Influence of pH on heavy metal speciation and removal from wastewater using micellar-enhanced ultrafiltration

Magnetic nanostructures functionalized with a derived lysine coating applied to simultaneously remove heavy metal pollutants from environmental systems

The pollution of environmental systems with heavy metals is becoming a serious problem worldwide. These contaminants are one of the main issues in sludge (which is considered waste) and can even have harmful effects if the sludge is not treated properly. Thus, the development of a novel functional magnetic nanoadsorbent based on a derived lysine is reported here, which can be efficiently applied for metal removal from sludge. Magnetic nanoparticles were coated with silica layer and further modified by covalent bonding of derived lysine. The morphology of the nanomaterial, its nano-size and the silica layer thickness were analyzed by transmission electron microscopy. The successful silanization of the lysine derivative to the silica-coated magnetic nanostructures was investigated by several physicochemical characterization techniques, while the magnetic properties were measured with a vibrating sample magnetometer. The synthesized nanostructures were used as adsorbents for simultaneous removal of most critical heavy metals (Cr, Zn, Cu) from real complex sludge suspensions. The main practical adsorption parameters, pH of the native stabilized sludge, adsorbent amount, time, and adsorbent regeneration were investigated. The results show promising adsorption properties among currently available adsorbents (the total equilibrium adsorption capacity was 24.5 mg/g) from the sludge with satisfactory nanoadsorbent reusability and its rapid removal. The stability of the nanoadsorbent in the sludge, an important but often neglected practical parameter for efficient removal, was verified. This work opens up new possibilities for the development of high-quality magnetic nanoadsorbents for metal pollutants applied in various complicated environmental fields and enables waste recovery.

Solution pH affects single, sequential and binary systems of sulfamethoxazole and cadmium adsorption by self-assembled cellulose: Promotion or inhibition?

A new self-assembled cellulose (SACS) containing multi-functional amine, carboxyl and hydroxyl groups was successfully obtained through etherification, cross-linking and grafting processes. Then, the adsorption of sulfamethoxazole (SMZ) and Cd(II) onto SACS at pH values of 3, 5.7 and 7.5 was systematically investigated by batch experiments of single, sequential and binary systems, characterization and density functional theory (DFT) calculations. The presence of Cd(II) decreased the adsorption of SMZ because of hydrophilic site competition, while SMZ inversely increased the adsorption of Cd(II), which was attributed to bridging and especially to electrostatic shielding effects; moreover, both the inhibitory and synergistic effects were more obvious in the binary system and at a pH of 7.5. There was a dynamic balance between the inhibitory and synergistic effects that depended on the system, pH value and concentration ratio. DFT results further indicated that SMZ^- more easily coordinated with Cd(II) at sulfonyl oxygen and nitrogen sites, and the cationic bridge of Cd(II) with SMZ^- mainly occurred in the sequential system. Moreover, a complexation-decomplexation-complexation balance of SMZ^- and Cd(II) probably occurred in the binary system.

Ion exchange based approach for rapid and selective Pb(II) removal using iron oxide decorated metal organic framework hybrid

Polyacrylic acid capped Fe₃O₄ - Cu-MOF (i-MOF) hybrid was prepared for rapid and selective lead removal, with 93% removal efficiency, exceptional selectivity, and adsorption capacity of 610 mg/g and 91% of i-MOF hybrid could be easily separated from the contaminated water using magnetic separation. The adsorption process followed a pseudo-second-order model and the adsorption efficiency decreased from 93% to 83% on raising the temperature from 25 °C to 40 °C. The change in equilibrium adsorption capacity with respect to equilibrium adsorbate concentration followed the Langmuir isotherm model. i-MOF had a high selectivity coefficient and removal efficiency for lead ions even when exposed simultaneously with naturally abundant cations (Na(I), Ca(II), Mg(II)). Release of Cu(II) ions from the i-MOF after Pb(II) removal suggested suggested ion-exchange to be the dominant removal mechanism. This new finding for Pb(II) removal with excellent adsorption performance using i-MOF through ion exchange based approach is a viable option for treating lead contaminated water.

Effect of pyrolysis temperature on the bioavailability of heavy metals in rice straw-derived biochar

The aim of this study was to investigate the stability of heavy metals in biochar derived from rice straw with heavy metal enrichment, and the relationship between pyrolysis temperature and the stability of heavy metals in biochar. The concentrations of heavy metals of rice straw and biochar (pyrolyzed at 300 °C, 500 °C, 700 °C, and 900 °C) were measured. The experiments of extraction and leaching were conducted to evaluate the effect of pyrolysis temperature on the stability of heavy metals in biochar. A pot experiment was conducted to investigate the environmental risk of heavy metals from biochar. The pyrolysis temperature affected the pH, total C, total N, surface structure, functional groups, and the concentrations of heavy metals in biochar. After being pyrolyzed, the bioavailable DTPA fraction of total Cu, Zn, Cd, and Pb of BC500, BC700, BC900, and BC900 decreased by 72.87%, 69.45%, 48.09% and 15.89%, respectively, in comparison with levels in rice straw. In addition, the leaching potential of heavy metals in biochar was significantly reduced. The pot experiment and the correlation analysis indicated that the pyrolysis temperature was not significantly related to the accumulation of heavy metals in aerial parts of rice seedlings. Increase in the pyrolysis temperature had a positive effect on increasing the stability and decreasing the mobility of heavy metals in biochar. However, the variations in the pyrolysis temperature were not the main factor to affect the uptake of heavy metals originated from biochar into the aerial parts of rice seedlings.

Long-term continuous and real-time in situ monitoring of Pb(II) toxic contaminants in wastewater using solid-state ion selective membrane (S-ISM) Pb and pH auto-correction assembly

Lead (Pb) contaminants in wastewater have inhibited microbial activities and thus exerted high energy consumption in wastewater treatment plants (WWTPs). Current Pb monitoring has been conducted ex situ and off line, unable to affect real-time proactive control and operation. This study targets the crucial challenge of better and faster Pb monitoring by developing novel mm-sized screen-printed solid-state ion-selective membrane (S-ISM) Pb sensors with low-cost, high accuracy and long-term durability and that enable real-time in situ monitoring of Pb(II) ion contamination down to low concentrations (15 ppb-960 ppb) in wastewater. An innovative pH auto-correction data-driven model was built to overcome the inextricable pH inferences on Pb(II) ISM sensors in wastewater. Electrochemical impedance spectroscopy (EIS) and cyclic voltammograms (CV) analysis showed (3,4-ethylenedioxythiophene, EDOT) deposited onto the mm-sized screen-printed carbon electrodes using electropolymerization effectively alleviated the interferences from dissolved oxygen and improved long-term stability in wastewater. Monte Carlo simulation of the nitrification process predicted that real-time, and high accurate in situ monitoring of Pb(II) in wastewater and swift feedback control could save ∼53 % of energy consumption by alleviating the errors from pH and DO impacts in WWTPs.

Heavy metal-immobilizing bacteria combined with calcium polypeptides reduced the uptake of Cd in wheat and shifted the rhizosphere bacterial communities

In situ stabilization techniques for the "remediation" of heavy metal-contaminated soil are a novel and inexpensive technology. However, the mechanisms underlying the interaction of exogenous passivators with the bacterial community in wheat rhizosphere soil remain unclear. Soil static culture and pot experiments were conducted to evaluate the effects and mechanisms of the heavy metal-immobilizing bacterium Enterobacter bugandensis TJ6 and calcium polypeptides (CPPs) and their association with Cd uptake in wheat, soil quality and the rhizobacterial community structure. The results showed that compared with the control treatment (CK), the TJ6, CPP, and TJ6+CPP treatments significantly decreased the diethylenetriaminepentaacetic acid (DTPA)-extractable Cd (25.2%-60.1%) content and increased the pH, organic matter content and urease activity in the wheat rhizosphere soil, which resulted in decreases in the Cd (21.5%-77.8%) content in wheat tissues (grain, straw, and roots). In particular, the TJ6+CPP treatment was more effective at decreasing Cd accumulation in grains. Furthermore, the TJ6+CPP treatment improved the diversity of the soil bacterial community in the wheat rhizosphere, and the relative abundances of Proteobacteria, Firmicutes, Arthrobacter, Microvirga, Ensifer, Brevundimonas, Devosia and Pedobacter were enriched. These results suggest that the TJ6+CPP treatment decreased the uptake of Cd in wheat by i) providing essential elements (N and C sources), ii) increasing the pH and reducing the bioavailable Cd content in wheat rhizosphere soil, iii) allowing colonization to promote plant growth and Cd-resistant bacteria, and iv) increasing the abundance of genes associated with ABC transporters, carbon metabolism and oxidative phosphorylation in the rhizosphere bacterial community. Our results showed that the heavy metal-immobilizing bacterium TJ6 combined with CPPs decreased the Cd content and increased the bacterial community diversity of wheat rhizosphere soil. Our results also highlight the potential of using heavy metal-immobilizing bacteria and CPPs to ensure the safe production of crops growing on heavy metal-polluted soils.

Constructing Positively Charged Thin-Film Nanocomposite Nanofiltration Membranes with Enhanced Performance

Using polyethylenimine (PEI) as the aqueous reactive monomers, a positively charged thin-film nanocomposite (TFN) nanofiltration (NF) membrane with enhanced performance was developed by successfully incorporating graphene oxide (GO) into the active layer. The effects of GO concentrations on the surface roughness, water contact angle, water flux, salt rejection, heavy metal removals, antifouling property, and chlorine resistance of the TFN membranes were evaluated in depth. The addition of 20 ppm GO facilitated the formation of thin, smooth, and hydrophilic nanocomposite active layers. Thus, the TFN-PEI-GO-20 membrane showed the optimal water flux of 70.3 L·m-2·h-1 without a loss of salt rejection, which was 36.8% higher than the thin-film composite (TFC) blank membrane. More importantly, owing to the positively charged surfaces, both the TFC-PEI-blank and TFN-PEI-GO membranes exhibited excellent rejections toward various heavy metal ions including Zn2+, Cd2+, Cu2+, Ni2+, and Pb2+. Additionally, compared with the negatively charged polypiperazine amide NF membrane, both the TFC-PEI-blank and TFN-PEI-GO-20 membranes demonstrated superior antifouling performance toward the cationic surfactants and basic protein due to their hydrophilic, smooth, and positively charged surface. Moreover, the TFN-PEI-GO membranes presented the improved chlorine resistances with the increasing GO concentration.

MXene-Carbon Nanotube Hybrid Membrane for Robust Recovery of Au from Trace-Level Solution

The use of precious metals in many areas, such as printed circuit boards, catalysts, and targeted drugs, is increasing due to their unique physical and chemical properties, but their recovery remains a great challenge. Here, we report a sandwiched Ti₃C₂T_x MXene/carbon nanotube (CNT) hybrid membrane, where the CNT isolates and supports the MXene sheets, which act as a reducing agent. The hybrid membrane shows excellent ability to capture precious metal ions in solution with a high flux. The water permeability of the membrane reaches 437.6 L m^-2 h^-1 bar^-1 (2.46 × 10^-18 m²), about 202 times higher than that of a pure Ti₃C₂T_x membrane, and captures 99.8% Au(III) from a solution with an extremely low concentration of 20 ppm. The desirable precious metal trapping capability of the Ti₃C₂T_x-CNT film is due to the high redox activity of C-Ti-OH. This work provides an efficient way for the recovery of precious metal ions from wastewater.

Micellar-Enhanced Ultrafiltration Using a Plant-Derived Surfactant for Dye Separation in Wastewater Treatment

Micellar-enhanced ultrafiltration (MEUF) is one of several membrane methods used for the removal of trace organic pollutants from aqueous streams. In this process, a surfactant is added to a polluted aqueous solution at a concentration higher than its critical micelle concentration (CMC). Unlike synthetic surfactants, natural surfactants, from plants such as the saponin, while ecologically adaptable as surfactants in MEUF systems, are also biodegradable, renewable, and environmentally safe. This study applied Sapindus rarak extract as the natural surfactant in MEUF for Remazol dye separation. It was found that the presence of Sapindus rarak extract increased separation of Remazol red and blue dyes by up to 97.02% and 99.42%, respectively. However, the addition of surfactant decreased permeate fluxes due to membrane fouling and concentration polarization. In addition, loading micelle (Lm), representing the performance of the surfactant micelle for dye separation, as well as the blocking mechanism, was investigated. Lm was found to be in the range of 0.002-0.068 mM dyes/mM saponin. Ultrafiltration blocking mechanisms, as confirmed by the Hermia model, were: standard blocking, for cases without the addition of surfactant; cake formation, for cases with surfactant below the CMC; and complete blocking, for cases with surfactant above the CMC.

Granule-based immobilization and activity enhancement of anammox biomass via PVA/CS and PVA/CS/Fe gel beads

Granule-based immobilization of anammox biomass assisted by polyvinyl alcohol/chitosan (PVA/CS) and PVA/CS/Fe gel beads was studied, via the operation of three identical up-flow reactors (R1 without gel beads, R2 with PVA/CS, R3 with PVA/CS/Fe) for 203 days. In the end, the nitrogen removal rates (NRR) were 5.3 ± 0.4, 10.0 ± 0.3 and 13.9 ± 0.5 kg-N m^-3 d^-1 for R1, R2 and R3, respectively. The porous PVA/CS and PVA/CS/Fe created a suitable eco-niche for anammox bacteria to grow and attach, thus being retained in the reactor. The EPS entangles newly grown cells within the gel beads, resulting in compact aggregation. The interaction between Fe ions added to PVA/CS/Fe gel beads and negatively charged EPS groups strongly promoted granule strength and compactness. The immobilization method proposed by this study was found to effectively improve biomass retention in the reactors, which is promising for advanced anammox process applications.

Flexible Pt3Ni-S-Deposited Teflon Membrane with High Surface Mechanical Properties for Efficient Solar-Driven Strong Acidic/Alkaline Water Evaporation

Solar-driven water evaporation provides a promising solution to the energy crisis and environmental issues. Capitalizing on the high photothermal conversion efficiency and excellent resistance to strong acids or strong alkalis of Pt₃Ni-S nanowires, we strategically design and prepare a flexible Pt₃Ni-S-deposited Teflon (PTFE) membrane for achieving efficient strong acid/alkaline water evaporation under simulated sunlight irradiation (1 sun). By comparing the surface morphology, mechanical properties, and water evaporation performance of the as-prepared three different membranes, we have screened out a high-performance photothermal membrane that has good hydrophobicity (water contact angle = 106°), strong mechanical properties, high light-to-heat conversion efficiency (η = 80%), and excellent durability (10 cycles in a range of pH = 1.2-12). In particular, we explore the mechanism of high surface mechanical properties of the as-prepared membrane using density functional theory. The results demonstrate that the related mechanism can be ascribed to two main reasons: (1) hydrogen bonds can be formed between the 2-pyrrolidone ring and PTFE-3 and (2) the O atom in PTFE-3 carries more negative charge (-0.19 |e|) than PTFE-1 (-0.16 |e|) and PTFE-2 (-0.15 |e|). Our work highlights the great potentials of a Pt₃Ni-S-deposited PTFE membrane as a device for implementing solar energy-driven evaporation of industrial wastewater with strong acidity or alkalinity and provides a new strategy for improving the surface mechanical properties of a photothermal membrane.

Micellar-Enhanced Ultrafiltration to Remove Nickel Ions: A Response Surface Method and Artificial Neural Network Optimization

Nickel ions from aqueous solutions were removed by micellar-enhanced ultrafiltration (MEUF), using the surfactant sodium dodecyl sulfate (SDS) as a chelating agent. Process variables and indicators were modeled and optimized by a response surface methodology (RSM), using the Box–Behnken design (BBD). The generated quadratic models described the relationship between a performance indicator (nickel rejection rate or permeate flux) and process variables (pressure, nickel concentration, SDS concentration, and molecular weight cut-off (MWCO)). The analysis of variance (ANOVA) showed that both models are statistically significant. To remove 1 mM of nickel ions, the optimal condition for maximum nickel removal and flux were: pressure = 30 psi, CSDS = 10.05 mM, and MWCO = 10 kDa, resulting in a rejection rate of 98.16% and a flux of 119.20 L/h∙m2. Experimental verification indicates that the RSM model could adequately describe the performance indicators within the examined ranges of the process variables. An artificial neural network (ANN) modelling followed to predict the MEUF performance and validate the RSM results. The obtained ANN models showed good fitness to the experimental data.

Optimization of process using carboxymethyl chitosan for the removal of mixed heavy metals from aqueous streams

This paper highlights the efficacy of carboxymethyl chitosan (CMCh), a bio-degradable water-soluble derivative of chitosan for the separation of a mixture of heavy metal ions such as copper, nickel, zinc and lead from aqueous streams, as they constitute, the major industrial pollutants present in wastewater. The experimental studies are conducted using commercially available ultrafiltration module using synthetic solutions of the contaminants. The design of experiments was performed by Response Surface Methodology (RSM) with split-plot D-optimal design. Parametric studies were carried out using initial pH of the feed solution, loading ratio (P/M) and initial metal ion concentration to assess the percentage rejection and recovery of metal ions. The maximum percentage rejection of Cu(II), Ni(II), Zn(II) and Pb(II) with CMCh were found to be 100%, 100%, 95%, and 98% respectively under optimum conditions. Subsequently the metal ions were recovered collectively by reversing the pH to 2. The results show that CMCh could be an effective size enhancing species for the removal and recovery of mixture of metals by SEUF.

Hybrid ceramic membrane reactor combined with fluidized adsorbents and scouring agents for hazardous metal-plating wastewater treatment

In this study, a ceramic membrane consisting of aluminum oxide in the support and active layer with a surface pore size of 0.1 μm was applied with a real hazardous metal-plating wastewater. Alumina membrane was submerged directly into a fluidized membrane reactor specially designed for fluidizing the granular activated carbon (GAC) particles along membrane surface by recirculating a bulk wastewater through the reactor to improve fouling control and removal efficiency of contaminants. Zeolite particle which has the similar size to the GAC was also tested to compare membrane performance. Neutralizing a wastewater pH resulted in the agglomeration of particulate and colloidal materials, leading to the significant deposit of the fouling layer on membrane surface. The external fouling layer formed on membrane surface enhanced the removal efficiency of the heavy metal ions due to its role as secondary membrane. In addition to the fouling control by mechanical scouring actions, fluidizing the GAC particles on membrane was more beneficial to improve organic removal efficiency than zeolite. The increase in GAC dosage from 10 to 30 v/v% did not result in any beneficial effect on both fouling reduction and organic removal efficiency.

Role of sepiolite for cadmium (Cd) polluted soil restoration and spinach growth in wastewater irrigated agricultural soil

Metals that contaminate soil are one of the major problems seriously affecting sustainable agriculture worldwide. Cadmium (Cd) toxicity to agricultural crops is a global problem. Mobility of Cd in contaminated soil can be minimized by the amendment of soil passivators which will ultimately reduce its movement from soil to plants. A pot study was performed to evaluate the impact of sepiolite from 1% to 5% on Cd solubility and its accumulation in spinach tissues. Soil pH, Cd fractionation, Cd accumulation in spinach tissue and Cd adsorption mechanism were determined. Results were recorded that soil pH was increased from 0.3 to 1.0 units with the increasing rate of sepiolite from 1% to 5%. Similarly, Cd contents in acid soluble phase was decreased by 42.8% and increased in residual phase by 35.8% at 5% rate, relative to control. Moreover, the significant reduction in Cd uptake by spinach shoots and roots was occurred by 26.2% and 30.6% at 5% rate, respectively. Furthermore, the maximum Cd adsorption capacity 37.35 mg g^-1 was recorded at 5% rate relative to control. The analysis of FTIR, XRD and SEM also confirm the ability of sepiolite for Cd polluted soil restoration and thereby, reduces its phytoavailability in polluted soil to alleviate food security challenges.

Rhizoremediation of Cu(II) ions from contaminated soil using plant growth promoting bacteria: an outlook on pyrolysis conditions on plant residues for methylene orange dye biosorption

Rhizoremediation is one of the most accepted, cost-effective bioremediation techniques focusing on the application of rhizospheric microorganisms in combination with plants for the remediation of organic and inorganic pollutants from the contaminated sites. This work focuses on isolation and identification of metal resistant bacteria to grow on medium with the copper ion concentration of 1500 mg/L. The resistant isolate was identified as Pantoea dispersa by a 16S rRNA sequencing. The bioaccumulation of Cu(II) ions in plant is high at the concentration of Cu(II) ion is 125 mg/L in soil. In Sphaeranthus indicus the Cu(II) ion translocation factor has expanded with an expansion of grouping of Cu(II) ion in the soil and the most extreme TF factor was acquired at the centralization of Cu(II) ion is 150 mg/L in soil. Surface morphology of biochar was characterized by Scanning Electron Microscopy (SEM) analysis. The adsorption performance of biochar (Sphaeranthus indicus biomass) and mechanism for the removal of Cu(II) ion were investigated. This study resolves that pyrolysis is promising technology for the conversion of metal ion contaminated plant residues from phytoremediation into valuable products.

The effect of sepiolite application on rice Cd uptake - A two-year field study in Southern China

Sepiolite (SEP) is a clay mineral with great potential to stabilize soil heavy metals. A two-year field experiment was conducted to explore the optimum use of SEP to immobilize soil Cd and to promote the consumption safety of rice grown in a typical paddy field in Southern China. SEP was applied once or twice over the two-year study at three levels (0.1, 0.5, and 1%, w/w) before rice planting. The results showed that SEP effectively reduced rice grain Cd concentrations by 47-49% in the first year and by 44-50% in the second year due to the residue effect. Application of SEP for two consecutive years reduced the rice grain Cd concentration by up to 75%, achieving a safe level (<0.2 mg kg^-1). SEP also reduced Zn concentrations in rice grains (by 6-10%), while the Cd/Zn ratios of rice grains were decreased by 24-72% over the two years, implying it was much safer for consumption. SEP significantly increased the soil pH (0.9-1.8 units) and available phosphorus, and it reduced the soil available Cd (by 20-95%) and Zn concentrations (by 30-99%). In brief, SEP effectively stabilized soil Cd and reduced uptake by rice; the effect was dose-dependent and 0.5% (w/w) was optimum in the present study. The main mechanism of SEP to stabilize soil Cd is the increase in soil pH analogous to liming. This study shows that SEP application can be an efficient way to remediate Cd contaminated rice paddies and fulfill the goal of safe production of rice and thereby reduce the health risks associated with consuming rice.

Removal of heavy metal ions from multi-component aqueous solutions by eco-friendly and low-cost composite sorbents with anisotropic pores

Copper, nickel, zinc, chromium, and iron ions are the prevailing contaminants in the aqueous effluents resulting from the photo-etching industry. In this context, we investigate here the metal ion sorption performance of an ion-imprinted cryogel (IIC), consisting of low-cost materials coming from renewable resources, towards multi-component metal ion solutions. The IIC sorbent, which is based on a chitosan matrix embedding a natural zeolite, was synthesized using a straightforward strategy by coupling copper-imprinting and unidirectional ice-templating methods. As consequence, the 1D-orientation and the interconnectivity of flow-channels sustain the fast metal ion diffusion within the IIC anisotropic structure. The removal efficiency of IIC sorbent reached 50% after 30 min, and the sorption equilibrium was attained within 150 min. For assessing the successful formation of imprinted cavities with well-defined sizes controlled by the radius of copper ions used as template, selectivity studies were performed on binary, ternary, and five-component synthetic mixtures. The efficiency of IIC as sorbent was further evaluated on real-life aqueous effluents discharged from photo-etching processes; thus, an IIC dosage of 6 g L^-1 was found to remove 98.89% of Cu²⁺, 94.56% of Fe³⁺, 91.67% of Ni²⁺, 92.24% of Zn²⁺, and 82.76% of Cr³⁺ ions from this type of industrial wastewaters.

Role of concentration polarization in cross flow micellar enhanced ultrafiltration of cadmium with low surfactant concentration

Concentration polarization is an important issue in micellar enhanced ultrafiltration (MEUF) of wastewater containing heavy metal ions at low surfactant concentrations. In this paper, we studied removal of Cd(Ⅱ) by cross flow MEUF at low sodium dodecyl sulfate (SDS) concentration levels, and the role of concentration polarization in flux decline and Cd(Ⅱ) rejection was emphasized. Concentration polarization resistance and SDS concentration near membrane were calculated to characterize concentration polarization. The results showed that SDS concentration near membrane was 13 mM when feed concentration was merely 0.8 mM. By combining phase diagram of SDS, structures of SDS micelles in concentration polarization layer were deduced and thin layer structure transformed to porous structure formed by accumulated globular micelles when SDS concentration increased. Although micelles formed in concentration polarization layer was responsible for flux decline, they also provided adsorption sites for Cd(Ⅱ).

Roles of surfactants in pressure-driven membrane separation processes: a review

Surfactants widely exist in various kinds of wastewaters which could be treated by pressure-driven membrane separation (PDMS) techniques. Due to the special characteristics of surfactants, they may affect the performance of membrane filtration. Over the last two decades, there are a number of studies on treating wastewaters containing surfactants by PDMS. The current paper gives a review of the roles of surfactants in PDMS processes. The effects of surfactants on membrane performance were discussed via two aspects: influence of surfactants on membrane fouling and enhanced removal of pollutants by surfactants. The characteristics of surfactants in solution and at solid-liquid interface were summarized. Surfactants in membrane filtration processes cause membrane fouling mainly through adsorption, concentration polarization, pore blocking, and cake formation, and fouling degree may be influenced by various factors (feed water composition, membrane properties, and operation conditions). Furthermore, surfactants may also have a positive effect on membrane performance. Enhanced removal of various kinds of pollutants by PDMS in the presence of surfactants has been summarized, and the removal mechanism has been revealed. Based on the current reports, further studies on membrane fouling caused by surfactants and enhanced removal of pollutants by surfactant-aided membrane filtration were also proposed.

Speciation Distribution of Heavy Metals in Uranium Mining Impacted Soils and Impact on Bacterial Community Revealed by High-Throughput Sequencing

This study investigated the influence of heavy metals on bacterial community structure in a uranium mine. Soils from three differently polluted ditches (Yangchang ditch, Zhongchang ditch, and Sulimutang ditche) were collected from Zoige County, Sichuan province, China. Soil physicochemical properties and heavy metal concentrations were measured. Differences between bacterial communities were investigated using the high-throughput sequencing of the 16S rRNA genes. The obtained results demonstrated that bacterial richness index (Chao and Ace) were similar among three ditches, while the highest bacterial diversity index was detected in the severely contaminated soils. The compositions of bacterial communities varied among three examined sites, but Proteobacteria and Acidobacteria were abundant in all samples. Redundancy analysis revealed that soil organic matter, Cr and pH were the three major factors altering the bacterial community structure. Pearson correlation analysis indicated that the most significant correlations were observed between the contents of non-residual Cr and the abundances of bacterial genera, including Thiobacillus, Nitrospira, and other 10 genera. Among them, the abundances of Sphingomonas and Pseudomonas were significant and positively correlated with the concentrations of non-residual U and As. The results highlighted the factors influencing the bacterial community in uranium mines and contributed a better understanding of the effects of heavy metals on bacterial community structure by considering the fraction of heavy metals.

Pollution, Source, and Relationship of Trace Metal(loid)s in Soil-Wheat System in Hebei Plain, Northern China

To study the complex migration and transformation of trace metal(loid)s in a soil–wheat system, 225 pairs of surface soil and wheat samples were collected from the Taihang Mountains front plain, Hebei Province, northern China. The concentrations and pools (F1, water-soluble; F2, exchangeable; F3, carbonate-bound; F4, humic acid-bound; F5, Fe–Mn oxide-bound; F6, organic matter-bound; and F7, residual) of Cu, Pb, Zn, Cr, Ni, Cd, and Hg, and the soil properties of the samples were analyzed. The sum of the F1, F2, F3, and F4 proportions of Cd was higher than that of the other trace metal(loid)s, implying that Cd has greater mobility. We found a significant correlation (p < 0.01) between pools of trace metal(loid)s and the corresponding elements in wheat and a significant correlation (p < 0.01) between pools of trace metal(loid)s and pH, cation exchange capacity, clay, and total organic carbon. The results of principle component analysis (PCA)indicated that Cr, Ni and As mainly come from natural sources and Cu, Pb, Zn, and Cd from mixed groups related to farming and industry, Hg come from the coal burning. In addition, the total target hazard quotients showed the presence of harmful levels of trace metal(loid)s in wheat.

Characterization of the Fouling Layer on the Membrane Surface in a Membrane Bioreactor: Evolution of the Foulants' Composition and Aggregation Ability

In this study, the characteristics of membrane foulants were analyzed with regard to morphology, composition, and aggregation ability during the three stages of transmembrane pressure (TMP) development (fast-slow-fast rise in TMP) in a steady operational membrane bioreactor (MBR). The results obtained show that the fouling layer at the slow TMP-increase stage possessed a higher average roughness (71.27 nm) and increased fractal dimension (2.33), which resulted in a low membrane fouling rate (0.87 kPa/d). A higher extracellular DNA (eDNA) proportion (26.12%) in the extracellular polymeric substances (EPS) resulted in both higher zeta potential (-23.3 mV) and higher hydrophobicity (82.3%) for initial foulants, which induced and increased the protein proportion in the subsequent fouling layer (74.11%). Furthermore, the main composition of the EPS shifted from protein toward polysaccharide dominance in the final fouling layer. The aggregation test confirmed that eDNA was essential for foulant aggregation in the initial fouling layer, whereas ion interaction significantly affected foulant aggregation in the final fouling layer.

Arsenate removal from aqueous solutions using micellar-enhanced ultrafiltration

In this study, arsenate (As-V) removal using micellar enhanced ultrafiltration (MEUF) modified by cationic surfactants was studied by a dead-end polyacrylonitrile (PAN) membrane apparatus. The UF membrane has been produced by a phase inversion process. The prepared membrane was characterized and analyzed for morphology and membrane properties. The influence of operating parameters such as initial concentrations of As-V, surfactants, pH, membrane thickness, and co-existing anions on the removal of As-V, surfactant rejection, and permeate flux have been studied. The experimental results show that from the two different cationic surfactants used the CPC (cetyl-pyridinium chloride) efficiency (91.7%) was higher than that of HTAB (hexadecyltrimethyl-ammonium bromide) (83.7%). The highest As-V removal was 100%, and was achieved using initial feed concentrations of 100-1000 μg/L, at pH 7 with a membrane thickness of 150 μm in a dead-end filtration system. This efficiency for As-V removal was similar to that obtained using a cross-flow system. Nevertheless, this flux reduction was less than the reduction achieved in the dead-end filtration process. The PAN fabricated membrane in comparison to the RO and NF processes selectively removed the arsenic and the anions, in the water taken from the well, and had no substantial effect on the cations.

Enhancement of Detoxification of Petroleum Hydrocarbons and Heavy Metals in Oil-Contaminated Soil by Using Glycine-β-Cyclodextrin

Soil contamination with petroleum hydrocarbons and heavy metals is a widespread environmental problem. In recent years, cyclodextrin has attracted research interest because of its special hole structure that can form inclusion complexes with certain small molecules. However, the solubility of β-cyclodextrin (β-CD) in water is low and it crystallizes easily, leading to its low utilization in practice. In this experiment, we connected β-CD with glycine under alkaline conditions to prepare glycine-β-cyclodextrin (G-β-CD), which is water soluble, has stronger coordinating ability with heavy metals, and is more suitable for treating oil-contaminated soil. The results show that G-β-CD provides better desorption of petroleum hydrocarbons and heavy metals in soils with low organic matter content (1%) and NaNO₃ of 0.25 mol/L at 70 g/L G-β-CD under mildly acidic (pH 5⁻6) conditions. The results indicate that petroleum hydrocarbons and heavy metals were removed simultaneously by means of pretreatment with G-β-CD, and the results can provide a theoretical basis for remediation of petroleum-contaminated soil.

Super rapid removal of copper, cadmium and lead ions from water by NTA-silica gel

A silica gel material modified with nitrilotriacetic acid (NTA-silica gel) was sensibly designed and prepared via a simple method for the super rapid removal of Cu²⁺, Cd²⁺ and Pb²⁺ from water.

Sustainable technologies for water purification from heavy metals: review and analysis

We review and analyze current water purification technologies in the context of sustainability, and we introduce the Ranking Efficiency Product (REP) index, to evaluate their efficiency and implementation in this broader perspective.

Pb speciation in rare earth minerals and use of entropy and fuzzy clustering methods to assess the migration capacity of Pb during mining activities

Ionic rare earth mining is of strategic importance in China; however, the heavy metal pollution it causes is of great concern, both at home and abroad. Data on Pb speciation at a rare earth mine were used to analyse the migration capacity of Pb during the process of ore leaching. The major factors thought to influence the migration capacity of Pb, including the clay mineral content, soil pH, oxidation-reduction potential, organic matter content, and cation exchange capacity (CEC), were quantified. Entropy and fuzzy clustering methods were used to identify the most predictive factors. The results showed that the clay mineral content of the soil exerts the strongest influence on the migration capacity of Pb in rare earth mines; the other factors demonstrated relatively weak influences on the transport processes. The results obtained from the classification of Pb per the potential migration index (PMI) approximated those obtained from fuzzy clustering, and the results of entropy analyses were consistent with those of fuzzy clustering. Thus, the methods upon which the proposed model is based are suitable for predicting the migration capacity of Pb in ore during the mining of ionic rare earth minerals.

Modeling arsenic (V) removal from water by micellar enhanced ultrafiltration in the presence of competing anions

With increasing arsenic (As) contamination incidents reported around the world, better processes for As removal from industrial wastewater and other contaminated waters are required to protect drinking water sources. Complexation of As with cetylpyridinium chloride (CPC) cationic surfactant micelles, coupled with ultrafiltration (UF), has the potential to improve As removal, but competition from other anions could be a limiting factor. Using a binary-system ion-exchange model, the selectivity coefficients for binding of the monovalent and divalent forms of arsenate (As (V)) to cationic cetylpyridinium (CP⁺) micelles, relative to Cl^-, were determined to be 0.55 for H₂AsO₄^- and 0.047 mol L^-1 for HAsO₄^2-, respectively. The affinity sequence for binding of commonly occurring monovalent anions by CP⁺ micelles was found to be NO₃^- > Cl^- > HCO₃^- > H₂AsO₄^-, and for divalent anions, SO₄^2- > HAsO₄^2-. Distribution of As (V) between the micellar and aqueous phases was explored using ion exchange isotherms, with higher pH and lower concentrations of competing anions increasing rejection of As (V) across UF membranes. A model accounting for these effects, based on mass balances across UF membranes and selectivity coefficients for binding of anions to the CP⁺ micelles, was used to predict As (V) removal during micellar-enhanced ultrafiltration (MEUF) of mixtures of competing anions. Model predictions agreed well with experiment results for both artificial and spiked natural river water samples. Arsenic (≈0.1 mM) removals of 91% and 84% were achieved from artificial waters and spiked natural river waters, respectively, by adding 20 mM CPC prior to UF.

A novel natural chitosan/activated carbon/iron bio-nanocomposite: Sonochemical synthesis, characterization, and application for cadmium removal in batch and continuous adsorption process

The natural chitosan was synthesized using shrimp shells via sonochemical method, and activated carbon produced from grape stalks biomass. The novel bio-nanocomposite of chitosan/activated carbon/iron nanoparticles was synthesized via the sonochemical method and characterized using FTIR, SEM, and BET techniques. This bio-based nanocomposite was utilized to cadmium removal from dilute solution. The adsorption process via batch method was optimized, and the impacts of pH of feed, the dosage of adsorbent, and concentration of cadmium were analyzed. The kinetics and equilibrium analysis was done, and results indicate the predomination of chemical absorption and the single-layer adsorption process. Langmuir data indicates that the synthesized bio-nanocomposite can adsorb 344 mg cadmium per each gram. To evaluate the ability of the synthesized nanocomposite in the industrial application, the adsorption tests were done in a continuous adsorption system in three cycles.

Promoting the productivity and quality of brinjal aligned with heavy metals immobilization in a wastewater irrigated heavy metal polluted soil with biochar and chitosan

Depleting aquifers, lack of planning and low socioeconomic status of Pakistani farmers have led them to use wastewater (WW) for irrigating their crops causing food contamination with heavy metals and ultimately negative effects on human health. This study evaluates the effects of chitosan (CH) and biochar (BC) on growth and nutritional quality of brinjal plant together with in situ immobilization of heavy metals in a soil polluted with heavy metals due to irrigation with wastewater (SPHIW) and further irrigated with the same WW. Both CH and BC were applied at three different rates i.e. low rate [(LR), BC0.5%, CH0.5% and BC0.25%+CH0.25%], medium rate [(MR), BC1%, CH1% and BC0.5%+CH0.5%] and high rate [(HR), BC1.5%, CH1.5% and BC0.75%+CH0.75%]. Result revealed that brinjal growth, antioxidant enzymes, and fruit nutritional quality significantly improved from LR to HR for each amendment, relative to control. However, these results were more prominent with BC alone and BC+CH, compared with CH alone at each rate. Similarly, with few exceptions, significant reduction in Ni, Cd, Co, Cr and Pb concentrations in the root, shoot and fruit were found in sole CH treatment both at LR and MR but in both CH and BC+CH treatments at HR, relative to control. Interestingly, the concentrations of Fe in the roots, shoots and fruit were more pronounced at BC treatments relative to CH and BC+CH treatments at each rate, compared to control. Overall, the BC+CH treatment at HR was the most effective treatment for in situ immobilization of heavy metals in SPHIW and further irrigated with the same WW, compared to rest of the treatments. This study indicates that BC0.75%+CH0.75% treatment can be used to reduce mobility and bioavailability of heavy metals in SPHIW and facilitates plant growth by improving the antioxidant system. However, the feasibility of BC0.75%+CH0.75% treatment should also be tested at the field scale.

Efficient removal of Cd2+ and Pb2+ from aqueous solution with amino- and thiol-functionalized activated carbon: Isotherm and kinetics modeling

In order to address the increasingly severe pollution issue caused by heavy metals, activated carbon-based absorbents have gained considerable attention. Herein, two novel adsorbents, amino-functionalized activated carbon (N-AC) and thiol-functionalized activated carbon (S-AC), were successfully synthesized by stepwise modification with tetraethylenepentamine (TEPA), cyanuric chloride (CC) and sodium sulfide. The pristine and synthesized materials were characterized by BET analysis, SEM, FTIR spectroscopy, elemental analysis and zeta-potential analyzer. Meanwhile, their adsorption properties for Cd²⁺ and Pb²⁺ and the effects of various variables on the adsorption processes were systematically investigated. The findings confirmed that amino-groups and thiol-groups endowed the AC with a strong affinity for metal ions and that the pH of solution affected the uptake efficiencies of the adsorbents by influencing their surface charges. Furthermore, six isotherm models (Langmuir, Freundlich, Temkin, Dubinin-Radushkevich, Sips and Redlich-Peterson) and four kinetic models (pseudo-first-order, pseudo-second-order, Intra-particle diffusion and Elovich) were applied to interpret the adsorption process at three different temperatures (288 K, 298 K and 308 K). The results indicated that temperature played an important role and that the rate-limiting step was chemosorption. A better fit for all adsorption systems was obtained with Langmuir model, with the maximum adsorption capacities at 298 K of 79.20 mg Cd²⁺/g and 142.03 mg Pb²⁺/g for N-AC, 130.05 mg Cd²⁺/g and 232.02 mg Pb²⁺/g for S-AC, respectively. Subsequently, the thermodynamic parameters revealed the nature of the adsorption was endothermic and spontaneous under the experimental condition. The possible adsorption procedures and the underlying mechanisms comprising physical and chemical interactions were proposed. Moreover, the as-synthesized adsorbents exhibited excellent regeneration performance after five adsorption/desorption cycles. The overall results demonstrated that both N-AC and S-AC could be the promising efficient candidates for removing Cd²⁺ and Pb²⁺ from contaminated water.

Lead and copper removal from groundwater by spherical agglomeration using a biosurfactant extracted from Yucca decipiens Trel

The spherical agglomeration technique (SAT) has emerged as an innovative alternative for the removal of heavy metals from water at optimum levels of surfactant addition. This technique has achieved high removal efficiencies. In the present study, Yucca decipiens extracts were applied as the biosurfactant for the removal of heavy metals from groundwater of a mining community using SAT. Aqueous models were generated to explain the removal of copper and lead in solutions. It was possible to remove 99.96% and 99.62% respectively. The highest concentrations of copper and lead 209.5 and 2 mg L^-1, respectively, were observed at a waterhole in the mining community. This sample was used to test the efficiency of SAT, using optimal conditions of the models. It was possible to remove 99.22% of copper and 91.50% of lead present in the groundwater. High concentrations of sodium and calcium were found. To reduce the residual sodium concentration, the pH was decreased from 11 to 9.5. 99.84% of copper and only 93.49% of lead were removed; the remaining concentrations did not exceed the limit of Mexican regulations (NOM-001-SEMARNAT-1996, 1996). It was demonstrated that the Yucca extracts are effective or the treatment of water with high concentrations of heavy metals under the conditions of SAT.

Impacts of sodium hydroxide and sodium hypochlorite aging on polyvinylidene fluoride membranes fabricated with different methods

This study compared the effects of chemical aging on the polyvinylidene fluoride (PVDF) membranes fabricated with the methods of non-solvent induced phase separation (NIPS) (named NIPS-PVDF) and thermally induced phase separation (TIPS) (named TIPS-PVDF). The chemical solutions of sodium hypochlorite (NaClO) and sodium hydroxide (NaOH) were chosen at the concentration of 5000mg/L. The equivalence of 5 and 10years was respectively selected as the time of aging. The physicochemical evolutions of membrane aging are characterized on the base of morphology analysis, chemical components, permeation ability and mechanical properties. The aging of NIPS-PVDF membrane led to the elimination of surface hydrophilic additives, while NaOH focused on the dehydrofluorination process resulting in the formation of conjugated chains of polyene on the skeleton structure. The chemical components of the surface of TIPS-PVDF membrane were removed continuously during the aging processes of both NaClO and NaOH, which was caused by the saponification of surface additives and the chain scissions of skeleton structure, but without producing any obvious conjugated chains of polyene. All the aging processes led to the increase of contact angle and the decrease of mechanical properties, and the permeability was reduced first and increased later due to the enlargement of surface membrane pores and membrane block. With the influence of membrane aging, selectivity of membrane was decreased (except coliform bacteria). At the beginning of filtration, the turbidity and particle count were at relatively high levels and declined with the filtration process.

A review of functionalized carbon nanotubes and graphene for heavy metal adsorption from water: Preparation, application, and mechanism

Carbon-based nanomaterials, especially carbon nanotubes and graphene, have drawn wide attention in recent years as novel materials for environmental applications. Notably, the functionalized derivatives of carbon nanotubes and graphene with high surface area and adsorption sites are proposed to remove heavy metals via adsorption, addressing the pressing pollution of heavy metal. This critical revies assesses the recent development of various functionalized carbon nanotubes and graphene that are used to remove heavy metals from contaminated water, including the preparation and characterization methods of functionalized carbon nanotubes and graphene, their applications for heavy metal adsorption, effects of water chemistry on the adsorption capacity, and decontamination mechanism. Future research directions have also been proposed with the goal of further improving their adsorption performance, the feasibility of industrial applications, and better simulating adsorption mechanisms.