Modified clays alter diversity and respiration profile of microorganisms in long-term hydrocarbon and metal co-contaminated soil

Clays and surfactant-modified clays (organoclays) are becoming popular as pollutant sorbents due to their high reactivity and low-cost availability. However, the lack of field testing and data on ecotoxicity limits their application. Considering such aspects, this study assessed the impact of clay amendments to polycyclic aromatic hydrocarbons (PAHs)/cadmium (Cd)-contaminated soil on microbial respiration profiles (active vs. inactive cells) using redox staining and the relative abundance and diversity of bacteria and archaea. These clay products are bentonite, cationic surfactant-modified bentonite and palmitic acid-grafted surfactant-modified bentonite). After 70 days, the addition of bentonite and its modified forms altered microbial community structure mainly among dominant groups (Actinobacteria, Proteobacteria, Firmicutes and Chloroflexi) with effects varying depending on material loading to soil. Among amendments, fatty acid (palmitic acid) tailored cationic surfactant-modified bentonite proved to be microbial growth supportive and significantly increased the number of respiration-active microbial cells by 5% at a low dose of material (e.g. 1%). Even at high dose (5%), the similarity index using operational taxonomic units (OTUs) also indicates that this modified organoclay-mixed soil provided only slightly different environment than control soil, and therefore, it could offer more biocompatibility than its counterpart organoclay at similar dose (e.g. cationic surfactant-modified bentonite). This study promotes designing ‘eco-safe’ clay-based sorbents for environmental remediation.