Removal of trace metals and improvement of dredged sediment dewaterability by bioleaching combined with Fenton-like reaction

Enhancement of sludge dewaterability using combined technology of bioleaching and Fenton: Microscopic structure and hydrophilic/hydrophobic properties of sludge particles

Bioleaching and Fenton technology are commonly used preconditioning techniques for sludge dewatering. This study compared the dewatering mechanisms of different conditioning technologies. The results showed that bound water, specific resistance to filtration (SRF), and capillary suction time decreased from 3.95 g/g, 6.16 × 1012 m/kg, and 130.6 s to 3.15 g/g, 2.81 × 1011 m/kg, and 33 s, respectively, under combined treatment condition. Moreover, the free radicals, including ·OH, O2-·and Fe (Ⅳ), further damaged the cell structure, thus increasing the concentration of DNA in the S-EPS layer. This intense degradation sludge particle size decreased by 15.6% and significantly increased zeta potential. Under the combined technology, the α-helix and β-sheet decreased by 42.2% and 56.5%, respectively, destabilizing the spatial structure of proteins and promoting the release of bound water. In addition, the combined technology decreased (Ala/Lys) ratio in the TB-EPS layer by 67.6%, indicating the weakening of protein water-holding capacity. Moreover, the conversion of oxygen-containing compounds to nonpolar hydrocarbons increased the hydrophobicity of the sludge under a combined treatment, thus enhancing dewatering performance.

Metal bioleaching from printed circuit boards by bio-Fenton process: Optimization and prediction by response surface methodology and artificial intelligence models

Recycling printed circuit boards (PCBs) in the e-waste stream is essential for ecological protection and metal recycling for a circular economy. Efficient metal recovery from PCBs is highly dependent on the determination of the optimum combination of inputs in the recycling process. In this study, optimization and predictive modelling of the bio-Fenton process were performed employing the response surface methodology (RSM) and the artificial intelligence (AI) models for efficient enzymatic metal bioleaching from discarded cellphone PCBs. The Box-Behnken design (BBD) of RSM was chosen as the design matrix. Further, two AI models, i.e., support vector machine (SVM) and artificial neural network (ANN) were employed to predict complex metal bioleaching process. Experiments were performed based on variations of four input process parameters, namely, glucose oxidase (GOx) content (100-1000 U/L), Fe²⁺ content (10-50 mM), PCB pulp density (1-10 g/L), and shaking speed (150-450 rpm). Results revealed that the maximum simultaneous enzymatic metal extraction of 100% Cu, 70% Ni, 40% Pb, and 100% Zn was attained at the optimized conditions: GOx content: 300 U/L, Fe²⁺ content: 10 mM, pulp density: 1 g/L, and shaking speed: 335 rpm. A comparative analysis of the AI models suggested that the ANN-based model predicting more accurate results than the SVM-based model with coefficient of determination values > 0.99 for all the targeted metals. The FTIR analysis confirmed the partial disintegration of PCB polymeric base by OH radicals (OH•), which helped in liberating and exposing the embedded metals to the bio-Fenton solution. Further, the oxidation of metals by ferric ions produced from GOx-mediated oxidation of ferrous ions ensued efficient enzymatic metal bioleaching. Selective metal recovery of >99% was obtained by the chemical precipitation of bioleachate.

Resource Utilization of Lake Sediment to Prepare “Sponge” Light Aggregate: Pore Structure and Water Retention Mechanism Study

Nitrogen, phosphorus, and metals’ pollutants discharged from industrial sources eventually accumulate in lake sediment, hence increasing the difficulty of sediment treatment and disposal. In this work, the water storage ceramsite is prepared from dredged lake sediment and cyano-bacterial powder. The effects of pyrolysis temperature and cyanobacterial sediment on the porosity of ceramsite were investigated. The results showed that the pyrolysis of organic matter and the de-composition of compounds or salts can produce gas, causing a rich pore structure inside the ceramsite. When the temperature increased to 1150 °C, vitrification would collapse the pore structure inside the material. At the cyanobacterial-to-sediment ratio of 3:7, the porosity and water absorption of the material could reach 81.82% and 92.45% when the pyrolysis temperature was 500 and 1050 °C, respectively. The internal macropore structure of ceramsite improved the water absorption performance, and the mesoporous structure was responsible for its long water release time and stable water release structure. The ceramsite exhibited a superior metals’ retention effect. Under different pH and temperature conditions, the consolidation rates of Fe, Ni, Mn, Cr, and Pb in ceramsite were all more than 99%, suggesting the safety of the material in environmental applications. This study demonstrates the feasibility of the resourceful production of water storage ceramsite from lake sediment and cyanobacterial slurry, which helps to reduce the impact of solid waste on the environment. Thus, this work provides a practical basis for guiding water storage ceramsite in the construction of sponge cities.

Species Diversity of Mycoplankton on the Background of Selected Indicators of Water Quality in Stratified Mesotrophic Lakes

The aim of the study was to determine mycoplankton species diversity in relation to the physico-chemical parameters of lake waters. The research was carried out in the summer months in 15 mesotrophic lakes and showed a high ecological significance index for Rhodotorula glutinis, Epicoccum nigrum, Fusarium sporotrichioides, and Trichophyton violaceum. Mycoplankton abundance and species diversity decreased with the depth of water, which coincided with a decrease in oxygen content and organic matter concentration. A high concentration of nitrogen compounds (total nitrogen-TN and dissolved nitrogen-DN) limited the development of mycobiota in the hypolimnion. In the metalimnion, the intensive development of organisms, especially bacteria, limited mycoplankton abundance despite perfect physical and chemical conditions for its development. Finally, mycoplankton functioned the best in slightly alkaline waters.

Heavy Metal Pollution and Health Risk Assessment of Vegetable-Soil Systems of Facilities Irrigated with Wastewater in Northern China

Soil pollution by heavy metals is a major concern in China and has received much attention in recent years. Aiming to investigate the status of heavy metal pollution and the safety of vegetables in the soil of wastewater-irrigated facilities, this study investigated the distribution and migration characteristics of heavy metals in vegetable−soil systems of facilities in a typical sewage irrigation area of the Xi River, Shenyang City, northern China. Health risks due to the fact of exposure to heavy metals in the vegetable soil of facilities and ingrown vegetables through different exposure pathways were evaluated. Spatial interpolation and a potential ecological risk assessment were applied to evaluate the soil quality. Bioaccumulation factors (BCFs) were used to analyze the absorption and transportation capacity of Cd, Cu, Pb, and Zn by different parts of different vegetables. The results showed that the average concentration of Cd exceeded the standard values by 1.82 times and accumulated by 11 times, suggesting that Cd poses the most severe pollution among the four metals in the soil of facilities in the Xi River sewage irrigation area. In the city, a significant accumulation of Cd in the soil was identified with different spatial distributions. Cd also contributed the most in terms of the estimated potential ecological risk index, while the impacts of the other three metals were relatively small. The concentrations of heavy metals were mostly lower than the limit set by the corresponding Chinese standards. Various BCFs were observed for the four metals in the order Cd > Zn > Cu > Pb. Vegetables also demonstrated different BCFs in the order of leaf vegetables > Rhizome vegetable > Solanaceae vegetable. The magnitude of the noncarcinogenic risk for all four heavy metals was less than one for all three exposure routes and did not cause significant noncarcinogenic health effects in humans. However, the carcinogenic risk of Cd from some vegetables via dietary intake was considered higher. Protection measures should be taken to implement better pollution control and land use planning.

Application of Advanced Oxidation Technology in Sludge Conditioning and Dewatering: A Critical Review

Sludge dewatering is an important link in sludge treatment. In practical engineering, the dewatering effect of unconditioned sludge is very poor. The use of advanced oxidation technology can improve sludge dewatering performance, reduce sludge capacity, and remove micro-pollutants, which is beneficial for sludge post-treatment and disposal. Based on the current status of sludge conditioning and dehydration, the characteristics of the advanced oxidation method for sludge dehydration were systematically explained using various free radical reaction mechanisms and dehydration conditions. The effects of various advanced oxidation technologies on sludge conditioning and dewatering has been extensively discussed. Finally, the application prospects of the advanced oxidation technology in sludge conditioning and dewatering are presented.

Role of hypochlorite in the harmless treatment of cyanide tailings through slurry electrolysis

The efficient and harmless treatment of cyanide tailings is necessary for gold extraction processes. The present study reports the effects of ClO^- generation in a slurry electrolysis system containing NaCl on the removal rate of cyanide and heavy metal ions in cyanide tailings. The chemical dissolution of metallic minerals and the reaction mechanisms were investigated by Fourier-transform infrared (FT-IR) and X-ray diffraction (XRD) analyses. The obtained results evidenced the key role of ClO^- in the removal of cyanide and heavy metal ions through the slurry electrolysis system with NaCl addition. Furthermore, ClO^- formation increased with the NaCl concentration, resulting in a higher removal rate of cyanide and heavy metal ions and enhanced metallic mineral dissolution. The cyanide tailings showed the best harmless effect with a NaCl concentration of 15 g/L. With this condition, the removal rates of CN_T, CN^-, Cu, Zn and Fe were 96.15%, 98.34%, 98.62%, 99.32% and 79.31%, respectively; furthermore, Cu and Fe precipitated at the cathode. The relative hematite content decreased by 3.12%. Under the effect of an electric field, the cyanide and metal cyanide complexes in the cyanide tailings oxidised and decomposed to release metal cations by the strongly oxidising ClO^- generated at the anode. The metal cations and hematite were reduced at the cathode, while the metal oxide mineral hematite in the electrolyte underwent chemical dissolution. In the toxic degradation of cyanide tailings, the comprehensive recovery of metals and destruction of metallic minerals in tailings will provide favourable conditions for subsequent comprehensive utilisation.

Fungi Can Be More Effective than Bacteria for the Bioremediation of Marine Sediments Highly Contaminated with Heavy Metals

The contamination of coastal marine sediments with heavy metals (HMs) is a widespread phenomenon that requires effective remediation actions. Bioremediation based on the use of bacteria is an economically and environmentally sustainable effective strategy for reducing HM contamination and/or toxicity in marine sediments. However, information on the efficiency of marine-derived fungi for HM decontamination of marine sediments is still largely lacking, despite evidence of the performance of terrestrial fungal strains on other contaminated matrixes (e.g., soils, freshwater sediments, industrial wastes). Here, we carried out for the first time an array of parallel laboratory experiments by using different combinations of chemical and microbial amendments (including acidophilic autotrophic and heterotrophic bacteria, as well as filamentous marine fungi) for the bioremediation of highly HM-contaminated sediments of the Portman Bay (NW Mediterranean Sea), an area largely affected by long-term historical discharges of mine tailings. Our results indicate that the bioleaching performance of metals from the sediment is based on the addition of fungi (Aspergillus niger and Trichoderma sp.), either alone or in combination with autotrophic bacteria, was higher when compared to other treatments. In particular, fungal addition allowed obtaining bioleaching yields for As eight times higher than those by chemical treatments and double compared with the addition of bacteria alone. Moreover, in our study, the fungal addition was the only treatment allowing effective bioleaching of otherwise not mobile fractions of Zn and Cd, thus overtaking bacterial treatments. We found that the lower the sediment pH reached by the experimental conditions, as in the case of fungal addition, the higher the solubilization yield of metals, suggesting that the specific metabolic features of A. niger and Trichoderma sp. enable lowering sediment pH and enhance HM bioleaching. Overall, our findings indicate that fungi can be more effective than acidophilic autotrophic and heterotrophic bacteria in HM bioleaching, and as such, their use can represent a promising and efficient strategy for the bioremediation of marine sediments highly contaminated with heavy metals.

Novel combination of bioleaching and persulfate for the removal of heavy metals from metallurgical industry sludge

The objective of this study was to remove heavy metals from metallurgical industry sludge by bioleaching alone and bioleaching combined with persulfate (PDS). The results showed that the removal of Cu, Zn, Pb, and Mn reached 70%, 83.8%, 25.2%, and 76.9% by bioleaching alone after 18 days, respectively. The experiment of bioleaching combined with PDS was carried out in which the optimal additive dosage of K₂S₂O₈, 8 g/L, was added to bioleaching after 6 d. After 1 h, the removal of four heavy metals reached 75.1, 84.3, 36.7, and 81.6%, respectively. Compared with bioleaching alone, although the increase in removal efficiency was only slightly increased, the treatment cycle was distinctly shortened from 18 to 6 days + 1 h. The scanning electron microscopy (SEM) results showed that the surface morphology of the sludge was changed significantly by the combined treatment. The content of heavy metals was significantly reduced after bioleaching combined with PDS by energy dispersive X-ray spectroscopy (EDX). Through electron paramagnetic resonance (EPR) and free radical quenching experiments, it was indicated that sulfate radicals [Formula: see text] plays a leading role in the combined treatment. The treated sludge mainly existed in a stable form, and the bioavailability was reduced with European Community Bureau of Reference (BCR) morphology analysis. This study proved that the combination of bioleaching and PDS could not only shorten the treatment cycle but also further improve the efficiency of heavy metal leaching. It provides a novel treatment method for the removal of heavy metals from metallurgical industry sludge.

Metallic contamination of global river sediments and latest developments for their remediation

This review article represents the comparative study of heavy metal concentration in water and sediments of 43 important global rivers. The review is a solitary effort in the area of heavy metal contamination of river-sediments during last ten years. The interpretation of heavy metal contamination in sediments has been verified with different indices, factors, codes and reference guidelines, which is based on geochemical data linked to background value of metals. It is observed that health hazards arise due to dynamics of movement of metals between water and sediments, which is primarily influenced by several factors such as physical, chemical, biological, hydrological and environmental. Also, the reason behind accumulation and assimilation of heavy metals on river water system is explained with appropriate mechanisms. Several factors e.g. pH, ORP, organic matter etc. are mainly involved in the distribution, accumulation and assimilation of metals in the sediment phase to water phase. Remediation technologies such as in-situ and ex-situ have been discussed for the removal of heavy metals from contaminated sediments. We have also compared the performance efficiencies of the technologies adopted by different researchers during the period 2003 to 2019 for the removal of metal bound sediments. Many researchers have preferred in-situ over ex-situ remediation due to low cost and time saving remediation effects. In this work we have also incorporated the safety measures and strategies which can prevent the metal accumulation in sediments of river system.

Bioleaching for environmental remediation of toxic metals and metalloids: A review on soils, sediments, and mine tailings

Owing to industrial evolution, a huge mass of toxic metals, including Co, Cu, Cr, Mn, Ni, Pb, and Zn, and metalloids, such as As and Sb, has inevitably been released into the natural environment and accumulated in soils or sediments. Along with modern industrialization, many mineral mines have been explored and exploited to provide materials for industries. Mining industries also generate a vast amount of waste, such as mine tailings, which contain a high concentration of toxic metals and metalloids. Due to the low economic status, a majority of mine tailings are simply disposed into the surrounding environments, without any treatment. The mobilization and migration of toxic metals and metalloids from soils, sediments, and mining wastes to water systems via natural weathering processes put both the ecological system and human health at high risk. Considering both economic and environmental aspects, bioleaching is a preferable option for removing the toxic metals and metalloids because of its low cost and environmental safety. This chapter reviews the recent approaches of bioleaching for removing toxic metals and metalloids from soils, sediments, and mining wastes. The comparison between bioleaching and chemical leaching of various waste sources is also discussed in terms of efficiency and environmental safety. Additionally, the advanced perspectives of bioleaching for environmental remediation with consideration of other influencing factors are reviewed for future studies and applications.

Removal behavior and chemical speciation distributions of heavy metals in sewage sludge during bioleaching and combined bioleaching/Fenton-like processes

The removal and chemical speciation changes of heavy metals in the sewage sludge during the single bioleaching and combined bioleaching/Fenton-like processes were compared in this study. The improvement in the dewaterability of the treated sludge was also investigated. The single bioleaching led to a removal of Zn, Cu, Cd, Cr, Mn, Ni, As and Pb of 67.28%, 50.78%, 64.86%, 6.32%, 56.15%, 49.83%, 20.78% and 10.52% in 10 days, respectively. The chemical speciation analysis showed that the solubilization of heavy metals in mobile forms (exchangeable/acid soluble and reducible forms) and oxidizable form was the main reason for their removal. Subsequent Fenton-like treatment was carried out at different bioleaching stages when the bioleached sludge dropped to certain pH values (4.5, 4.0 and 3.0), by adding H₂O₂ at different dosages. The highest removal ratio of Zn, Cu, Cd, Cr, Mn and Ni could reach 75.53%, 52.17%, 71.91%, 11.63%, 66.29% and 65.19% after combined bioleaching/Fenton-like process, respectively, with appropriate pH and H₂O₂ dosages in less than 6 days. The solubilization efficiencies of these heavy metals in mobile forms were further improved by Fenton-like treatment. The removal efficiencies of As and Pb decreased due to their transformation into insoluble forms (mostly residual fraction) after Fenton treatment. The capillary suction times (CST) of the raw sludge (98.7 s) decreased by 79.43% after bioleaching and 87.44% after combined process, respectively.

A critical review on metal complexes removal from water using methods based on Fenton-like reactions: Analysis and comparison of methods and mechanisms

Metals mainly exist in the form of complexes in urban wastewater, fresh water and even drinking water, which are difficult to remove and further harm human health. Fenton-like reaction has been used for the removal of metal complexes. Effective removal of metal complexes using Fenton-like reaction requires the removal of both metals and organic ligands, meanwhile, the fate of metals and organic pollutions must be clearly understood. Thus, this review summarizes the relevant research on metal complex removal from using Fenton-like reactions in the past ten years, with the detailed removal approaches and mechanisms analyzed. Electro-, photo-, microwave/ultrasound-Fenton reactions or the synergistic Fenton reaction have been shown to exhibit excellent metal complex treatment capabilities. Furthermore, various catalysts, such as transition metals, bimetals and metal-free catalytic systems can expand the potential applications of Fenton-like reactions. Novel Fenton reaction methods without the addition of metals or H₂O₂, with construction of a dual active center catalyst, or with the introduction of other free radicals, are all worthy of further investigation. Due to increasing levels of environmental metal and organic pollutions remediation requirements, more research is required for the development of economical and efficient novel Fenton-like processes.

Insight into sludge dewatering by advanced oxidation using persulfate as oxidant and Fe2+ as activator: Performance, mechanism and extracellular polymers and heavy metals behaviors

This study established a Fe²⁺/persulfate oxidation system to dewater sludge in WWTPs. Dewatering performance, persulfate consumption and the variations of sludge pH, TN and TP during dewatering process were monitored. EPS and ζ-potential behaviors for ameliorating sludge dewatering was investigated. Transformation, leaching toxicity and environmental risk of heavy metals in sludge during dewatering were determined. Results demonstrated that after treated by Fe²⁺/persulfate oxidation system with 0.6 mmol/g-VS of persulfate at Fe²⁺/persulfate molar ratio 0.6, WC decreased to 53.5% and SCST increased to 4.15, which implied an excellent improvement of sludge dewatering. The fast persulfate consumption, the decrease of sludge pH and the increase of TN illustrated the positive effects of Fe²⁺ in activating persulfate and the decomposition of EPS by the activation products, SO₄^•- and ^•OH. Another product (Fe³⁺) generated during persulfate activation could decrease the content of phosphorus-containing matter (released from EPS decomposition) through the precipitation reaction with PO₄^3-. The decrease of TOC and UV-254 happened in HPO-A, HPO-N and TPI-A organic substance of EPS (mainly contained in TB-EPS fraction) indicated that the destruction of hydrophobic organic matter of EPS would stimulate the release of bound water, which was beneficial to dewater sludge. The largest protein loss in TB-EPS (from 24.5 to 10.7 mg/L) indicated that the effective decomposition of TB-EPS could significantly ameliorate sludge dewatering. The increase of ζ-potential indicated the degradation of organic matter in EPS with negative charge. To sum up, the destruction of protein-like substances in hydrophobic organic matter of TB-EPS was the main mechanism for improving sludge dewatering by Fe²⁺/persulfate oxidation system. 3D-EEM fluorescence spectroscopy analysis proved that these protein-like substances were mainly tryptophan protein and humic acid. Moreover, due to the disruption of EPS, the contents of heavy metals in sludge, and their leaching toxicity and environmental risk were reduced. Therefore, Fe²⁺/persulfate oxidation system has potential and application prospects to improve sludge dewatering and optimize sludge management in WWTPs.

Port Sediments: Problem or Resource? A Review Concerning the Treatment and Decontamination of Port Sediments by Fungi and Bacteria

Contamination of marine sediments by organic and/or inorganic compounds represents one of the most critical problems in marine environments. This issue affects not only biodiversity but also ecosystems, with negative impacts on sea water quality. The scientific community and the European Commission have recently discussed marine environment and ecosystem protection and restoration by sustainable green technologies among the main objectives of their scientific programmes. One of the primary goals of sustainable restoration and remediation of contaminated marine sediments is research regarding new biotechnologies employable in the decontamination of marine sediments, to consider sediments as a resource in many fields such as industry. In this context, microorganisms—in particular, fungi and bacteria—play a central and crucial role as the best tools of sustainable and green remediation processes. This review, carried out in the framework of the Interreg IT-FR Maritime GEREMIA Project, collects and shows the bioremediation and mycoremediation studies carried out on marine sediments contaminated with ecotoxic metals and organic pollutants. This work evidences the potentialities and limiting factors of these biotechnologies and outlines the possible future scenarios of the bioremediation of marine sediments, and also highlights the opportunities of an integrated approach that involves fungi and bacteria together.

Mobilisation of hazardous elements from arsenic-rich mine drainage ochres by three Aspergillus species

Natural ferric ochres that precipitate in streambeds at abandoned mining sites are natural scavengers of various metals and metalloids. Thus, their chemical and structural modification via microbial activity should be considered in evaluation of the risks emerging from probable spread of contamination at mining sites. Our results highlight the role of various aspergilli strains in this process via production of acidic metabolites that affect mobility and bioavailability of coprecipitated contaminants. The Mössbauer analysis revealed subtle structural changes of iron in ochres, while the elemental analysis of non-dissolved residues of ochres that were exposed to filamentous fungi suggest coinciding bioextraction of arsenic and antimony with extensive iron mobilisation. However, the zinc bioextraction by filamentous fungi is less likely dependent on iron leaching from ferric ochres. The strain specific bioextraction efficiency and subsequent bioaccumulation of mobilised metals resulted in distinct tolerance responses among the studied soil fungal strains. However, regardless the burden of bioextracted metal(loid)s on its activity, the Aspergillus niger strain has shown remarkable capability to decrease pH of its environment and, thus, bioextract significant and environmentally relevant amounts of potentially toxic elements from the natural ochres.

Simultaneous heavy metal removal and sludge deep dewatering with Fe(II) assisted electrooxidation technology

A hybrid sludge conditioning strategy with electrooxidation and Fe(II) addition was used for heavy metal removal from sewage sludge and industrial sludge, with simultaneous sludge dewatering and stabilization. With the addition of 82 mg/g DS Fe(II) and treatment time of 4.5 h, heavy metal removals of 72.95% and 78.49% for Cu, 66.29% and 84.26% for Zn, and 36.52% and 36.99% for Pb were achieved from sewage sludge and industrial sludge samples respectively. The system pH decreased to 2.33 and 2.98 and the oxidation-reduction potential (ORP) values increased to 435.90 mV and 480.60 mV in sewage sludge and industrial sludge samples, respectively, which was conducive to the desorption and dissolution of heavy metals from sludge structures and the degradation of the organic compounds that complexed with heavy metals. In addition, the hybrid conditioning process demonstrated excellent dewatering performance due to the efficient electrochemical disintegration of sludge flocs together with the coagulation of sludge particles by Fe(III) generated via electrooxidation. The strong acidic and oxidative environment produced by the enhanced electrooxidation process was also responsible for pathogen inactivation.

Use of polysorbate 20 and sodium thiosulfate to enhance sewage sludge dewaterability by bioleaching

Dewatering of sludge is a key problem that must be solved in the sewage sludge disposal industry. In this study, a series of process optimization tests were conducted to learn how to improve sludge treatment. The optimum process of sludge leaching treatment was studied in a specially designed 100-L reactor system. Four factors were investigated and nine batches of bioleaching tests were run at three levels of these factors. Orthogonal experiments showed that the effect of sludge return ratio and aeration rate on the sludge moisture content was significant and hydraulic retention time (HRT) had a clear effect, but nutrient types had a reduced effect on the moisture content of sludge. The primary and secondary order of each factor is reflux ratio > aeration rate > HRT > nutrient type. Under the optimal process, three batches of sludge were processed and the moisture content of the filter press cake was reduced to less than 60%, the organic matter content reduced to below 5%, and the concentration of heavy metals (Cu, Zn, Pb, and Cr) was much lower than the agricultural standard limit, which is suitable for landscaping, composting, and incineration power generation and other resource applications.

Megamerger of biosorbents and catalytic technologies for the removal of heavy metals from wastewater: Preparation, final disposal, mechanism and influencing factors

Heavy metal pollution, because of its high toxicity, non-biodegradability and biological enrichment, has been identified as a global aquatic ecosystems threat in recent decades. Due to the high efficiency, low cost, satisfactory recyclability, easy storage and separation, biosorbents have exhibited a promising prospect for heavy metals treatment in aqueous phase. This article comprehensively summarized different types of biosorbents derived from available low-cost raw materials such as agricultural and forestry wastes. The raw materials obtained are treated with conventional pretreatment or novel methods, which can greatly enhance the adsorption performance of the biosorbents. The suitable immobilization methods can not only further enhance the adsorption performance of the biosorbents, but also facilitate the process of separating the biosorbents from the wastewater. In addition, once biosorbents are put into large-scale use, the final disposal problems cannot be avoided. Therefore, it is necessary to review the currently accepted final disposal methods of biosorbents. Moreover, through the analysis of the adsorption and desorption mechanisms of biosorbents, it is not only beneficial to find the better methods to improve the adsorption performance of the biosorbents, but also better to explain the influencing factors of adsorption effect for biosorbents. Especially, different from many researches focused on biosorbents, this work highlighted the combination of biosorbents with catalytic technologies, which provided new ideas for the follow-up research direction of biosorbents. Finally, the purpose of this paper is to inject new impetus into the future development of biosorbents.

Mixotrophic acidophiles increase cadmium soluble fraction and phytoextraction efficiency from cadmium contaminated soils

A profound concern in developing microbially-assisted phytoextraction is that introduced microbes not only remove heavy metals from contaminated soils but also enhance metal uptake into plant tissues from the treated soils. Cadmium (Cd) removal efficiencies were compared after leaching with deionized water (CK), acidified basal salts medium (acid control), cell-free spent medium (spent bioleaching) and mixotrophic acidophiles (two-step bioleaching). Two-step bioleaching using the mixotrophic acidophiles removed 34% of total Cd and 87% of available Cd, significantly more than CK (3% and 4%), acid control (12% and 51%) and spent bioleaching (26% and 75%). Pot experiments of water spinach growing in four treated soils were conducted to evaluate the Cd uptake performance in plants. Notably, the mixotrophic acidophiles increased Cd concentration in plant tissues by 78% compared to the CK group. More interestingly, the mixotrophic acidophiles were not colonized in soils but caused the compositional increase of indigenous microbes such as the genera of Alicyclobacillus, Clostridium sensu strict and Streptacidiphilus. Meanwhile, two-step bioleaching had little effects on soil structure and physicochemical properties determined by the spectroscopy characteristics analysis. These results implied that the mixotrophic acidophiles facilitated the development of microbially-assisted phytoextraction technology.

Profiles and potential health risks of heavy metals in soil and crops from the watershed of Xi River in Northeast China

Heavy metal soil contamination is a major environmental concern in China and has drawn significant public attention in recent years. The present study assessed the human health risk of four heavy metals (cadmium, lead, mercury, and zinc) through soil exposure and dietary intake in the watershed of Xi River, west of Shenyang, Liaoning Province, northeast China. Concentrations of Cd, Pb, Hg, and Zn were determined in 411 soil samples and 59 crop samples collected from this area. The health risks of heavy metal exposure via soil ingestion, inhalation, dermal contact, and dietary intake of crops were further estimated; total hazard index values were 1.15, 1.33, and 2.03 for the adult male, adult female, and children, respectively, and the total cancer risk was 3.7 × 10^-3 for the local residents. Among the four investigated heavy metals, Cd was the primary pollutant posing health risks. The estimated non-carcinogenic and carcinogenic risks posed by the studied heavy metals were above the acceptable levels proposed by the U.S. EPA. Dietary intake of contaminated crops was identified as the predominant pathway for human heavy metal exposure in this area, suggesting that soil heavy metals can cause more harm when transported to the food chain. Overall, our findings suggest that the Xi River watershed is unsuitable for growing agricultural products, and more effective controls should be taken to curb future heavy metal pollution in the area.

Dredged-Sediment-Promoted Synthesis of Boron-Nitride-Based Floating Photocatalyst with Photodegradation of Neutral Red under Ultraviolet-Light Irradiation

A novel floating photocatalyst (BN-DS-7) has been successfully synthesized by calcining the mixture of boron nitride (BN) and dredged sediment (DS) with a specific mass ratio (3:7) at 1100 °C for a half hour. BN is synthesized for the first time using an oxygen-limited method, which consists of a nanoplate ∼30 nm in size and has a bandgap at 3.94 eV. The as-synthesized BN can degrade NR under ultraviolet (UV) light irradiation. For BN-DS-7, X-ray photoelectron spectroscopy analysis suggests that BN mainly interacts with DS through the strong coordination between these N atoms in BN and these Si and Al atoms in DS. This leads to BN-DS-7 having good compression strength (∼9 MPa). Thermogravimetric analysis for BN shows that a few BN (∼13%) synthesized via an oxygen-limited method will pyrolyze at 1100 °C and the released gas can be sealed in the inside of DS at 1100 °C, resulting in that BN-DS-7 can float on the water surface. Photodegradation results show that BN-DS-7 can degrade 84% of NR (20 mg/L) under UV-light irradiation for 5 h, and the active species are •OH and photoinduced hole. Total organic carbon analysis for NR solution before and after photodegradation show that ∼70% of NR has been mineralized into inorganic carbons. This work is helpful to develop a new type of BN-based floating material and enlarge the application field of DS.

A novel bioreactor system for simultaneous mutli-metal leaching from industrial pyrite ash: Effect of agitation and sulphur dosage

Simultaneous multi-metal leaching from industrial pyrite ash is reported for the first time using a novel bioreactor system that allows natural diffusion of atmospheric O₂ and CO₂ along with the required temperature maintenance. The waste containing economically important metals (Cu, Co, Zn & As) was leached using an adapted consortium of meso-acidophilic Fe²⁺ and S oxidising bacteria. The unique property of the sample supported adequate growth and activity of the acidophiles, thereby, driving the (bio) chemical reactions. Oxido-reductive potentials were seen to improve with time and the system's pH lowered as a result of active S oxidation. Increase in sulphur dosage (>1g/L) and agitation speed (>150rpm) did not bear any significant effect on metal dissolution. The consortium was able to leach 94.01% Cu (11.75% dissolution/d), 98.54% Co (12.3% dissolution/d), 75.95% Zn (9.49% dissolution/d) and 60.80% As (7.6% dissolution/d) at 150rpm, 1g/L sulphur, 30°C in 8days.

Laboratory assessment of bioleaching of shallow eutrophic sediment by immobilized photosynthetic bacteria

Eutrophic sediment is a serious problem in ecosystem restoration, especially in shallow lake ecosystems. We present a novel bioleaching approach to treat shallow eutrophic sediment with the objective of preventing the release of nitrate, phosphate, and organic compounds from the sediment to the water column, using porous mineral-immobilized photosynthetic bacteria (PSB). Bioactivity of bacteria was maintained during the immobilization process. Immobilized PSB beads were directly deposited on the sediment surface. The deposited PSB utilized pollutants diffused from the sediment as a nutritive matrix for growth. We evaluated the effects of light condition, temperature, initial pH, amount of PSB beads, and frequency of addition of PSB beads for contaminant removal efficiency during bioleaching operations. The presented study indicated that immobilized PSB beads using porous minerals as substrates have considerable application potential in bioremediation of shallow eutrophic lakes.

Chemical oxidation of bis(2-chloroethyl) ether in the Fenton process: Kinetics, pathways and toxicity assessment

Bis(2-chloroethyl) ether (BCEE) is a common chemical material and a frequently detected contaminant in groundwater. It has a strong toxicity and some other chemicals such as poly(vinyl chloride-co-isobutyl vinyl ether) contain similar chloroaliphatic ether structure. So the effective degradation method and transformation pathways for BCEE need to be learned. The present study compared the degradation rate of BCEE by Fenton's reagent and other common oxidation methods, and optimized the reaction conditions. Oxidation intermediates and pathways were also proposed and toxicities of the intermediates were investigated. Results showed that Fenton was highly effective to degrade BCEE. pH, Fe²⁺ and H₂O₂ concentration all affected the oxidation rate, among which Fe²⁺ was the most significant variable. A total of twelve chlorinated intermediates were detected. Three main reaction pathways involved cleavage of the ether bond, hydroxyl substitution for hydrogen, and radical coupling. The pathways could be well interpreted and supported by theoretical calculations. The reaction mixture showed a decreasing trend in TOC concentration and toxicity until totally harmless to Vibrio fischeri after 15 min, but it was noteworthy that toxicities of some dimeric intermediates were stronger than BCEE by calculation.

Synergistic effect of biogenic Fe3+ coupled to S° oxidation on simultaneous bioleaching of Cu, Co, Zn and As from hazardous Pyrite Ash Waste

Pyrite ash, a waste by-product formed during roasting of pyrite ores, is a good source of valuable metals. The waste is associated with several environmental issues due to its dumping in sea and/or land filling. Although several other management practices are available for its utilization, the waste still awaits and calls for an eco-friendly biotechnological application for metal recovery. In the present study, chemolithotrophic meso-acidophilic iron and sulphur oxidisers were evaluated for the first time towards simultaneous mutli-metal recovery from pyrite ash. XRD and XRF analysis indicated higher amount of Hematite (Fe₂O₃) in the sample. ICP-OES analysis indicated concentrations of Cu>Zn>Co>As that were considered for bioleaching. Optimization studies indicated Cu - 95%, Co - 97%, Zn - 78% and As - 60% recovery within 8days at 10% pulp density, pH - 1.75, 10% (v/v) inoculum and 9g/L Fe²⁺. The productivity of the bioleaching system was found to be Cu - 1696ppm/d (12% dissolution/d), Co - 338ppm/d (12.2% dissolution/d), Zn k 576ppm/d (9.8% dissolution/d) and As - 75ppm/d (7.5% dissolution/d). Synergistic actions for Fe²⁺ - S° oxidation by iron and sulphur oxidisers were identified as the key drivers for enhanced metal dissolution from pyrite ash sample.

Comparison of bioleaching and electrokinetic remediation processes for removal of heavy metals from wastewater treatment sludge

Heavy metals prevent the growing amount of sewage sludge from being disposed as fertilizeron land. The electrokinetic remediation and bioleaching technology are the promising methods to remove heavy metals. In recent years, some innovation has been made to achieve better efficiency, including the innovation of processes and agents. This paper reviews the development of the electrokinetic remediation and bioleaching technology and analyses their advantages and limitation, pointing out the need of the future research for the heavy metals-contaminated sewage sludge.

A new five-coordinated copper compound for efficient degradation of methyl orange and Congo red in the absence of UV–visible radiation

A five-coordinated compound, Cu₂(2,2-bipy)₂(pfbz)₄, displays excellent properties of degrading methyl orange and Congo red in the absence of UV–visible radiation.