Abstract: In response to the heavy metal contamination in oilfield wastewater residues and the on-site treatment requirement, a composite stabilizing agent composed of sodium sulfide (Na₂S), potassium dihydrogen phosphate (KH₂PO₄), and microbial residue was formulated. An orthogonal experimental design was employed to optimize the stabilizer formulation and determine its optimal mass composition. The results indicated that the optimal composition of the composite stabilizer was 0.015% Na₂S,0.12% KH₂PO₄, and 5% microbial residue. The composite stabilizer significantly reduced the leaching toxicity concentrations of heavy metals such as Cu, Zn, Cd, and Pb, with stabilization efficiencies ranging from 75% to 92%, and all leaching toxicity concentrations met the relevant regulatory limits. Tessier sequential extraction analysis showed that the stabilizer promoted the transformation of heavy metals from exchangeable and carbonate-bound fractions to oxidizable and residual fractions, thereby reducing their environmental mobility. Microbial compatibility analysis demonstrated that the stabilizer had no significant inhibitory effects on the abundance and morphology of indigenous microorganisms in the residues. Furthermore, multi-batch residue experiments confirmed the stabilization performance and applicability of the composite stabilizer.

Key words: oilfield wastewater residue, heavy metal stabilization, composite stabilizing agent, leaching toxicity, microbial compatibility