The researchers studied how adding phosphate and citric acid to lead-contaminated soil affects the stability of the soil and the availability of lead. They found that adding phosphate decreased the amount of lead that could be extracted from the soil, showing that phosphate immobilizes lead. However, adding citric acid had the opposite effect, increasing the extractable lead content. The researchers also found that the stability of lead passivated by phosphate alone was higher compared to when citric acid was also present.