG and expands the application field of microbially induced carbonate precipitation (MICP).