Effects of sulfur dosage on continuous bioleaching of heavy metals from contaminated sediment.

The bioleaching technology has been considered as a promising green technology for remediation of contaminated sediments in recent years. Bioleaching technology was generally conducted in the batch bioreactor; however, the continuous bioreactor should be developed for the application of bioleaching technology in the future. The purposes of this study were to establish a continuous bioleaching process, and to evaluate the effects of sulfur dosage on the efficiency of metal removal during this continuous bioleaching process. The obtained results show that the pH decrease, sulfate production and metal removal efficiency all increased with increasing sulfur dosage in the continuous bioleaching process due to high substrate concentration for sulfur-oxidizing bacteria. After 30 days of operation time, the maximum solubilization efficiencies for Zn, Ni, Cu and Cr were found to be 78%, 90%, 88% and 68%, respectively, at 5% of sulfur dosage. After the bioleaching process, heavy metals bound in the carbonates, Fe-Mn oxides and organics/sulfides in the sediment were effectively removed and the potential ecological and toxic risks of treated sediment were greatly reduced. The results of bacterial community analyses demonstrated that this continuous bioleaching process were dominated by several acidophilic sulfur-oxidizing bacteria; S. thermosulfidooxidans, At. thiooxidans/At. ferrooxidans, S. thermotolerans and At. albertensis, whereas the percentage of less-acidophilic sulfur-oxidizing bacteria (T. thioparus and T. cuprina) was lower than 15% of total bacteria. In addition, the cell numbers of sulfur-oxidizing bacteria increased as the sulfur dosage was increased in the continuous bioleaching process.

Heterologous expression of bacterial CotA-laccase, characterization and its application for biodegradation of malachite green.

Malachite green (MG) is used as fungicide/parasiticide in aquaculture, its persistence is detrimental as it exhibits carcinogenic effects to aquatic organisms. Bacterial laccase evaluated as the best enzyme at extreme condition for aquatic MG removal. Study aims to increase laccase concentration, CotA-laccase from Bacillus subtilis was cloned and overexpressed in Escherichia coli. Optimal catalysis for purified CotA-laccase were at pH 5.0, 60 °C, and 1 mM of (2,2-azino-di-[3-ethylbenzothiazoline-sulphonate-(6)]) with Km and Kcat 0.087 mM and 37.64 S-1 respectively. MG biodegradation by CotA-laccase in clam and tilapia pond wastewaters and cytotoxic effect of biodegraded products in grouper fin-1 cells were determined. MG degradation by CotA-laccase was equally efficient, exhibiting upto 90-94% decolorization at freshwater and saline conditions and treated solution was non-toxic to GF-1 cells. Thus, recombinant-CotA-laccase could be an environmentally-friendly enzyme for aquaculture to remove MG, thereby effective to reduce its accumulation in aquatic organisms and ensuring safe aquaculture products.

Effects of sulfur dosage and inoculum size on pilot-scale thermophilic bioleaching of heavy metals from sewage sludge.

Land application of sewage sludge has received significant attention in recent years but the presence of elevated heavy metals in the sludge limits its land application. The purposes of this study were to investigate the effects of sulfur dosage and inoculum size on the thermophilic bioleaching of heavy metals from sewage sludge in a pilot-scale bioreactor. The microbial communities in this thermophilic bioleaching process were also identified using real-time polymerase chain reaction (real-time PCR). The results showed that the oxidation of sulfur and metal solubilization decreased with the increasing sulfur dosage. When the sulfur dosage was greater than 2% (w/v), the sulfur oxidation and metal solubilization rates decreased, indicating that the thermophilic bioleaching was hindered by high levels of substrate. However, it was found that the efficiency of metal solubilization and solid degradation was increased with the increase of inoculum size in the range from 5% to 20%. At the end of bioleaching, the efficiency of Mn, Zn, Ni, Cu and Cr from the sewage sludge reached 73-100%, 51-60%, 38-52%, 17-43% and 1-38%, respectively, while SS and VSS were degraded by 33-48% and 47-67%, respectively. Based on the analysis of real-time PCR, Sulfobacillus acidophilus was observed to be the predominant species (13-67% of total bacteria), whereas the populations of Sulfobacillus thermosulfidooxidans and Acidithiobacillus caldus were accounted relatively low (<1%).

Bioleaching of heavy metals from harbor sediment using sulfur-oxidizing microflora acclimated from native sediment and exogenous soil.

The harbor sediment containing high concentration of heavy metals may pose serious impacts on the marine ecosystem and environmental quality. The bioleaching process has been considered as an environmentally friendly and cost-effective alternative for removing heavy metals from contaminated sediments. In this study, a series of experiments were performed to investigate the feasibility of bioleaching process for removing heavy metals from the contaminated harbor sediments. The performance of the bioleaching process inoculated with sulfur-oxidizing microflora acclimated from the native harbor sediment was compared with that acclimated from the exogenous soil. In the bioleaching experiment with inoculants from native sediment, the efficiency of Zn, Cu, Cr, Pb, and Ni (30 days) reached 39-100%, 21-94%, 8-63%, 5-74%, and 19-77%, respectively. While 59-100% of Zn, 22-100% of Cu, 0-95% of Cr, 0-100% of Pb, and 22-100% of Ni were respectively removed in the bioleaching experiment with inoculants from exogenous soil after 30 days of reaction time. The results show that the rate and efficiency of metal removal in the bioleaching process decreased with an increase of sediment solid content from 10 to 40 g/L. The efficiency of metal removal in the bioleaching process with inoculants from the native sediment was lower than those from the exogenous soil due to the bacterial activity. By the fractionation of metal in the harbor sediment, exchangeable, carbonate-bound, and Fe/Mn oxide-bound metals (mobile fractions) were found to be apparently reduced and even organic matter/sulfide-bound and residual metals (stable fractions) were slightly removed after the bioleaching experiment.

Bioregeneration of spent mercury bearing sulfur-impregnated activated carbon adsorbent.

Among various adsorbents studied, sulfur-impregnated activated carbon is one of the most promising adsorbents for mercury removal from flue gas. However, a large amount of spent activated carbons containing high content of mercury are generated after adsorption. To make the adsorption a more viable option, the regeneration and reuse of the spent activated carbon should be considered. The purpose of this study is to develop a novel technique for bioregeneration of sulfur-impregnated activated carbons after adsorption of mercury from flue gases by sulfur-oxidizing bacteria. The optimal operating parameters for this bioregeneration process were studied using central composite design (CCD) and response surface methodology (RSM). Results showed that the sulfur oxidation rate was increased with increasing activated carbon dosage. Furthermore, the increase of inoculum size only caused a slight increase of sulfur oxidation rate in the bioregeneration. The maximum mercury removal efficiency of more than 50% was obtained at 10% (w/v) activated carbon dosage and 20% (v/v) inoculum size. After the bioregeneration process, Brunauer-Emmett-Teller (BET) surface area and micropore volume of spent activated carbon increased due to the bio-oxidation of mercury bearing sulfur on the surface of activated carbons.

Relationship between microbial community dynamics and process performance during thermophilic sludge bioleaching.

Heavy metals can be removed from the sludge using bioleaching technologies at thermophilic condition, thereby providing an option for biotreatment of wasted sludge generated from wastewater treatment. The purposes of this study were to establish a molecular biology technique, real-time PCR, for the detection and enumeration of the sulfur-oxidizing bacteria during the thermophilic sludge bioleaching. The 16S rRNA gene for real-time PCR quantification targeted the bioleaching bacteria: Sulfobacillus thermosulfidooxidans, Sulfobacillus acidophilus, and Acidithiobacillus caldus. The specificity and stringency for thermophilic sulfur-oxidizing bacteria were tested before the experiments of monitoring the bacterial community, bacterial number during the thermophilic sludge bioleaching and the future application on testing various environmental samples. The results showed that S. acidophilus was identified as the dominant sulfur-oxidizing bacteria, while A. caldus and S. thermosulfidooxidans occurred in relatively low numbers. The total number of the sulfur-oxidizing bacteria increased during the thermophilic bioleaching process. Meanwhile, the decrease of pH, production of sulfate, degradation of SS/VSS, and solubilization of heavy metal were found to correlate well with the population of thermophilic sulfur-oxidizing bacteria during the bioleaching process. The real-time PCR used in this study is a suitable method to monitor numbers of thermophilic sulfur-oxidizing bacteria during the bioleaching process.

Biodegradation of tetramethylammonium hydroxide (TMAH) in completely autotrophic nitrogen removal over nitrite (CANON) process.

This study conducted a completely autotrophic nitrogen removal over nitrite (CANON) process in a continuous anoxic upflow bioreactor to treat synthetic wastewater with TMAH (tetramethylammonium hydroxide) ranging from 200 to 1000mg/L. The intermediates were analyzed for understanding the metabolic pathway of TMAH biodegradation in CANON process. In addition, (15)N-labeled TMAH was used as the substrate in a batch anoxic bioreactor to confirm that TMAH was converted to nitrogen gas in CANON process. The results indicated that TMAH was almost completely biodegraded in CANON system at different influent TMAH concentrations of 200, 500, and 1000mg/L. The average removal efficiencies of total nitrogen were higher than 90% during the experiments. Trimethylamine (TMA) and methylamine (MA) were found to be the main biodegradation intermediates of TMAH in CANON process. The production of nitrogen gas with (15)N-labeled during the batch anaerobic bioreactor indicated that CANON process successfully converted TMAH into nitrogen gas.

Adsorption of fluoride to UiO-66-NH2 in water: Stability, kinetic, isotherm and thermodynamic studies.

To provide safe drinking water, fluoride in water must be removed and adsorption processes appear to be the most widely used method. Metal organic frameworks (MOFs) represent a new class of adsorbents that have been used in various adsorption applications. To study the adsorption mechanism of fluoride to MOFs in water and obtain related adsorption parameters, we synthesized a zirconium-based MOF with a primary amine group on its ligand, named UiO-66-NH2. The kinetics, adsorption isotherm and thermodynamics of fluoride adsorption to UiO-66-NH2 were investigated. The crystalline structure of UiO-66-NH2 remained intact and the local structure of zirconium in UiO-66-NH2 did not change significantly after being exposed to fluoride. The kinetics of the fluoride adsorption in UiO-66-NH2 could be well represented by the pseudo second order rate law. The enthalpy of the adsorption indicates that the F(-) adsorption to UiO-66-NH2 was classified as a physical adsorption. However, the comparison between the adsorption capacities of UiO-66-NH2 and UiO-66 suggests that the fluoride adsorption to UiO-66-NH2 might primarily involve a strong interaction between F(-) and the metal site. The fluoride adsorption capacity of UiO-66-NH2 was found to decrease when pH>7. While the presence of chloride/bromide ions did not noticeably change the adsorption capacity of UiO-66-NH2, the ionic surfactants slightly affected the adsorption capacity of UiO-66-NH2. These findings provide insights to further optimize the adsorption process for removal of fluoride using zirconium-based MOFs.

Removal of oil droplets from water using carbonized rice husk: enhancement by surface modification using polyethylenimine.

Carbonized rice husk (CRH) is a promising material to separate oil from water owing to its abundance, low-cost, and environmentally benign characteristics. However, CRH's performance is somewhat limited by its similar surface charge to that of oil, leading to repulsive interactions. To improve the separation efficiency of CRH, CRH was modified via impregnation with a cationic biocompatible polymer, polyethlyenimine (PEI) to form PEI-CRH. The modified sample exhibits a remarkably higher (10-50 times) oil/water (O/W) separation efficiency than that of the unmodified one. Small PEI-CRH particles (about 64 µm) are found to adsorb oil droplets faster and larger quantities than bigger particles (about 113 and 288 µm). PEI-CRH exhibits higher separation efficiency at high temperatures owing to the destabilization of the emulsion. It is also found that the oil adsorption mechanism involves a chemical interaction between PEI-CRH and oil droplets. The addition of NaCl considerably improves the separation efficiency, while the addition of a cationic surfactant has the opposite effect. In acidic emulsions, PEI-CRH adsorbs more oil than in neutral or basic conditions owing to favorable attractive forces between oil droplets and the surface of PEI-CRH. PEI-CRH can be easily regenerated by washing with ethanol. These promising features of PEI-CRH indicate that PEI-CRH could be an efficient and low-cost adsorbent for the O/W separation applications.

Thermophilic bioleaching of heavy metals from waste sludge using response surface methodology.

The bioleaching process is considered to be more efficient and environmentally friendly than conventional technologies for removal of heavy metals from waste sludge. The objective of this study was to develop an optimal thermophilic bioleaching process for the treatment of waste sludge containing high concentrations of heavy metals. In this study, two operating parameters, sludge solid content and sulfur (substrate) concentration, were studied based on a central composite design (CCD) for their metal solubilization and solid degradation performances. The optimal bioleaching operation conditions were then determined using the response surface methodology (RSM). The results indicated that an increase in sludge solid content range from 0.5% to 5.0% resulted in a decrease in the pH reduction rate due to the increase in buffering capacity. The rate of acidification corresponded to sulfur concentration until sulfur itself became inhibitory. At sulfur concentration beyond approximately 2.75%, the lower acidification rate was caused by a lower bacteria growth rate. Similar trends were also observed in the variations of ORP and sulfate concentrations during this thermophilic bioleaching process. At the optimum conditions of a sludge solid content of 0.5% and sulfur concentration of 2.5%, the thermophilic bioleaching process achieved the maximum solubilization of 97%, 99%, 99% and 78% for Cu, Zn, Ni and Pb, respectively. At the same time, the maximum SS and VSS destruction efficiency were 69% and 63%, respectively.

Simultaneous metal leaching and sludge digestion by thermophilic microorganisms: effect of solids content.

High concentrations of heavy metal in livestock manures limit land application of their sludges. A practical and economical method of sludge treatment is important for converting the livestock sludge into soil conditioners or fertilizers. In this study, the effect of solid contents on the simultaneous aerobic digestion and metal leaching at thermophilic condition were investigated in a batch reactor. Different solid contents in the range of 0.5-4% (dry-w/v) were studied. The results showed that an increase of solid content decreased the pH reducing rate. It was the result of increase in buffering capacity and possible microbial inhibition at a higher solid content. Similar results were also found in the variations of ORP and sulfate concentrations during this process. In most cases, this biological process is able to solubilize 82-99% of heavy metals from the livestock sludge. It was found that the efficiency and rate of metal solubilization decreased with increasing solid contents. In addition, 54-80% of organic matter in the sludge was degraded after 28 days of reaction. A low sludge digestion efficiency was found at a high solid content. Moreover, the dewaterability of sludge was improved and the fertility (N, P and K) of sludge did not change significantly after this bioprocess.

Enhancement of metal bioleaching from contaminated sediment using silver ion.

A silver-catalyzed bioleaching process was used to remove heavy metals from contaminated sediment in this study. The effects of silver concentration added on the performance of bioleaching process were investigated. High pH reduction rate was observed in the bioleaching process with silver ion. The silver ion added in the bioleaching process was incorporated into the lattice of the initial sulfide through a cationic interchange reaction. This resulted in the short lag phase and high metal solubilization in the bioleaching process. The maximum pH reduction rate and the ideal metal solubilization were obtained in the presence of 30 mg/L of silver ion. When the added silver ion was greater than 30 mg/L, the rates of pH reduction and metal solubilization gradually decreased. The solubilization efficiencies of heavy metals (Cu, Zn, Mn, Ni and Cr) were relatively high in the silver-enhanced bioleaching process, except Pb. No apparent effect of silver ion on the growth of sulfur-oxidizing bacteria was found in the bioleaching. These results indicate that the kinetics of metal solubilization can be enhanced by the addition of silver ion.

Variations of metal distribution in sewage sludge composting.

In the study, the variations of heavy metal distributions (of Cu, Mn, Pb, and Zn) during the sewage sludge composting process were investigated by sequential extraction procedures. The total content of Cu and Zn in the composted mixture increased after the composting process. Mn and Zn were mainly found in mobile fractions (exchangeable fraction (F1), carbonate fraction (F2), and Fe/Mn oxide fraction (F3)). Cu and Pb were strongly associated with the stable fractions (organic matter/sulfides fraction (F4) and residual fraction (F5)). These five metal fractions were used to calculate the metal mobility (bioavailability) in the sewage sludge and composted mixture. The mobility (bioavailability) of Mn, Pb, and Zn (but not Cu) increased during the composting process. The metal mobility in the composted mixture ranked in the following order: Mn>Zn>Pb>Cu.

Bioleaching of heavy metals from contaminated sediment by indigenous sulfur-oxidizing bacteria in an air-lift bioreactor: effects of sulfur concentration.

The effects of sulfur concentration on the bioleaching of heavy metals from the sediment by indigenous sulfur-oxidizing bacteria were investigated in an air-lift reactor. Increasing the sulfur concentration from 0.5 to 5 g/l enhanced the rates of pH reduction, sulfate production and metal solubilization. A Michaelis-Menten type equation was used to explain the relationships between sulfur concentration, sulfate production and metal solubilization in the bioleaching process. After 8 days of bioleaching, 97-99% of Cu, 96-98% of Zn, 62-68% of Mn, 73-87% of Ni and 31-50% of Pb were solubilized from the sediment, respectively. The efficiency of metal solubilization was found to be related to the speciation of metal in the sediment. From economical consideration, the recommended sulfur dosage for the bioleaching of metals from the sediment is 3g/l.

Bioleaching of heavy metals from livestock sludge by indigenous sulfur-oxidizing bacteria: effects of sludge solids concentration.

A technologically and economically feasible process called bioleaching was used for the removal of heavy metals from livestock sludge with indigenous sulfur-oxidizing bacteria in this study. The effects of sludge solids concentration on the bioleaching process were examined in a batch bioreactor. Due to the buffering capacity of sludge solids, the rates of pH reduction, ORP rise and metal solubilization were reduced with the increase of the solids concentration. No apparent influence of solids concentration on sulfate produced by sulfur-oxidizing bacteria was observed when the solids concentration was less than 4% (w/v). A Michaelis-Menten type of equation was able to well describe the relationship between solids concentration and rate of metal solubilization. Besides, high efficiencies of metal solubilization were achieved after 16 d of bioleaching. Therefore, the bioleaching process used in this study could be applied to remove heavy metals effectively from the livestock sludge.

Assessment of recoverable forms of sulfur particles used in bioleaching of contaminated sediments.

The use of recoverable sulfur particles will enhance the feasibility and reduce the cost of bioleaching process. Three different forms of sulfur particles, powder, pastilles and pellets were used to study the utilization and recovery of sulfur, used as energy source for thiobacilli in the bioleaching process. The Langmuir isotherm was used to explain the adsorption equilibrium existing between the sorbed and suspended bacteria and the maximum adsorption capacity obtained from the Langmuir isotherm was utilized to determine the specific surface area of the sulfur particles. The specific surface area of sulfur particles was found to be the determining factor in the bioleaching process and not the particle size. The rates of pH reduction, sulfate production and metal solubilization increased with increasing specific surface area of the particles. The pH reduction and metal solubilization were significantly enhanced by the reuse of recovered sulfur particles. The efficiency of metal solubilization with recovered sulfur pastilles was comparable to that with sulfur powder. This study revealed the practicability of reusing the recovered sulfur pastilles in the bioleaching process.