Toxicity evaluation of a heavy-metal-polluted river: Pollution identification and bacterial community assessment.

The Salt River is an important urban river in Kaohsiung, Taiwan. In this study, the source identification and risk and toxicity assessment of the heavy-metal-contaminated sediments in the Salt River were investigated. The geo-accumulation index (Igeo), enrichment factor (EF), sediment quality guidelines (SQGs), potential ecological risk index (RI), pollution load index (PLI), and toxic units (TU) were applied to determine effects of heavy metals on microbial diversities and ecosystems. Results from the ecological and environmental risk assessment show that high concentrations of Zn, Cr, and Ni were detected in the midstream area and the sum of toxic units (ΣTUs) in the midstream (7.2-32.0) is higher than in the downstream (14.0-19.7) and upstream (9.2-17.1). It could be because of the continuous inputs of heavy-metal-contained wastewaters from adjacent industrial parks. Results also inferred that the detected heavy metals in the upstream residential and commercial areas were possibly caused by nearby vehicle emissions, non-point source pollution, and domestic wastewater discharges. Results of metagenomic assays show that the sediments contained significant microbial diversities. Metal-tolerant bacterial phyla (Proteobacteria: 24.4%-46.4%, Bacteroidetes: 1.3%-14.8%, and Actinobacteria: 2.3%-11.1%) and pathogenic bacterial phyla (Chlamydiae: 0.5%-37.6% and Chloroflexi: 5.8%-7.2%) with relatively high abundance were detected. Metal-tolerant bacteria would adsorb metals and cause the increased metal concentrations in sediments. Results indicate that the bacterial composition in sediment environments was affected by anthropogenic pollution and human activities and the heavy-metal-polluted ecosystem caused the variations in bacterial communities. PRACTITIONER POINTS: Microbial community in sediments is highly affected by heavy metal pollution. Wastewaters and vehicle traffic contribute to river sediments pollution by heavy metals. Proteobacteria, Bacteroidota, and Actinobacteria are dominant heavy-metal-tolerant bacterial phyla in sediments. Toxicity assessment is required to study risk levels of heavy-metal contained sediments.

Bioremediation of trichloroethylene-polluted groundwater using emulsified castor oil for slow carbon release and acidification control.

In this study, the emulsified castor oil (ECO) substrate was developed for a long-term supplement of biodegradable carbon with pH buffering capacity to anaerobically bioremediate trichloroethylene (TCE)-polluted groundwater. The ECO was produced by mixing castor oil, surfactants (sapindales and soya lecithin [SL]), vitamin complex, and a citrate/sodium phosphate dibasic buffer system together for slow carbon release. Results of the emulsification experiments and microcosm tests indicate that ECO emulsion had uniform small droplets (diameter = 539 nm) with stable oil-in-water characteristics. ECO had a long-lasting, dispersive, negative zeta potential (-13 mv), and biodegradable properties (viscosity = 357 cp). Approximately 97% of TCE could be removed with ECO supplement after a 95-day operational period without the accumulation of TCE dechlorination byproducts (dichloroethylene and vinyl chloride). The buffer system could neutralize acidified groundwater, and citrate could be served as a primary substrate. ECO addition caused an abrupt TCE adsorption at the initial stage and the subsequent removal of adsorbed TCE. Results from the next generation sequences and real-time polymerase chain reaction (PCR) indicate that the increased microbial communities and TCE-degrading bacterial consortia were observed after ECO addition. ECO could be used as a pH-control and carbon substrate to enhance anaerobic TCE biodegradation effectively. PRACTITIONER POINTS: Emulsified castor oil (ECO) contains castor oil, surfactants, and buffer for a slow carbon release and pH control. ECO can be a long-term carbon source for trichloroethylene (TCE) dechlorination without causing acidification. TCE removal after ECO addition is due to adsorption and reductive dechlorination mechanisms.