A promising approach for simultaneous removal of ammonia and multiple heavy metals from landfill leachate by carbonate precipitating bacterium

Combined effect of zinc and cadmium ions on nitrification performance during the biological nitrogen removal of simulated livestock breeding wastewater

Zinc and cadmium ions are usually found in livestock breeding wastewater, and the mixed ions will have an impact on the biological nitrogen removal. Nitrification performance plays an important role in biological nitrogen removal. In order to investigate the combined effect of zinc and cadmium ions on nitrification performance and to reveal the interactions between zinc and cadmium ions, three concentration ratios of zinc and cadmium ions, as well as 18 different concentration gradients were designed with the direct equipartition ray and the dilution factor method. The effect of pollutants on the nitrification performance of biological nitrogen removal was analyzed by the nonlinear regression equation, and the concentration-addition model was conducted to probe into the relationship between the mixed pollutants and the nitrification performance. The results showed that the effect on nitrification performance increased significantly with the increase of reaction duration and pollutant concentration, which indicated that the effects are concentration-dependent and time-dependent. The concentration-addition model suggested that the interactions between zinc and cadmium ions with different concentration ratios were mainly antagonistic, and as the percentage of cadmium ions in the mixtures increased, the antagonism between the mixtures became stronger. This study will provide a relevant theoretical basis for the regulation of the ratios and concentrations of heavy metal ions during the biological treatment of livestock breeding wastewater.

Development of an electrochemical separation-Rhodopseudomonas palustris electrolysis cell-coupled system for resourceful treatment of mature leachate landfill

Electrochemical technology is a promising technique for separating ammonia from mature landfill leachate. However, the accompanying migration and transformation of coexisting pollutants and strategies for further high-value resourceful utilization of ammonia have rarely received attention. In this study, an electrochemical separation-Rhodopseudomonas palustris electrolysis cell coupled system was initially constructed for efficient separation and conversion of nitrogen in mature landfill leachate to microbial protein with synchronously tracking the transport and conversion of coexisting heavy metals accompanying the process. The results revealed that ammonia concentration in the cathode increased from 40.3 to 49.8% with increasing the current density from 20 to 40 mA/cm2, with less than 3% of ammonia transformation to NO2--N and NO3--N. During ammonia separation, approximately 95% of HM-DOMs (Cr, Cu, Ni, Pb, and Zn) were released into the anolyte due to humus degradation and further diffused to the cathode. A significant correlation was observed between the releases of HM-DOMs. Cu-DOMs accounted for 70.2% of the total Cu content, which was the highest proportion among the heavy metals (HMs). Among the HMs in anolyte, 57.4% of Pb, 52.5% of Ni, and 50.6% of Zn diffused to the cathode, and most of the HMs were removed in the form of hydroxide precipitations due to heavy alkaline catholyte. Compared with the open-circuit condition, the utilization efficiency of NH4+-N in the R. palustris electrolysis cell increased by 445.1% with 47% and 50% increases in final NH4+-N conversion rate and R. palustris biomass, respectively, due to bio-electrochemical enhanced phototrophic metabolism and acid generation for buffering the strong alkalinity of the electrolyte to maintain suitable growth conditions for R. palustris.

Engineered Bacillus subtilis Biofilm@Biochar living materials for in-situ sensing and bioremediation of heavy metal ions pollution

The simultaneous sensing and remediation of multiple heavy metal ions in wastewater or soil with microorganisms is currently a significant challenge. In this study, the microorganism Bacillus subtilis was used as a chassis organism to construct two genetic circuits for sensing and adsorbing heavy-metal ions. The engineered biosensor can sense three heavy metal ions (0.1-75 μM of Pb2+ and Cu2+, 0.01-3.5 μM of Hg2+) in situ real-time with high sensitivity. The engineered B. subtilis TasA-metallothionein (TasA-MT) biofilm can specifically adsorb metal ions from the environment, exhibiting remarkable removal efficiencies of 99.5% for Pb2+, 99.9% for Hg2+and 99.5% for Cu2+ in water. Furthermore, this engineered strain (as a biosensor and absorber of Pb2+, Cu2+, and Hg2+) was incubated with biochar to form a hybrid biofilm@biochar (BBC) material that could be applied in the bioremediation of heavy metal ions. The results showed that BBC material not only significantly reduced exchangeable Pb2+ in the soil but also reduced Pb2+ accumulation in maize plants. In addition, it enhanced maize growth and biomass. In conclusion, this study examined the potential applications of biosensors and hybrid living materials constructed using sensing and adsorption circuits in B. subtilis, providing rapid and cost-effective tools for sensing and remediating multiple heavy metal ions (Pb2+, Hg2+, and Cu2+).

The competitive and selective adsorption of heavy metals by struvite in the Pb(II)-Cd(II)-Zn(II) composite system and its environmental significance

The prevalence of struvite and other phosphate minerals in eutrophic environments has a significant effect on the transport and transformation of environmental heavy metals, but their competitive immobilization characteristics and mechanisms for heavy metals remain unclear. Three different sources of struvite (BS, CSHS, and CSS) were obtained respectively by biosynthesis and chemical synthesis with or without humic acid to investigate their competitive immobilization characteristics and mechanism of heavy metals in the Pb(II)-Cd(II)-Zn(II) composite system. The results showed that the immobilization of heavy metals by struvite is physico-chemical adsorption and the affinity (in descending order) is Pb(II) >> Cd(II)/Zn(II). Cd(II) promotes the immobilization of Pb(II)/Zn(II) by BS. The order of the selective strength by struvite for Pb(II) is BS >> CSS ≈ CSHS. The study indicates that the difference between struvite holding heavy metal ions is related to the material composition and heavy metal types, and BS shows best selective immobilization for Pb(II) in the Pb(II)-Cd(II)-Zn(II) composite system. This study provides a theoretical basis for understanding the environmental geochemical role and eco-environmental effects of struvite.

Immobilization of Cd2+ in an aqueous environment using a two-step microbial-induced carbonate precipitation method

Previous researches indicate that the potent toxicity of cadmium hinders the efficacy of the microbial-induced carbonate precipitation (MICP) process for bioremediation of Cd2+ in aqueous environment. Increasing urea and calcium resource doses, introducing synergists, and utilizing urease-producing consortia can improve bio-immobilization performance of MICP. However, such measures may incur cost increases and/or secondary contamination. This study first verifies the substantial biotoxicity of Cd2+ for urease activity and then analyzes the practical limitation of traditional MICP using Bacillus pasteurii for bioremediation of Cd2+ in an aqueous environment containing 1-40 mM Cd2+ by a series tube tests and numerical simulation. Subsequently, a two-step MICP method, which separates urea hydrolysis and heavy metal precipitation, is introduced in this study to eliminate the inhibitory effect of heavy metal on urease activity. The concentrations of ammonium, Cd2+, and pH were monitored over time. The results indicate that the urease expression in B. pasteurii can be significantly inhibited by Cd2+ particularly at the concentration ranging from 10 to 40 mM, leading to pretty low efficacy of traditional MICP for bioremediation of Cd2+ (Cd2+ removal rate as low as 21.55-38.47% when the initial Cd2+ concentration = 40 mM). In contrast, when the two-step MICP method is applied, the Cd2+ can be almost completely immobilized, even though the concentration ratio of urea to Cd2+ is as low as 1.5:1.0, which is close to the theory minimum concentration ratio for the complete precipitation of carbonate to cadmium ions(1.0:1.0). Therefore, the cost-effective, environmentally sustainable, and straightforward two-step MICP method holds great potential for application in the bioremediation of Cd2+-contaminated solutions in high concentration.

Mn2+ recycling in hypersaline wastewater: unnoticed intracellular biomineralization and pre-cultivation of immobilized bacteria

Microbially induced manganese carbonate precipitation has been utilized for the treatment of wastewater containing manganese. In this study, Virgibacillus dokdonensis was used to remove manganese ions from an environment containing 5% NaCl. The results showed a significant decrease in carbonic anhydrase activity and concentrations of carbonate and bicarbonate ions with increasing manganese ion concentrations. However, the levels of humic acid analogues, polysaccharides, proteins, and DNA in EPS were significantly elevated compared to those in a manganese-free environment. The rhodochrosite exhibited a preferred growth orientation, abundant morphological features, organic elements including nitrogen, phosphorus, and sulfur, diverse protein secondary structures, as well as stable carbon isotopes displaying a stronger negative bias. The presence of manganese ions was found to enhance the levels of chemical bonds O-C=O and N-C=O in rhodochrosite. Additionally, manganese in rhodochrosite exhibited both + 2 and + 3 valence states. Rhodochrosite forms not only on the cell surface but also intracellularly. After being treated with free bacteria for 20 days, the removal efficiency of manganese ions ranged from 88.4 to 93.2%, and reached a remarkable 100% on the 10th day when using bacteria immobilized on activated carbon fiber that had been pre-cultured for three days. The removal efficiency of manganese ions was significantly enhanced under the action of pre-cultured immobilized bacteria compared to non-pre-cultured immobilized bacteria. This study contributes to a comprehensive understanding of the mineralization mechanism of rhodochrosite, thereby providing an economically and environmentally sustainable biological approach for treating wastewater containing manganese.