The effect of struvite and triple superphosphate on lead immobilization in contaminated soil in the presence of malic acid

Lead (Pb) contamination in soils poses a persistent environmental hazard due to its toxicity, mobility, and resistance to degradation. The application of phosphorus (P)-based amendments has recently been considered as an effective strategy for Pb immobilization. However, the impact of low-molecular-weight organic acids (LMWOAs) as a common rhizosphere exudate on this process is not fully understood. This study evaluated the efficiency of various P amendments on Pb immobilization, both individually and in combination with malic acid. The soil was spiked with 400 mg Pb kg⁻ 1 and six treatments were applied: The treatments included: S (struvite), T (triple superphosphate), M (malic acid), SM (struvite + malic acid), TM (triple superphosphate + malic acid, and C (control). Soil pH, organic carbon, basal respiration, microbial biomass carbon, extractable Pb with CaCl 2 , and available P were measured at 1, 3, 7, 14, 21, and 28 days of incubation. At the end of the incubation, Pb fractionation and X-ray diffraction (XRD) analysis were conducted. The lowest pH (4.75) during incubation was observed in the M treatment, while treatments containing S significantly increased soil pH. The greatest reduction in extractable Pb with CaCl 2 compared to the C was recorded in the S (44.1%), followed by SM (34.1%), T (30.1%), TM (24.5%), and M (16.7%). XRD analysis confirmed the formation of Pb phosphate mineral in treated soils. Struvite demonstrated superior performance over T for Pb immobilization as a recyclable P source, offering more benefits of environmental safety and resource recovery.