A pilot-scale sulfur-based sulfidogenic system for the treatment of Cu-laden electroplating wastewater using real domestic sewage as electron donor

Characteristics and mechanisms of phosphine production in sulfur-based constructed wetlands

Phosphine (PH3) is an important contributor to the phosphorus cycle and is widespread in various environments. However, there are few studies on PH3 in constructed wetlands (CWs). In this study, lab-scale CWs and batch experiments were conducted to explore the characteristics and mechanisms of PH3 production in sulfur-based CWs. The results showed that the PH3 release flux of sulfur-based CWs varied from 0.86±0.04 ng·m-2·h-1 to 1.88±0.09 ng·m-2·h-1. The dissolved PH3 was the main PH3 form in CWs and varied from 2.73 μg·L-1 to 4.08 μg·L-1. The matrix-bound PH3 was a staging reservoir for PH3 and increased with substrate depth. In addition, the sulfur-based substrates had a significant improvement on PH3 production. Elemental sulfur is more conducive to PH3 production than pyrite. Moreover, there was a significant positive correlation between PH3 production, the dsrB gene, and nicotinamide adenine dinucleotide (NADH). NADH might catalyze the phosphate reduction process. And the final stage of the dissimilatory sulfate reduction pathway driven by the dsrB gene might also provide energy for phosphate reduction. The migration and transformation of PH3 increased the available P (Resin-P and NaHCO3-P) from 35 % to 56 % in sulfur-based CW, and the P adsorption capacity was improved by 12 %. The higher proportion of available P increased the plant uptake rate of P by 17 %. This study improves the understanding of the phosphorus cycle in sulfur-based CW and provides new insight into the long-term stable operation of CWs.

Long-term stable and efficient degradation of ornidazole with minimized by-product formation by a biological sulfidogenic process based on elemental sulfur

Conventional biological treatment processes cannot efficiently and completely degrade nitroimidazole antibiotics, due to the formation of highly antibacterial and carcinogenic nitroreduction by-products. This study investigated the removal of a typical nitroimidazole antibiotic (ornidazole) during wastewater treatment by a biological sulfidogenic process based on elemental sulfur (S0-BSP). Efficient and stable ornidazole degradation and organic carbon mineralization were simultaneously achieved by the S0-BSP in a 798-day bench-scale trial. Over 99.8 % of ornidazole (200‒500 μg/L) was removed with the removal rates of up to 0.59 g/(m3·d). Meanwhile, the efficiencies of organic carbon mineralization and sulfide production were hardly impacted by the dosed ornidazole, and their rates were maintained at 0.15 kg C/(m3·d) and 0.49 kg S/(m3·d), respectively. The genera associated with ornidazole degradation were identified (e.g., Sedimentibacter, Trichococcus, and Longilinea), and their abundances increased significantly. Microbial degradation of ornidazole proceeded by several functional genes, such as dehalogenases, cysteine synthase, and dioxygenases, mainly through dechlorination, denitration, N-heterocyclic ring cleavage, and oxidation. More importantly, the nucleophilic substitution of nitro group mediated by in-situ formed reducing sulfur species (e.g., sulfide, polysulfides, and cysteine hydropolysulfides), instead of nitroreduction, enhanced the complete ornidazole degradation and minimized the formation of carcinogenic and antibacterial nitroreduction by-products. The findings suggest that S0-BSP can be a promising approach to treat wastewater containing multiple contaminants, such as emerging organic pollutants, organic carbon, nitrate, and heavy metals.

A nanofiber with a p-π conjugated structure designed based on the Jahn-Teller effect for the removal of cupric tartrate from wastewater

Copper organic complexes with strong chemical stability and high solubility in water are difficult to eliminate with traditional adsorbents. In this work, a novel amidoxime nanofiber (AO-Nanofiber) with the p-π conjugated structure was fabricated through homogeneous chemical grafting coupled with electrospinning and applied to capture cupric tartrate (Cu-TA) from aqueous solutions. The adsorption capacity of Cu-TA by AO-Nanofiber was 198.4 mg/g at an equilibrium time of 40 min, and the adsorption performance remains basically unchanged after 10 times adsorption-desorption cycles. The capture mechanism of Cu-TA by AO-Nanofiber was jointly validated by experiments and characterization such as Fourier Transform Infrared Spectrometer (FT-IR), X-ray Photoelectron Spectroscopy (XPS), and Density functional theory (DFT) calculations. These results demonstrated that the lone pair of electrons of the N atom from the amino groups and the O atom from hydroxyl groups in the AO-Nanofiber is partially transferred to the 3d orbital of the Cu(II) ions in Cu-TA, leading to the Jahn-Teller distortion of the Cu-TA and the more stable structure of AO-Nanofiber@Cu-TA was generated.

Hydroxylamine enhanced the degradation of diclofenac in Cu(II)/peracetic acid system: Formation and contributions of CH3C(O)O•, CH3C(O)OO•, Cu(III) and •OH

The slow reduction of Cu(II) into Cu(I) through peracetic acid (PAA) heavily limited the widespread application of Cu(II)/PAA system. Herein, hydroxylamine (HA) was proposed to boost the oxidative capacity of Cu(II)/PAA system by facilitating the redox cycle of Cu(I)/Cu(II). HA/Cu(II)/PAA system was quite rapid in the removal of diclofenac within a broad pH range of 4.5-9.5, with a 10-fold increase in the removal rate of diclofenac compared with the Cu(II)/PAA system at an optimal initial pH of 8.5. Results of UV-Vis spectra, electron paramagnetic resonance, and alcohol quenching experiments demonstrated that CH3C(O)O•, CH3C(O)OO•, Cu(III), and •OH were involved in HA/Cu(II)/PAA system, while CH3C(O)OO• was verified as the predominant reactive species of diclofenac elimination. Different from previously reported Cu-catalyzed PAA processes, CH3C(O)OO• mainly generated from the reaction of PAA with Cu(III) rather than CH3C(O)O• and •OH. Four possible elimination pathways for diclofenac were proposed, and the acute toxicity of treated diclofenac solution with HA/Cu(II)/PAA system significantly decreased. Moreover, HA/Cu(II)/PAA system possessed a strong anti-interference ability towards the commonly existent water matrix. This research proposed an effective strategy to boost the oxidative capacity of Cu(II)/PAA system and might promote its potential application, especially in copper-contained wastewater.

Enhanced selective copper precipitation by mechanochemically activated benzene tricarboxylic acid

Toward the treatment of waste solution containing heavy metals, direct precipitation of the metal ions from an acidic solution without alkaline neutralisation is still the greatest challenge. Based on the ligand properties of benzene tricarboxylic acid (BTC) to copper ions, a simple ball milling with 90 min at 400 rpm was used to activate BTC to enhance its capacity for copper removal from the pH of the original solution around 3-4. A set of analytical methods were used to characterise the activated BTC sample and BTC-Cu precipitate before and after copper precipitation. Compared with the raw BTC, the activated BTC could efficiently remove copper ions over 90% from an initial copper concentration of 100 mg/L in a shorter time from an acidic media with lower pH of around 2.60 and the maximum adsorption capacity can be stable at about 111.70 mg/g, resulting from probably the enhanced deprotonation effect for copper incorporation. Furthermore, at controlled dosage, the activated BTC demonstrated much high selectivity on precipitating copper ions from other heavy metals of Ni, Mn, Zn and Cd and provided a new approach for easy copper recycling from waste solution as secondary sources. This process may serve the purpose of recycling both metal and acidic solutions after the purification.

Reduction of S0 deposited on electroactive biofilm under an oxidative potential

The inevitable deposition of S⁰ on the electroactive biofilm (EAB) via anodic sulfide oxidation affects the stability of bioelectrochemical systems (BESs) when an accidental discharge of sulfide occurred, leading to the inhibition of electroacitivity, because the potential of anode (e.g., 0 V versus Ag/AgCl) is ~500 mV more positive than the redox potential of S^2-/S⁰. Here we found that S⁰ deposited on the EAB can be spontaneously reduced under this oxidative potential independent of microbial community variation, leading to a self-recovery of electroactivity (> 100 % in current density) with biofilm thickening (~210 μm). Transcriptomics of pure culture indicated that Geobacter highly expressed genes involving in S⁰ metabolism, which had an additional benefit to improve the viability (25 % - 36 %) of bacterial cells in biofilm distant from the anode and the cellular metabolic activity via electron shuttle pair of S⁰/S^2-(S_x^2-). Our findings highlighted the importance of spatially heterogeneous metabolism to its stability when EABs encountered with the problem of S⁰ deposition, and that in turn improved the electroactivity of EABs.

Sulfur disproportionation realizes an organic-free sulfidogenic process for sustainable treatment of acid mine drainage

Biological sulfidogenic processes (BSPs) have been considered effective biotechnologies for the treatment of organic-deficit acid mine drainage (AMD) and heavy metal recovery. However, high-rate sulfide production relies on the continuous addition of exogenous organic substrates as electron donors to facilitate dissimilatory sulfate reduction, which substantially increases the operational cost and CO₂ emission and also limits the wide application of BSPs in AMD treatment. In this study, we proposed a novel chemoautotrophic elemental sulfur disproportionation (SD) process as an alternative to conventional BSPs for treating AMD, in which sulfur-disproportionating bacteria (SDB) disproportionates sulfur to sulfide and sulfate without organic substrate supplementation. During the 393-day lab-scale test, we observed that the sulfur-disproportionating reactor (SDR) achieved a stable high-rate sulfide production, with a maximal rate of 21.10 mg S/L-h at an organic-substrate-free condition. This high rate of sulfide production suggested that the SD process could provide sufficient sulfide to precipitate metal ions from AMD. Thermodynamics analysis and batch tests further revealed that alkalinity rather than sulfate was the critical factor influencing the SD process, suggesting that the abundant sulfate present in AMD would not inhibit the SD process. The critical condition of SD in the SDR was therefore determined. Microbial community analysis showed that Dissulfurimicrobium sp. was the dominant SDB during the long-term operation regardless of dynamic sulfate and/or alkalinity concentrations, which provides evidence that SDB can be employed for sustainable and high-rate sulfide production for engineering purposes. A multi-stage AMD treatment system equipped with a SDR removed over 99% of the influent metals (i.e., Fe, Al, Zn, Cu, Pb) from AMD except for Mn. This study demonstrated that the novel SD process is a green and promising biotechnology for the sustainable treatment of organic-deficient metal-laden wastewater, such as AMD.

Recent Advances in Biotechnologies for the Treatment of Environmental Pollutants Based on Reactive Sulfur Species

The definition of reactive sulfur species (RSS) is inspired by the reactivity and variable chemical valence of sulfur. Sulfur is an essential element for life and is a part of global geochemical cycles. Wastewater treatment bioreactors can be divided into two major categories: sulfur reduction and sulfur oxidation. We review the origins of the definition of RSS and related biotechnological processes in environmental management. Sulfate reduction, sulfide oxidation, and sulfur-based redox reactions are key to driving the coupled global carbon, nitrogen, and sulfur co-cycles. This shows the coupling of the sulfur cycle with the carbon and nitrogen cycles and provides insights into the global material-chemical cycle. We also review the biological classification and RSS metabolic mechanisms of functional microorganisms involved in the biological processes, such as sulfate-reducing and sulfur-oxidizing bacteria. Developments in molecular biology and genomic technologies have allowed us to obtain detailed information on these bacteria. The importance of RSS in environmental technologies requires further consideration.

Experimental and modeling investigations on the unexpected hydrogen sulfide rebound in a sewer receiving nitrate addition: Mechanism and solution

Biogenic hydrogen sulfide is an odorous, toxic and corrosive gas released from sewage in sewers. To control sulfide generation and emission, nitrate is extensively applied in sewer systems for decades. However, the unexpected sulfide rebound after nitrate addition is being questioned in recent studies. Possible reasons for the sulfide rebounds have been studied, but the mechanism is still unclear, so the countermeasure is not yet proposed. In this study, a lab-scale sewer system was developed for investigating the unexpected sulfide rebounds via the traditional strategy of nitrate addition during 195-days of operation. It was observed that the sulfide pollution was even severe in a sewer receiving nitrate addition. The mechanism for the sulfide rebound can be differentiated into short-term and long-term effects based on the dominant contribution. The accumulation of intermediate elemental sulfur in biofilm resulted in a rapid sulfide rebound via the high-rate sulfur reduction after the depletion of nitrate in a short period. The presence of nitrate in sewer promoted the microorganism proliferation in biofilm, increased the biofilm thickness, re-shaped the microbial community and enhanced biological denitrification and sulfur production, which further weakened the effect of nitrate on sulfide control during the long-term operation. An optimized biofilm-initiated sewer process model demonstrated that neither the intermittent nitrate addition nor the continuous nitrate addition was a sustainable strategy for the sulfide control. To minimize the negative impact from sulfide rebounds, a (bi)monthly routine maintenance (e.g., hydraulic flushing with nitrate spike) to remove the proliferative microorganism in biofilm is necessary.

Efficient activation of peroxymonosulfate via Cu2+/Cu+ cycle enhanced by hydroxylamine for the degradation of Rhodamine B

The application of Cu²⁺/peroxymonosulfate (PMS) process for the elimination of refractory pollutants in industrial wastewater is limited by the slow transformation from Cu²⁺ to Cu⁺. In this research, hydroxylamine (HA) was employed to improve the degradation capacity of the Cu²⁺/PMS process. Rhodamine B (RhB) was selected as the target compound to indicate the performance of HA/Cu²⁺/PMS process. Compared with the Cu²⁺/PMS process, the reduction of Cu²⁺ to Cu⁺ was effectively promoted by HA in the HA/Cu²⁺/PMS process, which increased the decomposition rate of PMS by 29.2%, correspondingly, promoted the removal rate of RhB by 77.6%. The degradation of RhB followed pseudo-second-order kinetics in the proposed process. The active species analysis subsequently indicated hydroxyl radicals (·OH) and sulfate radicals (SO₄·^-) played important roles for degrading RhB with ·OH as the dominant active radical. The effects including initial pH, RhB concentration, PMS concentration, and Cu²⁺ concentration on the degradation of RhB were further investigated and discussed in detail. Additionally, the HA/Cu²⁺/PMS process exhibited effective RhB removal in simulated wastewater. From the perspective of waste utilization (Cu²⁺) and the remediation of organic contamination, the work would provide a valuable and promising process.

Treatment and remediation of metal-contaminated water and groundwater in mining areas by biological sulfidogenic processes: A review

Heavy metal pollution in the mining areas leads to serious environmental problems. The biological sulfidogenic process (BSP) mediated by sulfidogenic bacteria has been considered an attractive technology for the treatment and remediation of metal-contaminated water and groundwater. Notwithstanding, BSP driven by different sulfidogenic bacteria could affect the efficiency and cost-effectiveness of the treatment performance in practical applications, such as the microbial intolerance of pH and metal ions, the formation of toxic byproducts, and the consumption of organic electron donors. Sulfur-reducing bacteria (S⁰RB)-driven BSP has been demonstrated to be a promising alternative to the commonly used sulfate-reducing bacteria (SRB)-driven BSP for treating metal-contaminated wastewater and groundwater, due to the cost-saving in chemical addition, the high efficiency in sulfide production and metal removal efficiency. Although the S⁰RB-driven BSP has been developed and applied for decades, the present review works mainly focus on the developments in SRB-driven BSP for the treatment and remediation of metal-contaminated wastewater and groundwater. Accordingly, a comprehensive review for metal-contaminated wastewater treatment and groundwater remediation should be provided with the incorporation of the SRB- and S⁰RB-driven BSP. To identify the bottlenecks and to improve BSP performance, this paper reviews sulfidogenic bacteria presenting in metal-contaminated water and groundwater; highlight the critical factors for the metabolism of sulfidogenic bacteria during BSP; the ecological roles of sulfidogenic bacteria and the mechanisms of metal removal by sulfidogenic bacteria; and the application of the present sulfidogenic systems and their drawbacks. Accordingly, the research knowledge gaps, current process limitations, and future prospects were provided for improving the performance of BSP in the treatment and remediation of metal-contaminated wastewater and groundwater in mining areas.

Advances in elemental sulfur-driven bioprocesses for wastewater treatment: From metabolic study to application

Elemental sulfur (S0) is known to be an abundant, non-toxic material with a wide range of redox states (-2 to +6) and may serve as an excellent electron carrier in wastewater treatment. In turn, S0-driven bioprocesses, which employ S0 as electron donor or acceptor, have recently established themselves as cost-effective therefore attractive solutions for wastewater treatment. Numerous related processes have, to date, been developed from laboratory experiments into full-scale applications, including S0-driven autotrophic denitrification for nitrate removal and S0-reducing organic removal. Compared to the conventional activated sludge process, these bioprocesses require only a small amount of organic matter and produce very little sludge. There have been great efforts to characterize chemical and biogenic S0 and related functional microorganisms in order to identify the biochemical pathways, upgrade the bioprocesses, and assess the impact of the operating factors on process performance, ultimately aiming to better understand and to optimize the processes. This paper is therefore a comprehensive overview of emerging S0-driven biotechnologies, including the development of S0-driven autotrophic denitrification and S0-based sulfidogenesis, as well as the associated microbiology and biochemistry. Also reviewed here are the physicochemical characteristics of S0 and the effects that environmental factors such as pH, influent sulfur/nitrate ratio, temperature, S0 particle size and reactor configurations have on the process. Research gaps, challenges of process applications and potential areas for future research are further proposed and discussed.

Comprehensive evaluation of the long-term effect of Cu2+ on denitrifying granular sludge and feasibility of in situ recovery by phosphate

With increased industrial development, vast heavy metals are inevitably discharged into wastewater. Cu²⁺ is one of the most hazardous heavy metals in biotreatment. However, the potential effect of Cu²⁺ on denitrifying granular sludge is still unknown. This work assesses the response of denitrifying granular sludge to Cu²⁺ stress from multiple aspects. The denitrifying granular sludge could tolerate 5 mg L^-1 Cu²⁺, while the nitrogen removal efficiency decreased to 48.5% under 10 mg L^-1 Cu²⁺. Enzyme activity and carbohydrate metabolism were inhibited, and the denitrifying bacteria were washed out under Cu²⁺ stress. The resulting deteriorated state was reversed by phosphate. The nitrogen removal efficiency recovered to 99% after 10 days, and the enzyme activity also recovered to the original level. Membrane transport, transcription and cellular processes were promoted. Overall, the results of this work provide a feasible strategy to rapidly restore the metabolic activity of denitrifying granular sludge under Cu²⁺ stress.

Metal Sulfide Precipitation: Recent Breakthroughs and Future Outlooks

The interest in metal sulfide precipitation has recently increased given its capacity to efficiently recover several metals and metalloids from different aqueous sources, including wastewaters and hydrometallurgical solutions. This article reviews recent studies about metal sulfide precipitation, considering that the most relevant review article on the topic was published in 2010. Thus, our review emphasizes and focuses on the overall process and its main unit operations. This study follows the flow diagram definition, discussing the recent progress in the application of this process on different aqueous matrices to recover/remove diverse metals/metalloids from them, in addition to kinetic reaction and reactor types, different sulfide sources, precipitate behavior, improvements in solid–liquid separation, and future perspectives. The features included in this review are: operational conditions in terms of pH and Eh to perform a selective recovery of different metals contained in an aqueous source, the aggregation/colloidal behavior of precipitates, new materials for controlling sulfide release, and novel solid–liquid separation processes based on membrane filtration. It is therefore relevant that the direct production of nanoparticles (Nps) from this method could potentially become a future research approach with important implications on unit operations, which could possibly expand to several applications.

pH-dependent biological sulfidogenic processes for metal-laden wastewater treatment: Sulfate reduction or sulfur reduction?

Both biological sulfate reduction process and sulfur reduction process are attractive technologies for metal-laden wastewater treatment. However, the acidity stress of metal-laden wastewater could affect the sulfidogenic performance and the microbial community, weaken the stability, efficiency and cost-effectiveness of the biological sulfidogenic processes (BSP). In this study, long-term lab-scale trials were conducted with a sulfate-reducing bioreactor and a sulfur-reducing bioreactor to evaluate the effects of acidity on sulfidogenic activities and the microbial community of the BSP. In the 300-day trial, the sulfate-reducing bacteria (SRB)-driven BSP was stable in terms of sulfidogenic performance and microbial community with the decline of pH, while the sulfur-reducing bacteria (S⁰RB)-driven BSP achieved high-rate and low-cost sulfide production under neutral conditions but unstable under acidic conditions. With the decline of pH, the sulfide production rate (SPR) of the SRB-driven BSP stably increased from 30 to 83 mg S/L-h; while it decreased from 56 to 37 mg S/L-h in the S⁰RB-driven BSP with high fluctuation. The results of estimation were consistent with the thermodynamical calculations, in which the sulfur reduction process showed a better performance at pH 5-7, while the sulfate reduction process might gain more energy when pH<5. The stable sulfidogenic performance and microbial community diversity of the SRB-driven BSP could be attributed to the alkalinity produced in sulfate reduction to buffer the acidic stress. In comparison, the microbial community in the S⁰RB-driven BSP was significantly re-shaped by acidity stress, and the predominant sulfidogenic bacterium changed from Desulfovibrio at neutral condition, to Desulfurella at pH≤5.4. The stability of the microbial community significantly affected the SPR and the operational cost. Nevertheless, the organic consumption for sulfide production of the S⁰RB-driven BSP was still less than the SRB-driven BSP even in acidic conditions. Collectively, the S⁰RB-driven BSP was recommended under neutral or mild acid conditions, while the SRB-driven BSP was more suitable under fluctuating pH conditions, especially at low pH. Overall, this study presented the long-term performance of SRB- and S⁰RB-driven BSP under varying pH conditions, and provided guidance to determine the suitable BSP and operational cost for different metal-laden wastewater.

Cr(VI) bioremediation by active algal-bacterial aerobic granular sludge: Importance of microbial viability, contribution of microalgae and fractionation of loaded Cr

In this study, chromium (Cr) was used as an example of the most toxic heavy metals that threaten human health, and Cr(VI) bioremediation was implemented by using a new type of aerobic granular sludge (AGS), i.e., algal-bacterial AGS. Results showed that the total Cr removal efficiency by active algal-bacterial AGS was 85.1 ± 0.6% after 6 h biosorption at pH 6 and room temperature, which could be further improved to 93.8 ± 0.4% with external electron donor (glucose) supply. However, inactivation dramatically decreased the total Cr removal efficiency to 29.6 ± 3.5%, and no effect was noticed when external electron donor was provided. With an antibiotic (levofloxacin) or metabolic inhibitor (NaN₃) addition, the total Cr removal efficiency of bacterial AGS was inhibited by 16.0% or 10.1%, but this efficiency was maintained in the case of algal-bacterial AGS. Analysis of extracellular polymeric substances (EPS) composition revealed that under Cr(VI) exposure, more loosely bound EPS were secreted by algal-bacterial AGS, favoring Cr(VI) reduction. Results from chemical fractionation indicated that 90.5 ± 4.2% of the loaded Cr on algal-bacterial AGS was in an immobile form, reflecting the low environmental risk of Cr-loaded algal-bacterial AGS after biosorption of hazardous heavy metals from wastewater.

Elemental sulfur as electron donor and/or acceptor: Mechanisms, applications and perspectives for biological water and wastewater treatment

Biochemical oxidation and reduction are the principle of biological water and wastewater treatment, in which electron donor and/or acceptor shall be provided. Elemental sulfur (S⁰) as a non-toxic and easily available material with low price, possesses both reductive and oxidative characteristics, suggesting that it is a suitable material for water and wastewater treatment. Recent advanced understanding of S⁰-respiring microorganisms and their metabolism further stimulated the development of S⁰-based technologies. As such, S⁰-based biotechnologies have emerged as cost-effective and attractive alternatives to conventional biological methods for water and wastewater treatment. For instance, S⁰-driven autotrophic denitrification substantially lower the operational cost for nitrogen removal from water and wastewater, compared to the conventional process with exogenous carbon source supplementation. The introduction of S⁰ can also avoid secondary pollution commonly caused by overdose of organic carbon. S⁰ reduction processes cost-effectively mineralize organic matter with low sludge production. Biological sulfide production using S⁰ as electron acceptor is also an attractive technology for metal-laden wastewater treatment, e.g. acid mine drainage. This paper outlines an overview of the fundamentals, characteristics and advances of the S⁰-based biotechnologies and highlights the functional S⁰-related microorganisms. In particular, the mechanisms of microorganisms accessing insoluble S⁰ and feasibility to improve S⁰ bio-utilization efficiency are critically discussed. Additionally, the research knowledge gaps, current process limitations, and required further developments are identified and discussed.

The exacerbation of mercury methylation by Geobacter sulfurreducens PCA in a freshwater algae-bacteria symbiotic system throughout the lifetime of algae

Mine-polluted wastewater with mercury (Hg) poses severe environmental pollution since Hg(II) can be converted to highly neurotoxic methylmercury (MeHg) under anaerobic conditions. Previous studies on Hg methylation have focused on aquatic sediments, but few have investigated the MeHg formation in water layers containing algae. In this study, we investigated the dynamic effect of algae on Hg methylation throughout the lifetime of algae. We found that Chlorella pyrenoidosa was a non-methylating alga and exhibited good tolerance to Hg stress (1-20 μg/L); thus Hg(II) could not inhibit the process of eutrophication. However, the presence of C. pyrenoidosa significantly enhanced the Hg methylation by Geobacter sulfurreducens PCA. Compared to the control sample without algae, the MeHg production rate of algae-bacteria samples remarkably exacerbated by 62.3-188.3% with the algal growth period at cell densities of 1.5 × 10⁶-25 × 10⁶ cells/mL. The increase of algal organic matter and thiols with the algal growth period resulted in the exacerbation of MeHg production. The Hg methylation was also enhanced with the presence of dead algae, of which the enhancement was ~62.4% lower than that with the presence of live algae. Accordingly, the potential mechanism of Hg methylation in a freshwater algae-bacteria symbiotic system throughout the algal lifetime was proposed.