Electrochemical sulfide removal and recovery from paper mill anaerobic treatment effluent

Smartphone-assisted array discrimination of sulfur-containing compounds and colorimetric-fluorescence dual-mode sensor for detection of 1,4-benzenedithiol based on peroxidase-like nanozyme g-C3N4@Cu, N-CDs

Present work reported a novel nanozyme g-C3N4@Cu, N-CDs with excellent peroxidase-like activity obtained by loading Cu and N co-doped carbon dots on g-C3N4 (graphitic carbon nitride). g-C3N4@Cu, N-CDs can catalyze H2O2 to generate hydroxyl radical •OH, which oxidizes o-phenylenediamine to 2,3-diaminophenazine, which emits orange fluorescence under ultraviolet light irradiation. The experimental results confirmed that 1,4-benzenedithiol (BDT) could inhibit the peroxidase-like activity of g-C3N4@Cu, N-CDs. Based the principle above, a colorimetric-fluorescence dual-mode sensor for rapidly sensing of BDT was creatively constructed with assisting of a smartphone. The sensor showed excellent linearity over ranges of 0.75-132 μM and 0.33-60.0 μM with detection limits of 0.32 μM and 0.25 μM for colorimetric and fluorescence detection, respectively. Moreover, a smartphone-assisted colorimetric array sensor was constructed to distinguish six sulfur-containing compounds according to the difference in the degree of inhibition of nanozyme activity by different sulfur-containing compounds. The array sensor could distinguish sulfur-containing compounds at low concentration as low as 0.4 μM. The results validated that the designed sensor was a convenient and fast platform, which could be utilized as a reliably portable tool for the efficient and accurate detection of BDT and the discrimination of multiple sulfur compounds in real water samples.

Seawater electrolysis for fuels and chemicals production: fundamentals, achievements, and perspectives

Seawater electrolysis for the production of fuels and chemicals involved in onshore and offshore plants powered by renewable energies offers a promising avenue and unique advantages for energy and environmental sustainability. Nevertheless, seawater electrolysis presents long-term challenges and issues, such as complex composition, potential side reactions, deposition of and poisoning by microorganisms and metal ions, as well as corrosion, thus hindering the rapid development of seawater electrolysis technology. This review focuses on the production of value-added fuels (hydrogen and beyond) and fine chemicals through seawater electrolysis, as a promising step towards sustainable energy development and carbon neutrality. The principle of seawater electrolysis and related challenges are first introduced, and the redox reaction mechanisms of fuels and chemicals are summarized. Strategies for operating anodes and cathodes including the development and application of chloride- and impurity-resistant electrocatalysts/membranes are reviewed. We comprehensively summarize the production of fuels and chemicals (hydrogen, carbon monoxide, sulfur, ammonia, etc.) at the cathode and anode via seawater electrolysis, and propose other potential strategies for co-producing fine chemicals, even sophisticated and electronic chemicals. Seawater electrolysis can drive the oxidation and upgrading of industrial pollutants or natural organics into value-added chemicals or degrade them into harmless substances, which would be meaningful for environmental protection. Finally, the perspective and prospects are outlined to address the challenges and expand the application of seawater electrolysis.

The dual built-in electric fields across CoS/MoS2 heterojunctions for energy-saving hydrogen production coupled with sulfion degradation

Substituting the sluggish oxygen evolution reaction with the sulfur oxidation reaction can significantly reduce energy consumption and eliminate environmental pollutants during hydrogen generation. However, the progress of this technology has been hindered due to the lack of cost-effective, efficient, and durable electrocatalysts. In this study, we present the design and construction of a hierarchical metal sulfide catalyst with a gradient structure comprising nanoparticles, nanosheets, and microparticles. This was achieved through a structure-breaking sulfuration strategy, resulting in a "ball of yarn"-like core/shell CoS/MoS2 microflower with CoS/MoS2/CoS dual-heterojunctions. The difference in work functions between CoS and MoS2 induces an electron polarization effect, creating dual built-in electric fields at the hierarchical interfaces. This effectively modulates the adsorption behavior of catalytic intermediates, thereby reducing the energy barrier for catalytic reactions. The optimized catalyst exhibits outstanding electrocatalytic performance for both the hydrogen evolution reaction and the sulfur oxidation reaction. Remarkably, in the assembled electrocatalytic coupling system, it only requires a cell voltage of 0.528 V at 10 mA cm-2 and maintains long-term durability for over 168 h. This work presents new opportunities for low-cost hydrogen production and environmentally friendly sulfion recycling.

Electrochemical sulfate production from sulfide-containing wastewaters and integration with electrochemical nitrogen recovery

Electrochemical methods can help manage sulfide in wastewater, which poses environmental and health concerns due to its toxicity, malodor, and corrosiveness. In addition, sulfur could be recovered as fertilizer and commodity chemicals from sulfide-containing wastewaters. Wastewater characteristics vary widely among wastewaters; however, it remains unclear how these characteristics affect electrochemical sulfate production. In this study, we evaluated how four characteristics of influent wastewaters (electrolyte pH, composition, sulfide concentration, and buffer strength) affect sulfide removal (sulfide removal rate, sulfide removal efficiency) and sulfate production metrics (sulfate production rate, sulfate production selectivity). We identified that electrolyte pH (3 × difference, i.e., 25.1 to 84.9 μM h-1 in average removal rate within the studied pH range) and sulfide concentration (16 × difference, i.e., 82.1 to 1347.2 μM h-1 in average removal rate) were the most influential factors for electrochemical sulfide removal. Sulfate production was most sensitive to buffer strength (6 × difference, i.e., 4.4 to 27.4 μM h-1 in average production rate) and insensitive to electrolyte composition. Together, these results provide recommendations for the design of wastewater treatment trains and the feasibility of applying electrochemical methods to varying sulfide-containing wastewaters. In addition, we investigated a simultaneous multi-nutrient (sulfur and nitrogen) process that leverages electrochemical stripping to further enhance the versatility and compatibility of electrochemical nutrient recovery.

Improving the Sulfurophobicity of the NiS-Doping CoS Electrocatalyst Boosts the Low-Energy-Consumption Sulfide Oxidation Reaction Process

Producing sulfur from a sulfide oxidation reaction (SOR)-based technique using sulfide aqueous solution has attracted considerable attention due to its ecofriendliness. This study demonstrates that NiS-doped cobalt sulfide NiS-CoS-supported NiCo alloy foam can deliver the SOR with superior electrocatalytic activity and robust stability compared to reported non-noble metal-based catalysts. Only 0.34 V vs RHE is required to drive a current density of 100 mA cm-2 for the SOR. According to the experiment, the catalyst exhibits a unique sulfurophobicity feature because of the weak interaction between sulfur and the transition metal sulfide (low affinity for elemental sulfur), preventing electrode corrosion during the SOR process. More impressively, the chain-growth mechanism of the SOR from short- to long-chain polysulfides was revealed by combining electrochemical and spectroscopic in situ methods, such as in situ ultraviolet-visible and Raman. It is also demonstrated that electrons can transfer straight from the sulfion (S2-) to the active site on the anode surface during the low-energy-consumption SOR process. This work provides new insight into simultaneous energy-saving hydrogen production and high-value-added S recovery from sulfide-containing wastewater.

Fabrication and characterization of a novel Ba2+-loaded sawdust biochar doped with iron oxide for the super-adsorption of SO42- from wastewater

Biochar is a low-cost adsorbent used in the treatment of contaminated wastewater. We investigated the potential of an Fe-impregnated, Ba²⁺-loaded biochar (Fe-(Ba-BC)) for the removal of SO₄^2- from aqueous solutions. The Ba²⁺-loaded biochar was synthesized from sawdust impregnated with iron oxide via pyrolysis at 600 °C. The porous structure of the Fe-(Ba-BC) was identified by scanning electron microscopy before sulfate was adsorbed onto the adsorbent. Functional groups were determined by energy-dispersive spectrophotometry and Raman spectrometry.. The Fe-(Ba-BC) Raman peaks before the experiment were higher than after, suggesting the precipitation of BaSO₄. The presence of BaCl₂ on the surface of the biochar was confirmed by X-ray diffraction. Batch sorption results showed that Fe-(Ba-BC) strongly adsorbed aqueous SO₄^2- with a removal efficacy of 96.7% under the optimum conditions of 0.25 M BaCl₂, a contact time of 480 min, a pH of 9 and an adsorbent dose of 2 g. The optimum condition for removal and reaction rate kinetics analysis indicated that adsorption curve fitted well with PSO, k₂ 0.00015 confirmed the removal of SO₄^2- via chemisorption. Thus, Fe-(Ba-BC) was found to be a favorable adsorbent for removing SO₄^2-.

Implementation of a Sulfide-Air Fuel Cell Coupled to a Sulfate-Reducing Biocathode for Elemental Sulfur Recovery

Bio-electrochemical systems (BES) are a flexible biotechnological platform that can be employed to treat several types of wastewaters and recover valuable products concomitantly. Sulfate-rich wastewaters usually lack an electron donor; for this reason, implementing BES to treat the sulfate and the possibility of recovering the elemental sulfur (S⁰) offers a solution to this kind of wastewater. This study proposes a novel BES configuration that combines bio-electrochemical sulfate reduction in a biocathode with a sulfide–air fuel cell (FC) to recover S⁰. The proposed system achieved high elemental sulfur production rates (up to 386 mg S⁰-S L⁻¹ d⁻¹) with 65% of the sulfate removed recovered as S⁰ and a 12% lower energy consumption per kg of S⁰ produced (16.50 ± 0.19 kWh kg⁻¹ S⁰-S) than a conventional electrochemical S⁰ recovery system.

Overview of recent developments of resource recovery from wastewater via electrochemistry-based technologies

As the rapid increase of the worldwide population, recovering valuable resources from wastewater have attracted more and more attention by governments and academia. Electrochemical technologies have been extensively investigated over the past three decades to purify wastewater. However, the application of these technologies for resource recovery from wastewater has just attracted limited attention. In this review, the recent (2010-2020) electrochemical technologies for resource recovery from wastewater are summarized and discussed for the first time. Fundamentals of typical electrochemical technologies are firstly summarized and analyzed, followed by the specific examples of electrochemical resource recovery technologies for different purposes. Based on the fundamentals of electrochemical reactions and without the addition of chemical agents, metallic ions, nutrients, sulfur, hydrogen and chemical compounds can be effectively recovered by means of electrochemical reduction, electrochemical induced precipitation, electrochemical stripping, electrochemical oxidation and membrane-based electrochemical processes, etc. Pros and cons of each electrochemical technology in practical applications are discussed and analyzed. Single-step electrochemical process seems ineffectively to recover valuable resources from the wastewater with complicated constituents. Multiple-step processes or integrated with biological and membrane-based technologies are essential to improve the performance and purity of products. Consequently, this review attempts to offer in-depth insights into the developments of next-generation of electrochemical technologies to minimize energy consumption, boost recovery efficiency and realize the commercial application.

Effects of the Feeding Solution Composition on a Reductive/Oxidative Sequential Bioelectrochemical Process for Perchloroethylene Removal

Chlorinated aliphatic hydrocarbons (CAHs) are common groundwater contaminants due to their improper use in several industrial activities. Specialized microorganisms are able to perform the reductive dechlorination (RD) of high-chlorinated CAHs such as perchloroethylene (PCE), while the low-chlorinated ethenes such as vinyl chloride (VC) are more susceptible to oxidative mechanisms performed by aerobic dechlorinating microorganisms. Bioelectrochemical systems can be used as an effective strategy for the stimulation of both anaerobic and aerobic microbial dechlorination, i.e., a biocathode can be used as an electron donor to perform the RD, while a bioanode can provide the oxygen necessary for the aerobic dechlorination reaction. In this study, a sequential bioelectrochemical process constituted by two membrane-less microbial electrolysis cells connected in series has been, for the first time, operated with synthetic groundwater, also containing sulphate and nitrate, to simulate more realistic process conditions due to the possible establishment of competitive processes for the reducing power, with respect to previous research made with a PCE-contaminated mineral medium (with neither sulphate nor nitrate). The shift from mineral medium to synthetic groundwater showed the establishment of sulphate and nitrate reduction and caused the temporary decrease of the PCE removal efficiency from 100% to 85%. The analysis of the RD biomarkers (i.e., Dehalococcoides mccartyi 16S rRNA and tceA, bvcA, vcrA genes) confirmed the decrement of reductive dechlorination performances after the introduction of the synthetic groundwater, also characterized by a lower ionic strength and nutrients content. On the other hand, the system self-adapted the flowing current to the increased demand for the sulphate and nitrate reduction, so that reducing power was not in defect for the RD, although RD coulombic efficiency was less.

Treatment of Palm Oil Refinery Effluent Using Tannin as a Polymeric Coagulant: Isotherm, Kinetics, and Thermodynamics Analyses

The refining of the crude palm oil (CPO) generates the palm oil refinery effluent (PORE). The presence of high contents of biochemical oxygen demand (BOD), chemical oxygen demand (COD), turbidity, and suspended solids (SS) in PORE encourages the determination of an effective treatment process to minimize the environmental pollution and preserve aquatic life. In the present study, a biodegradable natural polymer, namely tannin, was utilized as a coagulant to treat PORE. The coagulation experiment was conducted using a jar test apparatus. The tannin coagulation efficiency was evaluated based on the BOD, COD, turbidity, and SS removal from PORE by varying the tannin dose (50-300 mg/L), pH (pH 4-10), treatment time (15-90 min), and sedimentation time (15-90 min). It was found that the maximum removal of BOD, COD, turbidity, and SS was 97.62%, 88.89%, 93.01%, and 90.21%, respectively, at pH 6, a tannin dose of 200 mg/L, 60 min of coagulation time, and 60 min of sedimentation time. Analyses of isotherm models revealed that the Freundlich isotherm model was well fitted with the coagulation study. Kinetics studies show that the pseudo-second-order kinetics model was the well-fitted kinetics model for the BOD, COD, turbidity, and SS removal from PORE using tannin as a polymeric coagulant. The determination of thermodynamics parameters analyses revealed that BOD, COD, turbidity, and SS removal from PORE was spontaneous, exothermic, and chemical in nature. The finding of the present study shows that tannin as a natural polymeric coagulant would be utilized in PORE treatment to avoid toxic sludge generation.

Experimental study on volatile sulfur compound inhibition using a single-chamber membrane-free microbial electrolysis cell

Odor emissions from sewer systems and wastewater treatment plants have attracted much attention due to the potential negative effects on human health. A single-chamber membrane-free microbial electrolysis cell was proposed for the removal of sulfides in a sewer system. The feasibility of the use of volatile sulfur compounds and their removal efficiency in liquid and headspace gas phases were investigated using synthetic wastewater with real sewer sediment and Ru/Ir-coated titanium electrodes. The results indicate that hydrogen sulfide and volatile organic sulfur compounds were effectively inhibited in the liquid phase upon electrochemical treatment at current densities of 1.55, 2.06, and 2.58 mA/cm², and their removal rates reached up to 86.2-100%, except for dimethyl trisulfide, the amount of which increased greatly at 1.55 mA/cm². In addition, the amount of volatile sulfur compounds in the headspace decreased greatly; however, the total theoretical odor concentration was still high, and methanethiol and ethanethiol greatly contributed to the total strength of the odor concentration due to their low odor threshold concentrations. The major pathway for sulfide removal in the single-chamber membrane-free microbial electrolysis cell is biotic oxidation, the removal rate of which was 0.4-0.5 mg/min, 4-5 times that of indirect electrochemical oxidation.

A novel coupling process with partial nitritation-anammox and short-cut sulfur autotrophic denitrification in a single reactor for the treatment of high ammonium-containing wastewater

In this study, a novel coupling process with partial nitritation-anaerobic ammonium oxidation (anammox) (PNA) and sulfur autotrophic denitrification (SAD) was studied using an upflow biofilm reactor with mechanical vibration. At a lower dissolved oxygen (DO) concentration (0.40 ± 0.20 mg L^-1), ammonia could be efficiently removed from synthetic wastewater by the coupling system with a total nitrogen removal efficiency (NRE) of 98% and an influent NH₄⁺-N concentration of 600 mg L^-1. In this system, the nitrate, which was produced during the anammox reaction, could be timely reduced by the SAD reaction. Compared with the conventional PNA and SAD processes, coupling the PNA and SAD processes in a single reactor prevented nitrite accumulation in the SAD reaction and reduced the total sulfate production by 59%. The high-throughput sequencing analysis supported that the SAD bacteria (Thiobacillus) and anammox bacteria (Candidatus Kuenenia) could coexist on the elemental sulfur stone. Additionally, sulfur consumption and sulfate production were increased under a high DO concentration. The sulfate production/nitrate reduction ratio and changing profile of the substrate suggested that the short-cut SAD process mainly occurred in this coupling system. Otherwise, batch experiments also suggested that the nitrite removal rate in the anammox process was 34.5 times higher than that in the SAD process. The outcomes of these experiments revealed that most of the nitrite, as an intermediate product in the SAD reaction, served as an electron acceptor for the anammox reaction. A stoichiometric calculation of this coupling process indicated that the novel reaction scheme with a high NRE was successfully achieved. Under an ideal short-cut SAD process, almost 55% of the sulfur consumption could be reduced in this coupling system. The coupling system provides a new perspective for nitrogen removal in a single reactor and further promotes anammox and SAD performance in wastewater treatment processes.

Reductive/Oxidative Sequential Bioelectrochemical Process for Perchloroethylene Removal

An innovative bioelectrochemical reductive/oxidative sequential process was developed and tested on a laboratory scale to obtain the complete mineralization of perchloroethylene (PCE) in a synthetic medium. The sequential bioelectrochemical process consisted of two separate tubular bioelectrochemical reactors that adopted a novel reactor configuration, avoiding the use of an ion exchange membrane to separate the anodic and cathodic chamber and reducing the cost of the reactor. In the reductive reactor, a dechlorinating mixed inoculum received reducing power to perform the reductive dechlorination of perchloroethylene (PCE) through a cathode chamber, while the less chlorinated daughter products were removed in the oxidative reactor, which supported an aerobic dechlorinating culture through in situ electrochemical oxygen evolution. Preliminary fluid dynamics and electrochemical tests were performed to characterize both the reductive and oxidative reactors, which were electrically independent of each other, with each having its own counterelectrode. The first continuous-flow potentiostatic run with the reductive reactor (polarized at −450 mV vs SHE) resulted in obtaining 100% ± 1% removal efficiency of the influent PCE, while the oxidative reactor (polarized at +1.4 V vs SHE) oxidized the vinyl chloride and ethylene from the reductive reactor, with removal efficiencies of 100% ± 2% and 92% ± 1%, respectively.

Recovery of elemental sulfur with a novel integrated bioelectrochemical system with an electrochemical cell

Several industrial activities produce wastewater with high sulfate content that can cause significant environmental issues. Although bioelectrochemical systems (BESs) have recently been studied for the treatment of sulfate contained in this wastewater, the recovery of elemental sulfur with BESs is still in its beginnings. This work proposes a new reactor configuration named BES-EC, consisting of the coupling of a BES with an electrochemical cell (EC), to treat this type of wastewater and recover elemental sulfur. The reactor consisted of four electrodes: i) an abiotic anode, ii) a biocathode for the autotrophic sulfate reduction, iii) an anode of an electrochemical cell (EC) for the partial oxidation of sulfide to elemental sulfur (the biocathode and the EC anode were placed in the same chamber) and iv) an abiotic EC cathode. Several cathode potentials and sulfate loads were tested, obtaining high sulfate removal rates (up to 888 mg SO₄^2--S L^-1 d^-1 at -0.9 V vs. SHE with a specific energy consumption of 9.18 ± 0.80 kWh kg^-1 SO₄^2--S). Exceptionally high theoretical elemental sulfur production rates (up to 498 mg S⁰-S L^-1 d^-1) were achieved with the EC controlled at a current density of 2.5 A m^-2. Electron recovery around 80% was observed throughout most of the operation of the integrated system. In addition, short experiments were performed at different current densities, observing that sulfate removal did not increase proportionally to the higher applied current density. However, when the BES was controlled at 30 A m^-2 and the EC at 7.5 A m^-2, the proportion of elemental sulfur produced corresponded to 92.9 ± 1.9% of all sulfate removed.

Electrochemical removal of sulfide on porous carbon-based flow-through electrodes

Electrochemical oxidation of hydrogen sulfide and its separation from the waste stream in the form of sulfur was studied at low-cost carbon-based porous materials, activated carbon felt (ACF) and graphite felt (GF). Both materials were capable of selective HS^- oxidation to elemental sulfur in low-conductivity solutions (i.e., <1 mS cm^-1), as well as in raw sewage. The HS^- removal rate was ten times faster at ACF compared with GF electrode due to the higher surface area and chemisorption of HS^-. To address the electrode passivation with the electrodeposited sulfur, different electrochemical recovery strategies were tested. GF could be only partially regenerated (i.e., 30% efficiency) using cathodic polarization. Also, both anodic and cathodic polarization improved the sulfide removal in the subsequent working cycle due to the introduction of new redox-active oxygen containing functional groups. Sulfur deposited at the ACF electrode could not be recovered by any of the investigated strategies. Thus, sulfur was incorporated into the carbon matrix and strongly bonded with the carbon functional groups at both GF and ACF electrodes. Although carbon-based electrodes have been widely investigated for electrochemical sulfide removal, this study demonstrates that their application is limited by low regeneration efficiency of the electrodeposited sulfur.

Electrochemical monitoring of Acidithiobacillus thiooxidans biofilm formation on graphite surface with elemental sulfur

Inorganic wastewaters and sediments from the mining industry and mineral bioleaching processes have not been fully explored in bioelectrochemical systems (BES). Knowledge of interfacial changes due to biofilm evolution under acidic conditions may improve applications in electrochemical processes, specifically those related to sulfur compounds. Biofilm evolution of Acidithiobacillus thiooxidans on a graphite plate was monitored by electrochemical techniques, using the graphite plate as biofilm support and elemental sulfur as the only energy source. Even though the elemental sulfur was in suspension, S⁰ particles adhered to the graphite surface favoring biofilm development. The biofilms grown at different incubation times (without electric perturbation) were characterized in a classical three electrode electrochemical cell (sulfur and bacteria free culture medium) by non-invasive electrochemical impedance spectroscopy (EIS) and cyclic voltammetry. The biofilm structure was confirmed by Environmental Scanning Electrode Microscopy, while the relative fractions of exopolysaccharides and extracellular hydrophobic compounds at different incubation times were evaluated by Confocal Laser Scanning Microscopy. The experimental conditions chosen in this work allowed the EIS monitoring of the biofilm growth as well as the modification of Extracellular Polymeric Substances (EPS) composition (hydrophobic/ exopolysaccharides EPS ratio). This strategy could be useful to control biofilms for BES operation under acidic conditions.

Treatment of real flue gas desulfurization wastewater in an autotrophic biocathode in view of elemental sulfur recovery: Microbial communities involved

Sulfur oxide emissions can lead to acidic precipitation and health concerns. Flue gas desulfurization (FGD) systems treat these emissions generating a wastewater with high-sulfate content. This work is the first attempt to treat this effluent with bioelectrochemical systems (BES) in order to recover elemental sulfur, a technology that allows the treatment of several wastewaters that lack of electron donor. The sulfate treatment and elemental sulfur recovery have been studied in a biocathode with simultaneous sulfate reduction to sulfide and partial sulfide oxidation, comparing the performance obtained with synthetic and real wastewater. A decrease of the sulfate removal rate (SRR) from 108 to 73mgS-SO₄^2-L^-1d^-1 was observed coupled to an increase in the elemental sulfur recovery from 1.4 to 27mgS-S⁰L^-1d^-1. This elemental sulfur recovered as a solid from the real wastewater represented a 64% of the theoretical elemental sulfur produced (the elemental sulfur corresponded to a 72% of the solid weight). In addition, microbial communities analysis of the membrane and cathode biofilms and planktonic biomass showed that the real wastewater allowed a higher growth of sulfur oxidizing bacteria (SOB) adapted to more complex waters as Halothiobacillus sp. while decreasing the relative abundance of sulfate reducing bacteria (SRB).

A microbial fuel cell configured for the remediation of recalcitrant pollutants in soil environment

A pristine soil environment supports a healthy soil biodiversity, which is often polluted with recalcitrant compounds. The bioelectrochemical remediation of the contaminated soils using bioelectrochemical systems (BESs) is gaining significant attention with respect to the restoration of the soil ecosystem. In this direction, a microbial fuel cell (MFC, an application of BES), was employed for the treatment of total petroleum hydrocarbons (TPHs) in a soil microenvironment at three ranges of pollution (loading conditions – 320, 590 and 840 mg TPH per L). TPHs degraded effectively in the soil-electrode vicinity in the range of 158 mg TPH_R per L (320 mg TPH per L) and 356 mg TPH_R per L (840 mg TPH per L). The study also demosntrated a maximum bioelectrogenesis of 286.7 mW m⁻² (448 mV at 100 Ω) at the highest TPH loading concentration studied (840 mg TPH per L). The conditions prevailing in the soil MFC also facilitated the removal of sulfates (114 mg SO₄²⁻ per L; 62.64%) and the removal of total dissolved solids (910 mg TDS per L, 12.08%) at an 840 mg TPH per L loading condition. The pH of the outlet wastewater prevailing in the mild alkaline range of 7.6 and 8.4, along with improved sulfate and TPH removal in the respective conditions suggested suitable conditions for sulfate-reducing bacteria (SRB). This study also signified the sustainability of the process for the effective treatment of hydrocarbon contaminated soil that also generates green energy.

Bioelectroremediation of petroleum-based hydrocarbons contaminated soil was successfully performed through microbial fuel cells (MFCs).

Interfacing anaerobic digestion with (bio)electrochemical systems: Potentials and challenges

For over a century, anaerobic digestion has been a key technology in stabilizing organic waste streams, while at the same time enabling the recovery of energy. The anticipated transition to a bio-based economy will only increase the quantity and diversity of organic waste streams to be treated, and, at the same time, increase the demand for additional and effective resource recovery schemes for nutrients and organic matter. The performance of anaerobic digestion can be supported and enhanced by (bio)electrochemical systems in a wide variety of hybrid technologies. Here, the possible benefits of combining anaerobic digestion with (bio)electrochemical systems were reviewed in terms of (1) process monitoring, control, and stabilization, (2) nutrient recovery, (3) effluent polishing, and (4) biogas upgrading. The interaction between microorganisms and electrodes with respect to niche creation is discussed, and the potential impact of this interaction on process performance is evaluated. The strength of combining anaerobic digestion with (bio)electrochemical technologies resides in the complementary character of both technologies, and this perspective was used to distinguish transient trends from schemes with potential for full-scale application. This is supported by an operational costs assessment, showing that the economic potential of combining anaerobic digestion with a (bio)electrochemical system is highly case-specific, and strongly depends on engineering challenges with respect to full-scale applications.

Preparation and characterization of γ-Fe2O3 nanoparticles and investigation of its adsorption performance for sulfide, sulfite and thiosulfate from aqueous solutions using ultrasonic assisted method: Modeling and optimization

Maghemite nanoparticles, as an adsorbent, was used for the removal of sulfur species including sulfide, sulfite and thiosulfate from waste water samples by ultrasonic-assisted adsorption method. The characterization of the prepared nanoparticles was carried out by Fourier transform infrared spectroscopy, scanning electron microscopy, X-ray diffraction and BET technique. The nanoparticles well dispersed in the water. The adsorbent was easily separated magnetically from the solution after loading with adsorbate. According to central composite design, the best experimental conditions including initial pH, the dosage of adsorbent and sonication time were obtained for sulfide, sulfite and thiosulfate. After optimization of the parameters, the removal of analytes in these conditions lead to the highest analytes removal efficiency (above 98%). The adsorption capacity was evaluated using different adsorption isotherm models. The maximum predicted adsorption capacities for sulfide, sulfite and thiosulfate were obtained as 148.5, 122.5 and 119.6mgg^-1, respectively. Then, desorption process of the adsorbed thiosulfate was also investigated using sodium hydroxide solution as the solvent and the other conditions affect to desorption were optimized.

A novel bioelectrochemical system for chemical-free permanent treatment of acid mine drainage

The mining sector is currently under unprecedented pressure due to stringent environmental regulations. As a consequence, a permanent acid mine drainage (AMD) treatment is increasingly being regarded as a desirable target with direct benefits for the environment and the operational and economic viability of the resources sector. In this study we demonstrate that a novel bioelectrochemical system (BES) can deliver permanent treatment of acid mine drainage without chemical dosing. The technology consists of a two-cell bioelectrochemical setup to enable the removal of sulfate from the ongoing reduction-oxidation sulfur cycle to less than 550 mg L^-1 (85 ± 2% removal from a real AMD of an abandoned silver mine), thereby also reducing salinity at an electrical energy requirement of 10 ± 0.3 kWh kg^-1 of SO₄^2--S removed. In addition, the BES operation drove the removal and recovery of the main cations Al, Fe, Mg, Zn at rates of 151 ± 0 g Al m^-3 d^-1, 179 ± 1 g Fe m^-3 d^-1, 172 ± 1 g Mg m^-3 d^-1 and 46 ± 0 g Zn m^-3 d^-1 into a concentrate stream containing 263 ± 2 mg Al, 279 ± 2 mg Fe, 152 ± 0 mg Mg and 90 ± 0 mg Zn per gram of solid precipitated after BES fed-rate control treatment. The solid metal-sludge was twice less voluminous and 9 times more readily settleable than metal-sludge precipitated using NaOH. The continuous BES treatment also demonstrated the concomitant precipitation of rare earth elements together with yttrium (REY), with up to 498 ± 70 μg Y, 166 ± 27 μg Nd, 155 ± 14 μg Gd per gram of solid, among other high-value metals. The high-REY precipitates could be used to offset the treatment costs.

Evaluation of key parameters on simultaneous sulfate reduction and sulfide oxidation in an autotrophic biocathode

Bioelectrochemical systems (BESs) are being studied as an alternative technology for the treatment of several kinds of wastewaters with a lack of electron donor such as high-strength sulfate wastewaters. This study evaluates different parameters that influence the simultaneous sulfate reduction and sulfide oxidation in an autotrophic biocathode: ion-exchange membrane (IEM), cathodic pH and cathode potential. Two different membranes were studied to evaluate sulfate and sulfide adsorption and diffusion from the cathode to the anode, observing that a cation-exchange membrane (CEM) widely decreased these effects. Three different cathode pH (5.5, 7 and 8.5) were studied in a long-term operation observing that pH = 7 was the optimal for sulfate removal, achieving reduction rates around 150 mg S-SO₄^2- L^-1 d^-1. Microbial community analysis of the cathode biofilm demonstrated a high abundance of sulfate-reducing bacteria (SRB, 67% at pH 7, 60% at pH 8.5 and 42% at pH 5.5), mainly Desulfovibrio sp. at pH 5.5 and 7 and Desulfonatronum sp. at pH 8.5. The cathode potential also was studied from -0.7 to -1.2 V vs. SHE achieving sulfate removal rates higher than 700 mg S-SO₄^2- L^-1 d^-1 at cathode potentials from -1.0 to -1.2 V vs. SHE. Also, the highest cathodic recovery and the highest sulfur species imbalance were observed at a cathode potential of -1.0 V vs. SHE, which indicated a higher elemental sulfur production.

Hydrogen sulfide emission sources, regulations, and removal techniques: a review

This review highlights the recent technologies of H2S removal from wastewater in the petroleum refinery. H2S is a harmful, putrid, and hazardous gaseous compound. The main processes such as physicochemical, chemical, biological, and electrochemical methods were compared and discussed in detail. The effects of various parameters and adsorbent characteristics were highlighted and correlated with the adsorption capacities. Surface functional groups and porosity surface area play a crucial role in the process of single-phase and composite adsorbents. Composite materials impregnated with some metals showed high removal efficiencies. It was found that the adsorption process is the most relevant way for H2S removal due to its high removal efficiency, low cost, eco-friendly, and operational simplicity. This study serves as a useful guideline for those who are interested in H2S removal.

Electrochemical oxidation of iron and alkalinity generation for efficient sulfide control in sewers

The addition of iron salts is one of the most commonly used dosing strategies for sulfide control in sewers. However, iron salts decrease the sewage pH which not only reduces the effectiveness of sulfide precipitation but also enhances the release of residual sulfide to the sewer atmosphere. Equally important, concentrated iron salt solutions are corrosive and their frequent transport, handling, and on-site storage often come with Occupational Health and Safety (OH&S) concerns. Here, we experimentally demonstrated a novel sulfide control approach using electrochemical systems with parallel placed iron electrodes. This enabled combining anodic dissolved iron species release with cathodic hydroxyl anion production, which alleviates all the aforementioned concerns. A long-term experiment was successfully carried out achieving an average sulfide removal efficiency of 95.4 ± 4.4% at low voltage input of 2.90 ± 0.54 V over the course of 8 weeks. This electrochemical method was demonstrated to successfully achieve efficient sulfide control. In addition, it increases the sewage pH, thereby overcoming the drawbacks associated with the pH decrease in the case of conventional iron salt dosing. Ferrous ions were produced at an overall coulombic efficiency (CE) of 98.2 ± 1.2%, whereas oxygen evolution and direct sulfide oxidation were not observed. Short-term experiments showed that increasing either inter-electrode gap or current density increased the cell voltage associated with the increase in the ohmic drop of the system. Overall, this study highlights the practical potential of in-situ generation of dissolved iron species and simultaneous hydroxyl anion generation for efficient sulfide control in sewers.

Bioelectrochemical approaches for removal of sulfate, hydrocarbon and salinity from produced water

Produced water (PW) is the largest liquid waste stream generated during the exploration and drilling process of both the conventional hydrocarbon based resources like crude oil and natural gas, as well as the new fossil resources like shale gas and coal bed methane. Resource management, efficient utilization of the water resources, and water purification protocols are the conventionally used treatment methods applied to either treat or utilize the generated PW. This review provides a comprehensive overview of these conventional PW treatment strategies with special emphasises on electrochemical treatment. Key considerations associated with these approaches for efficient treatment of PW are also discussed. After a thorough assessment of the salient features of these treatment platforms, we propose a new strategy of uniquely integrating bioelectrochemical processes with biological system for more effective PW treatment and management.

Electrochemical sulfide removal and caustic recovery from spent caustic streams

Spent caustic streams (SCS) are produced during alkaline scrubbing of sulfide containing sour gases. Conventional methods mainly involve considerable chemical dosing or energy expenditures entailing high cost but limited benefits. Here we propose an electrochemical treatment approach involving anodic sulfide oxidation preferentially to sulfur coupled to cathodic caustic recovery using a two-compartment electrochemical system. Batch experiments showed sulfide removal efficiencies of 84 ± 4% with concomitant 57 ± 4% efficient caustic production in the catholyte at a final concentration of 6.4 ± 0.1 wt% NaOH (1.6 M) at an applied current density of 100 A m(-2). Subsequent long-term continuous experiments showed that stable cell voltages (i.e. 2.7 ± 0.1 V) as well as constant sulfide removal efficiencies of 67 ± 5% at a loading rate of 47 g(S) L(-1) h(-1) were achieved over a period of 77 days. Caustic was produced at industrially relevant strengths for scrubbing (i.e. 5.1 ± 0.9 wt% NaOH) at current efficiencies of 96 ± 2%. Current density between 0 and 200 A m(-2) and sulfide loading rates of 50-200 g(S) L(-1) d(-1) were tested. The higher the current density the more oxidized the sulfur species produced and the higher the sulfide oxidation. On the contrary, high loading rate resulted in a reduction of sulfide oxidation efficiency. The results obtained in this study together with engineering calculations show that the proposed process could represent a cost-effective approach for sodium and sulfur recovery from SCS.

Use of bauxite residue (red mud) as a low cost sorbent for sulfide removal in polluted water remediation

Sulfide is an important pollutant in aqueous systems. Sulfide removal from polluted waters is required prior to discharge. Red mud (RM) is a solid waste of bauxite processing that is rich in reactive iron oxides and consequently has the potential to be used to remove sulfide from aqueous systems. A series of experiments was undertaken using raw and sintered RM to remove sulfide from waters. RM was highly efficient at sulfide removal (average 75% sulfide removal at initial concentration of ∼5 mg L(-1), with 500 mg L(-1) RM addition) due to both physical adsorption (high specific area) and chemical reaction (with amorphous Fe). Sintered RM, which has a lower surface area and lower mineral reactivity, was much less efficient at removing sulfide (∼20% removal under equivalent experimental conditions). Furthermore, concomitant metal release from raw RM was lower than for sintered RM during the sulfide removal process. The results showed that raw RM is a potentially suitable material for sulfide removal from polluted waters and consequently could be used as a low cost alternative treatment in certain engineering applications.

Effect of cathode electron acceptors on simultaneous anaerobic sulfide and nitrate removal in microbial fuel cell

The current investigation reports the effect of cathode electron acceptors on simultaneous sulfide and nitrate removal in two-chamber microbial fuel cells (MFCs). Potassium permanganate and potassium ferricyanide were common cathode electron acceptors and evaluated for substrate removal and electricity generation. The abiotic MFCs produced electricity through spontaneous electrochemical oxidation of sulfide. In comparison with abiotic MFC, the biotic MFC showed better ability for simultaneous nitrate and sulfide removal along with electricity generation. Keeping external resistance of 1,000 Ω, both MFCs showed good capacities for substrate removal where nitrogen and sulfate were the main end products. The steady voltage with potassium permanganate electrodes was nearly twice that of with potassium ferricyanide. Cyclic voltammetry curves confirmed that the potassium permanganate had higher catalytic activity than potassium ferricyanide. The potassium permanganate may be a suitable choice as cathode electron acceptor for enhanced electricity generation during simultaneous treatment of sulfide and nitrate in MFCs.

Using sulfite pretreatment to improve the biodegradability of waste activated sludge

Significant improvement (∼51%) in biodegradability of waste activated sludge with sulfite pretreatment.

Electrochemical removal of sulfate from petroleum produced water

Petroleum produced water (PPW) is a waste-stream that entails huge cost on the petroleum industry. Along with other suspended and dissolved solids, it contains sulfate, which is a major hurdle for its alternative use intended toward enhanced oil recovery. This study proposes a two-step process for sulfate removal from PPW. A synthetic PPW was designed for the study using response surface methodology. During the first step, sulfate present in PPW was reduced to sulfide by anaerobic fermentation with 80% efficiency. In the second step, more than 70% of the accumulated sulfide was electrochemically oxidized. This integrated approach successfully removed sulfate from the synthetic wastewater indicating its applicability in the treatment of PPW and its subsequent applications in other oil field operations.

A novel electrochemical process for the recovery and recycling of ferric chloride from precipitation sludge

During wastewater treatment and drinking water production, significant amounts of ferric sludge (comprising ferric oxy-hydroxides and FePO4) are generated that require disposal. This practice has a major impact on the overall treatment cost as a result of both chemical addition and the disposal of the generated chemical sludge. Iron sulfide (FeS) precipitation via sulfide addition to ferric phosphate (FePO4) sludge has been proven as an effective process for phosphate recovery. In turn, iron and sulfide could potentially be recovered from the FeS sludge, and recycled back to the process. In this work, a novel process was investigated at lab scale for the recovery of soluble iron and sulfide from FeS sludge. Soluble iron is regenerated electrochemically at a graphite anode, while sulfide is recovered at the cathode of the same electrochemical cell. Up to 60 ± 18% soluble Fe and 46 ± 11% sulfide were recovered on graphite granules for up-stream reuse. Peak current densities of 9.5 ± 4.2 A m(-2) and minimum power requirements of 2.4 ± 0.5 kWh kg Fe(-1) were reached with real full strength FeS suspensions. Multiple consecutive runs of the electrochemical process were performed, leading to the successful demonstration of an integrated process, comprising FeS formation/separation and ferric/sulfide electrochemical regeneration.

Simultaneous sulfide removal and electricity generation with corn stover biomass as co-substrate in microbial fuel cells

Microbial fuel cells (MFCs), representing a promising method to treat combined pollutants with energy recovery, were utilized to remove sulfide and recover power with corn stover filtrate (CSF) as the co-substrate in present study. A maximum power density of 744 mW/m(2) was achieved with sulfide removal of 91% during 72 h operation when the CSF concentrations (mg-COD/l) and the electrolyte conductivity were set at 800 mg/l and 10.06 mS/cm, respectively, while almost 52% COD was removed due to the microbial degradation of CSF to the volatile organic carbons. CSF concentrations and electrolyte conductivities had significant effects on the performance of the MFCs. Simultaneous removals of inorganic pollutant and complex organic compounds with electricity generation in MFCs are reported for the first time. These results provide a good reference for multiple contaminations treatment especially sulfide containing wastewaters based on the MFC technology.

Precipitation of dissolved sulphide in pulp and paper mill wastewater by electrocoagulation

The precipitation of dissolved sulphide ions by electrocoagulation was studied at laboratory scale using pulp and paper mill wastewaters. Concentrations of dissolved organic carbon and phosphorus were analysed before and after the electrocoagulation process to examine the suitability of the process for treatment of sulphide odour from pulp and paper mill wastewater. The electrochemical cell used in this study was constructed from monopolar dissolving iron electrodes. The dissolved iron concentration was directly proportional to the applied electric charge (C/L) at the tested current densities. Electrochemically produced ferrous iron (Fe2+) precipitated dissolved sulphide ions efficiently. Electricity consumption of the treatment was 4-8 C/mg S(2-) while iron consumption was 1.1-2.2 mg/mg S(2-) during the initial phase of the sulphide precipitation when the applied electric charge was 10-60 C/L. When 60 C/L was applied, 88% of dissolved sulphides and 40% of phosphorus was precipitated. The reduction in DOC was low during the sulphide precipitation. According to these results, electrocoagulation can precipitate dissolved sulphides effectively and thereby reduce sulphide odours of pulp and paper mill wastewaters.