Microrespirometric characterization of activated sludge inhibition by copper and zinc

Co-based metal-organic frameworks for enhanced nickel adsorption and its impact on nitrifying microbial activity

The release of nickel "Ni(II)" into aquatic environments is of great concern because of environmental and health issues. Metal-organic frameworks (MOFs) are one of the most promising technologies for removing heavy metals from water. In this work, an octahedral Co-based MOF (Co-MOF) was synthesized with a high Ni(II) removal capacity (qmax of 1534.09 ± 45.49 mg g-1) in aqueous media. For the first time, the effect of Co-MOF alone and in co-exposure with Ni(II) on nitrifying microbial consortium was assessed using dynamic microrespirometry. A single concentration of Co-MOF had no significant effects on nitrifying microbial consortium, while the concentration of Ni(II) exerted non-competitive inhibition on the nitrifying microbial consortium with an IC50 of 1.67 ± 0.03 mg L-1. In addition, the theoretical speciation analysis showed a decrease of 40% of IC50 when the free Ni(II) concentration was considered. Co-exposure of Co-MOF and Ni(II) during the nitrifying process allowed us to conclude that Co-MOF is an effective adsorbent for Ni(II) and can be used to mitigate the inhibitory effects of nickel on nitrifying microbial consortia, which is crucial for maintaining the good operation of wastewater treatment and balance of nitrogen cycle.

Structural and functional bacterial biodiversity in a copper, zinc and nickel amended bioreactor: shotgun metagenomic study

BACKGROUND

At lower concentrations copper (Cu), zinc (Zn) and nickel (Ni) are trace metals essential for some bacterial enzymes. At higher concentrations they might alter and inhibit microbial functioning in a bioreactor treating wastewater. We investigated the effect of incremental concentrations of Cu, Zn and Ni on the bacterial community structure and their metabolic functions by shotgun metagenomics. Metal concentrations reported in previous studies to inhibit bacterial metabolism were investigated.

RESULTS

At 31.5 μM Cu, 112.4 μM Ni and 122.3 μM Zn, the most abundant bacteria were Achromobacter and Agrobacterium. When the metal concentration increased 2 or fivefold their abundance decreased and members of Delftia, Stenotrophomonas and Sphingomonas dominated. Although the heterotrophic metabolic functions based on the gene profile was not affected when the metal concentration increased, changes in the sulfur biogeochemical cycle were detected. Despite the large variations in the bacterial community structure when concentrations of Cu, Zn and Ni increased in the bioreactor, functional changes in carbon metabolism were small.

CONCLUSIONS

Community richness and diversity replacement indexes decreased significantly with increased metal concentration. Delftia antagonized Pseudomonas and members of Xanthomonadaceae. The relative abundance of most bacterial genes remained unchanged despite a five-fold increase in the metal concentration, but that of some EPS genes required for exopolysaccharide synthesis, and those related to the reduction of nitrite to nitrous oxide decreased which may alter the bioreactor functioning.

Dynamic modelling of tetrazolium-based microbial toxicity assay-a parametric proxy of traditional dose-response relationship

Microbial toxicity of test substances in tetrazolium assay is often quantified while referring to their IC₅₀ values. However, the implication of such an estimate is very limited and can differ across studies depending on prevailing test conditions. In this work, a factorial design-based end-point microbial toxicity assay was performed, which suggests a significant interaction (P= 0.041) between inoculum and tetrazolium dose on formazan production. Subsequently, a dynamic model framework was utilized to capture the nonlinearities in biomass, substrate, formazan profiles and to project the toxicant inhibition parameter as a robust alternative to IC₅₀ value. Microbial growth, glucose uptake and formazan production in the presence or absence of toxicant (Cu²⁺) from designed batch experiments were used for sequential estimation of model parameters, and their confidence intervals. A logistic growth model with multiplicative inhibition terms for formazan content and toxicant concentration fits the experimental data reasonably well (R²>0.96). Dynamic relative sensitivity analysis revealed that both microbial growth and formazan production profiles were sensitive to toxicant inhibition parameter. The modelling framework not only provides a better insight into the underlying toxic effect but also offers a stable toxicity index for the test substances that can be extended to design a versatile, robust in vitro assay system.

Multiparameter analysis of activated sludge inhibition by nickel, cadmium, and cobalt

Activated sludge processes are often inhibited by nickel, cadmium, and cobalt. The inhibitory effect of these heavy metals on a synthetic wastewater treatment process was tested through pulse microrespirometry; i.e., pulse of substrate injected in a microreactor system. The inhibitory effect was tested under different conditions including the heavy metals, substrate and biomass concentrations, and exposure time. The inhibitory effect was quantified by the percentage of inhibition, half saturation constant (K_S), inhibition constant (K_I), and maximum oxygen uptake rate (OUR_max). The results indicated that, in a range of concentration from 0 to 40 mg L^-1, the three heavy metals exerted an uncompetitive and incomplete inhibitory effect, with a maximum inhibition of 67, 57, and 53% for Ni, Co, and Cd, respectively. An increase of the biomass concentration by 620% resulted in a decrease of the inhibition by 47 and 69% for Co and Cd, respectively, while no effect was observed on Ni inhibition. An increase of the substrate concentration by 87% resulted in an increase of the inhibition by 24, 70, and 47% for Ni, Co and Cd, respectively. In the case of nickel and cadmium, an increase in the exposure time to the heavy metals also increased the inhibition.