Heavy Metals In Wastewater Research Articles

Customized Utilization Strategies of Industrial Lignin to Produce Adsorbents and Flocculants Based on Fractionation and Adequate Structural Interpretation.

Lignin, a by-product of pulping and biorefinery, has great potential to replace petrochemical resources for wastewater purification. However, the defects of lignin, such as severe heterogeneity, inferior reactivity and poor solubility, characterize the production process of lignin-based products by high energy consumption and serious pollution. In this study, several lignin fractions with relatively homogeneous structure were first obtained by organic solvent fractionation, and their structures were fully deciphered by various characterization techniques. Subsequently, each lignin component was custom-valued for wastewater purification based on their structural characteristics. Benefiting from the high reactivity and reaction accessibility, the lignin fraction (lignin-1) refined by dissolving in ethanol and n-butanol could been used as a raw material to produce cationic lignin-based flocculant (LBF) in a copolymerization system using green, cheap and recyclable ethanol as solvent. The lignin fraction (lignin-2) extracted by methanol and dioxane showed low reactivity and high carbon content, which was used to produce lignin-based activated carbon (LAC) with phosphoric acid as activator. Moreover, the influences of synthetic factors on the purification capacity were discussed, and the LBF and LAC produced under the optimal conditions showed distinguished purification effect on kaolin suspension and heavy metal wastewater, respectively. Furthermore, the corresponding purification mechanism and external factors were also elaborated. It is believed that this cleaner production strategy is helpful for the valorization of lignin in wastewater resources.

Development of nitric acid-modified activated carbon electrode for removal of Co2+/Mn2+/Ni2+ by electrosorption.

In this paper, nitric acid-modified activated carbon was used as an electrode in the electrosorption process for the removal of Co2+, Mn2+, and Ni2+ from wastewater. The effects of applied voltage, initial pH, and coexisting ions on removal efficiency were investigated. The adsorption process was evaluated by adsorption isotherm models. The results indicated that the electrosorption process was consistent with the Langmuir model, proving that the electrosorption process was a monolayer adsorption process. The maximum adsorption capacities of Co2+, Mn2+, and Ni2+ were 131.58mg/g, 102.04mg/g, and 103.09mg/g. Electrochemical tests revealed that the specific capacitance of AC-HNO3 was 54.11 F/g when the scanning rate was 5mV/s, while the specific capacitance of AC was 36.51 F/g. The Fourier transform infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS) confirmed that the content of oxygen groups on the surface of activated carbon increased after modification, which provided more adsorption sites for electrosorption. When the selected concentration of HCl was used as the eluent, the elution efficiency of Co2+, Mn2+, and Ni2+ could reach 94.23%, 93.65%, and 90.61%. The removal efficiency could reach more than 95% after three cycles. The results of the study can be used as a reference significance for the removal of cobalt, manganese, and nickel ions from heavy metal wastewater by electrosorption.

Progress in the Application of MOFs in the Field of Atmospheric Environment

Since its first appearance in 1995, metal-organic framework materials (MOFs) have become a research hotspot in the field of environmental treatment due to its advantages of large specific surface area, high porosity and adjustable pore size, especially in the treatment of dye wastewater and heavy metal wastewater. With the deepening of research, as a kind of crystalline material with porous structure, MOFs have a wide application prospect in the field of air purification materials due to their strong adsorption function. This paper summarizes the types and synthesis methods of MOFs, and reviews the application research progress of MOFs in gas separation, gas storage and particulate matter purification. At present, MOFs still have some problems, such as poor stability, low recycling efficiency, high cost and weak functionalization, etc. In the end of this paper, some research suggestions are put forward to solve these problems.

Utilization of biosynthesized silica-supported iron oxide nanocomposites for the adsorptive removal of heavy metal ions from aqueous solutions.

This study deals with heavy metal ions removal from simulated water using biosynthesized silica-supported iron oxide nanocomposites (nano-IOS). Agricultural and garden wastes have been utilized to prepare nano-IOS through a green synthesis process. Nano-IOS was characterized by XRD, SEM, FTIR, and zeta potential analysis. The nanocomposites were used to remove five heavy metals, viz., Pb2+, Cd2+, Ni2+, Cu2+, and Zn2+, with optimization of reaction parameters including pH, the concentration of heavy metals, adsorbent dosage, and contact time in batch mode experiments. The optimized dose of nano-IOS was 0.75g/L for the adsorption of Pb2+, Cd2+, Ni2+, Cu2+, and Zn2+ (10.0mg/L) with a contact duration of 70min at pH 5.0 for Pb2+, Cd2+, and Cu2+ and 6.0 for Ni2+ and Zn2+. The adsorption behavior of the nano-adsorbent was well described by Langmuir adsorption isotherm and pseudo-second-order kinetic model indicating chemisorption on the surface of nano-IOS. The adsorption was also found spontaneous and endothermic. Thus, the environmentally benign and bio-synthesized nano-IOS can be utilized as an effective nano-adsorbent for the rapid sequestration of heavy metal ionsfrom water and wastewater.

Shapeable amino-functionalized sodium alginate aerogel for high-performance adsorption of Cr(VI) and Cd(II): Experimental and theoretical investigations

Herein, an elastic polyethyleneimine/sodium alginate aerogel was designed by a one-step freeze-drying method. The 3D honeycomb-like structure endowed it excellent compressibility and plentiful exposed adsorption sites to accelerate Cr(VI) and Cd(II) adsorption. The adsorption processes matched the pseudo-second-order and Langmuir models, and the saturated adsorption capacity for Cr(VI) and Cd(II) was 678.67 and 464.23 mg/g, respectively. The obtained aerogel still expressed superior removal performance for Cr(VI) and Cd(II) even in the coexistent of multiple interfering factors and maintained stable adsorption capacity at least after 5 cycles. Importantly, the elastic cylindrical aerogel (∼0.59 g) could achieve dynamic filtration of industrial wastewater containing Cr(VI) and Cd(II) in fix-bed column system with corresponding effective treatment volume of 4.47 and 3.25 L, respectively. The reasonable mechanisms for the adsorption behaviors of heavy metals on the as-prepared aerogel was investigated via FTIR, XPS and Density functional theory (DFT), which could be attributed to the surface electrostatic adsorption, chelation and reduction effects. Eventually, the ecological assessment verified that eco-friendly SA/PEI aerogel had a powerful auxiliary effect on the anti-fouling growth of rice. Overall, such novel biomass aerogel adsorbent possessed great application potential and theoretical guiding significance for the purification of heavy metal wastewater.

Kinetics and Adsorption Isotherms of Amine-Functionalized Magnesium Ferrite Produced Using Sol-Gel Method for Treatment of Heavy Metals in Wastewater.

This study is focused on the kinetics and adsorption isotherms of amine-functionalized magnesium ferrite (MgFe2O4) for treating the heavy metals in wastewater. A sol-gel route was adopted to produce MgFe2O4 nanoparticles. The surfaces of the MgFe2O4 nanoparticles were functionalized using primary amine (ethanolamine). The surface morphology, phase formation, and functionality of the MgFe2O4 nano-adsorbents were studied using the SEM, UV-visible, FTIR, and TGA techniques. The characterized nanoparticles were tested on their ability to adsorb the Pb2+, Cu2+, and Zn2+ ions from the wastewater. The kinetic parameters and adsorption isotherms for the adsorption of the metal ions by the amine-functionalized MgFe2O4 were obtained using the pseudo-first-order, pseudo-second-order, Langmuir, and Freundlich models. The pseudo-second order and Langmuir models best described the adsorption kinetics and isotherms, implying strong chemisorption via the formation of coordinative bonds between the amine groups and metal ions. The Langmuir equation revealed the highest adsorption capacity of 0.7 mmol/g for the amine-functionalized MgFe2O4 nano-adsorbents. The adsorption capacity of the nanoadsorbent also changed with the calcination temperature. The MgFe2O4 sample, calcined at 500 °C, removed the most of the Pb2+ (73%), Cu2+ (59%), and Zn2+ (62%) ions from the water.

Preparation, Modification and Characterization of Chitosan-Oxalic Kola Nut Pod Adsorbent

Abstract: This work investigated the preparation, modification and characterization of prepared adsorbent from kola nut pod (agro waste).The kola nut pod waste was obtained from Ipetu-Ijesha farm, modified using chitosan-oxalic gel and characterized using Scanning Electron micrograph -Energy dispersive X-ray and FTIR spectrophotometer. The morphological study revealed that the chemically modified adsorbent has high porosity and large surface area for effective adsorption of metal ions in aqueous solution, The EDX analysis revealed that the chitosan-oxalic modified kola nut pod adsorbent contains C (0.01%), N (14.46%), K (7.76%), O (14.48%), Si (0.09%) and Ca (62.24%). The FTIR spectrum shows some adsorption peaks that indicate the functional groups on the adsorbent This further suggested that the adsorbent has potential adsorption sites as represented by the functional groups OH, C-H, C-C, and COOH. The results of Physico chemical properties showed that moisture; 8.32±0.07, Ash; 6.31±0.04, lignin; 12.97±0.03, cellulose; 23.53±0.04, Hemicellulose; 16.76±0.02, pectin; 7.74±0.03 and extractives 24.37±0.04. This results showed that this kola nut pod contains considerable amount of lignin, cellulose and hemicelluloses thus, will form a very good adsorbent for removal of dyes and heavy metals in wastewaters. Keyword: modification, characterization, chitosan-oxalic, kola nut pod waste, Physico chemical properties, morphological study.

Assessment of Physicochemical Parameters and Heavy Metals in Sagamu Abattoir Waste Water Ogun State, Nigeria

Abattoir wastewater is a great threat to environmental safety. This study was aimed at assessing the physiochemical constituents using standard methods and some heavy metals using Agilent Microwave Atomic Emission Spectrometer (MP-AES) 4200 model after sample digestion of wastewater from three (3) major abattoirs in Sagamu, Ogun State, Nigeria. The sample was digested using a mixture of nitric acid, hydrochloric acid and hydrogen peroxide. The digested samples were then analyzed for heavy metals. Data from the physicochemical characterization show that pH, TS, TDS, TSS and COD ranged from; (5.8 – 6.2), (11500 – 27733.33), (5500 – 9066.67), (6000 – 18666.67), and (1507.67 – 3671) respectively. The assessed heavy metals are in the range (mg/L): Zn (0.132-0.337), Cu (0.091 – 0.516), Mn (0.410 – 0.994), Fe (5.294 – 15.44), V (0.132 – 0.32), Cd (0.036 – 0.119), Ni (0.029 – 0.17), Pb (0.136 – 0.234), Cr (0.299 – 1.277), and Co (0.015 – 0.079). Zn, Cu and Fe are essential metals within tolerable limits except Mn that exceeded the limits of World Health Organization (0.04 mg/L) but below Federal Ministry of Environment (5 mg/L) while Ni, Pb and Chromium are above permissible limits in all samples. The pretreatment of the wastewater before discharging into environment is highly recommended and more research should be done on remediation to reduce the abattoir wastewater heavy metals concentration and the effect it is having on the ecosystem and the environment at large.

Modified Polymeric Biosorbents from Rumex acetosella for the Removal of Heavy Metals in Wastewater.

The contamination of water resources by effluents from various industries often contains heavy metals, which cause irreversible damage to the environment and health. The objective was to evaluate different biosorbents from the weed Rumex acetosella to remove metal cations in wastewater. Drying, grinding and sieving of the stems was carried out to obtain the biomass, retaining the fractions of 250 to 500 µm and 500 to 750 µm, which served to obtain the biosorbents in natura (unmodified), acidic, alkaline, and mixed. Proximal analysis, PZC, TOC, removal capacity, influence of pH, functional groups, thermal analysis, structural characteristics, adsorption isotherms, and kinetic study were evaluated. The 250 µm mixed treatment was the one that presented the highest removal percentages, mainly due to the OH, NH, -C-H, COOH, and C-O functional groups achieving the removal of up to 96.14% of lead, 36.30% of zinc, 34.10% of cadmium and 32.50% of arsenic. For contact times of 120 min and an optimum pH of 5.0, a loss of cellulose mass of 59% at 328 °C and a change in the surface of the material were also observed, which allowed for obtaining a topography with greater chelating capacity, and the Langmuir and pseudo-second order models were better fitted to the adsorption data. The new biosorbents could be used in wastewater treatment economically and efficiently.

Construction and performance analysis of a microbial electrochemical sensor for monitoring heavy metals in water environment

A microbial fuel cell (MFC)-based microbial electrochemical sensor was developed for real-time on-line monitoring of heavy metals in water environment. The microbial electrochemical sensor was constructed with staggered flow distribution method to optimize the parameters such as external resistance value and external circulation rate. The inhibition of concentration of simulated heavy metal wastewater on voltage under optimal parameters was analyzed. The results showed that the best performance of MFC electrochemical sensor was achieved when the external resistance value was 130 Ω and the external circulation rate was 1.0 mL/min. In this case, the microbial electrochemical sensors were responsive to 1-10 mg/L Cu2+, 0.25-1.25 mg/L Cd2+, 0.25-1.25 mg/L Cr6+ and 0.25-1.00 mg/L Hg2+ within 60 minutes. The maximum rejection rates of the output voltage were 92.95%, 73.11%, 82.76% and 75.80%, respectively, and the linear correlation coefficients were all greater than 0.95. In addition, the microbial electrochemical sensor showed a good biological reproducibility. The good performance for detecting heavy metals by the newly developed microbial electrochemical sensor may facilitate the real-time on-line monitoring of heavy metals in water environment.

Preparation of Magnetic Activated Carbon by Activation and Modification of Char Derived from Co-Pyrolysis of Lignite and Biomass and Its Adsorption of Heavy-Metal-Containing Wastewater

Adsorption with activated carbon (AC) is an important method for the treatment of heavy metal wastewater, but there are still certain challenges in the separation and reuse of activated carbon. The preparation of magnetic activated carbon (MAC) by modifying AC is one of the effective means to realize the separation of AC from solution after the adsorption process. In this work, lignite and poplar leaves were used as raw materials for co-pyrolysis, and the co-pyrolysis char was activated and modified to prepare MAC. The structure and properties were characterized by VSM, N2 adsorption, SEM, XRD, and FT-IR. At the same time, the adsorption performance of MAC on wastewater containing Pb and Cd ions was studied. The results show that the prepared MAC contains Fe3O4, and the saturation magnetization (Ms) of the MAC is 13.83 emu/g; the specific surface area of the MAC is 805.86 m2/g, and the micropore volume is 0.23 cm3/g; the MAC exhibited a good porous structure. When the pH value of the solution was 5, the adsorption time was 120 min, the dosage of MAC was 4 g/L, the initial concentration of Pb ion solution was 50 mg/L, and that of Cd ion solution was 25 mg/L, and the adsorption temperature was 30 °C, the adsorption efficiency of Pb, Cd ions were 84.40 and 78.80%, respectively, and the adsorption capacities were 10.55 and 4.93 mg/g, respectively. The adsorption of Pb and Cd ions by MAC conforms to the Langmuir adsorption model, which is a monolayer adsorption. The adsorption process is mainly chemical adsorption, which can be better described by the pseudo-second-order model. The adsorption thermodynamic analysis showed that the adsorption of Pb and Cd ions by MAC was a spontaneous reaction, and the higher the temperature, the stronger the spontaneity.

Floating-separation adsorbent for methylene blue and Pb(II) removal: Structure construction and adsorption mechanism

Inferior solid–liquid separation behavior and inadequate adsorption capacity are the main bottlenecks of current wastewater treatment adsorbents, especially in the purification of wastewater containing dyes and heavy metals. Accordingly, a novel floating separation adsorbent with polymeric chain structure, hollow glass sphere-g-(3-aminopropyl) triethoxysilane-g-poly(2-acrylamide-2-methyl-1-propanesulfonic acid) (APHGS-g-P(AMPS)), was fabricated through an electrophilic addition post-grafting strategy. The obtained APHGS-g-P(AMPS) exhibited satisfactory adsorption for methylene blue (MB) and lead ion (Pb(II)), as well as efficient flotation. Adsorption behavior analysis showed that the Pseudo-second-order kinetic model and Langmuir isotherm model were suitable for describing the adsorption process, and the maximum adsorption capacities for MB and Pb(II) were 249.33 and 345.93 mg·g−1 (313.15 K), respectively. Analysis of the adsorption mechanism indicated that the sulfonic acid group exhibited excellent universal adsorption for MB and Pb(II), which played a crucial role in the adsorption process. Furthermore, the efficient grafting of P(AMPS) polymerized chains onto the adsorbent was the pivotal factor to enhance adsorption. This study provided an efficient electrophilic addition post-grafting strategy for the structural design and functional modification of floating separation adsorbents. The results of this study are expected to have positive implications for the efficient purification of dyes and heavy metals wastewater.

Enhanced visible-light-driven photocatalytic activity of bi-phase titanium dioxide @ covalent organic framework Z-scheme system for photocatalytic removal of Cr (Ⅵ)

The discharge of industrial heavy metal wastewater not only leads to environmental pollution, but also harms human health, so it is imperative to control heavy metal pollution. In the current work, a sequence of anatase/brookite TiO2@COF with different mass ratios, bi-phase TiO2 and bare COF were successfully synthesized by simple hydrothermal synthesis. The photocatalytic activity of the prepared catalysts was assessed by reducing Cr(Ⅵ) in model pollutant (K2Cr2O7). Under the condition of pH = 3, the Cr(Ⅵ) reduction rate of TiO2@COF-4 was as high as 99.19% in 30 min. The photocatalytic activity of TiO2@COF-4 remained stable in four consecutive cycles, suggesting that it has a potential application prospect for the removal of Cr(Ⅵ) in the actual environment. The morphology and structure of photocatalysts were observed by different techniques (SEM/EDS, HRTEM, BET analysis, XRD, FTIR and XPS). The specific reasons for the enhancement of photocatalytic performance of materials were explored by different methods (PL, UV–vis DRS, Mott-Schottky plots, I-T curves and EIS). Finally, we concluded that the photocatalytic system of TiO2@COF-4 may follow the typical direct Z- scheme mechanism. This work provides an effective solution for the water treatment of heavy metal pollution.

Concentrations of Zn, Mn and Al in wood chips from wood-based manufacturing industries

The elemental composition of wood chip samples manufactured in the wood industries at Batu Pahat were analyzed for their heavy metals composition. This study was aimed to prove that heavy metals can accumulate in wood and trees themselves without adding any wood treatment additives. ICP-MS analysis was conducted on the wastewater at the same manufacturing industry to evince the occurrence of the leaching process that causes the presence of heavy metals in the wastewater. The results of the experiments showed that Mn, Zn and Al were among the highest metal traces found in the wood chips. As a result, Mn concentrations in three different samples of wood chips were 83.6 mg/L, 88.1 mg/L, and 80.9 mg/L, whereas Zn amounts were 38.8 mg/L, 40.2 mg/L, and 37.7 mg/L. Furthermore, the amounts of Al traces in the three samples were 28.2 mg/L, 29.6 mg/L, and 30.7 mg/L, respectively. The study also found that the highest metal trace in wastewater shows a proportional value to the highest concentration of heavy metals in wood chips. Therefore, it can be concluded that, even though the whole chipping production process is considered to be a dry process, the wastewater generated after cleaning surfaces, storage tanks and machinery, causes heavy metal leaching and contributes to the presence of heavy metals in wastewater.

Heavy metal toxicity of drinking water: A silent killer

Heavy metal toxicity has proven to be a major threat and there are several health risks associated with it. The toxic effects of these metals, even though they do not have any biological role, remain present in some or the other form harmful for the human body and its proper functioning. Heavy metals are generally referred to as those metals which possess a specific density of more than 5 g/cm3 and adversely affect the environment and living organisms. These metals are essential to maintain various biochemical and physiological functions in living organisms when in very low concentrations, however they become noxious when they exceed certain threshold concentrations. The most commonly found heavy metals in waste water include arsenic, cadmium, chromium, and copper, lead, nickel, and zinc, all of which cause risks for human health and the environment. Heavy metals enter the surroundings by natural means and through human activities. Various sources of heavy metals include soil erosion, natural weathering of the earth's crust, mining, industrial effluents, urban runoff, sewage discharge, insect or disease control agents applied to crops etc. Some heavy metals, i.e. arsenic, lead, mercury, cadmium, chromium, aluminium causes drastic harmful effect on the environment and living organisms, mainly human beings.

Efficient simultaneous removal of tetracycline hydrochloride and Cr(VI) through photothermal-assisted photocatalytic-Fenton-like processes with CuOx/γ-Al2O3

Advanced oxidation processes (AOPs) based on photo-Fenton reaction has been widely used in refractory organic wastewater treatment. However, the mineralization rate and H2O2 utilization in AOPs is relatively low. Herein, a photothermal-assisted photocatalytic-Fenton-like process with CuOx/γ-Al2O3 catalyst was designed to solve the above issues. The utilization rate of H2O2 and mineralization rate of tetracycline hydrochloride (TC) in the 3%-CuOx/γ-Al2O3/H2O2/full spectrum light system were significantly increased to 72.0% and 74.3%, respectively, which were ascribed to the synergy of photothermal effect and photocatalytic-Fenton-like reaction. During the simultaneous removal of TC and Cr(VI), the reaction efficiency of 3%-CuOx/γ-Al2O3/H2O2/light system was much higher than that of 3%-CuOx/γ-Al2O3/H2O2 system. In addition, the 3%-CuOx/γ-Al2O3/H2O2/light system was not only active in wider pH range (3–9), but also effective in various organic pollutants and Cr(VI) coexisted solutions as well as different water conditions. More importantly, a possible photothermal-assisted photocatalytic-Fenton-like reaction mechanism for the simultaneous removal of TC and Cr(VI) in the CuOx/γ-Al2O3/H2O2/light system was proposed. This work may pave a new way for efficient simultaneous removal of refractory organic pollutants and heavy metals in wastewater.

Compressible amino-modified carboxymethyl chitosan aerogel for efficient Cu(II) adsorption from wastewater

Copper ions (Cu(II)) in water pose a huge threat to human health and ecosystem security. Therefore, developing eco-friendly and sustainable strategies to eliminate the toxicity risk of Cu(II) in the aqueous environment remains a great challenge. In this work, an elastic polyethyleneimine-modified carboxymethyl chitosan aerogel was developed via a freeze-drying process for the adsorption of Cu(II). The adsorption kinetic and isotherm experiments demonstrated that the adsorption process was monolayer chemisorption and the maximum adsorption capacity calculated from Langmuir model was 175.56 mg g−1. The obtained aerogels exhibited preferential selectivity for Cu(II) even when the competing cations and organics existed. Besides, about 0.93 g of aerogel realized continuous fixed-bed column filtration of industrial wastewater with an effective purification volume of 4.14 L. Mechanistic analysis via FTIR and XPS revealed that the excellent adsorption performance for Cu(II) might be attributed to the strong electrostatic force, Vander Waals' force and coordination of N/O -containing functional groups. It is worth mentioning that the adsorbed Cu(II) ions in the aerogel were converted into a reusable photocatalyst, avoiding the secondary leakage of heavy metals. This work opened a door to synthesize carboxymethyl chitosan aerogel, promising for heavy metal wastewater treatment and realizing the concept of waste utilization.

Adsorptive Removal of Multi-Metal Ions in Wastewater Using Electrospun Cellulose Acetate / Iron-Modified Nanozeolite Nanofibrous Membrane

In this undertaking, nanofibrous membrane of cellulose acetate (CA) with varying concentrations of iron-modified nanozeolite (Fe-MNZ) were produced through electrospinning technique for the simultaneous adsorption of Ni2+, Co2+ and Cu2+ heavy metal ions in wastewater. The electrospun nanofibrous membranes (ENMs) produced underwent different characterization techniques to determine the effect of Fe-MNZ addition on the ENM of CA in terms of porosity and adsorption capacity. Porometry results and contact angle measurements confirmed the increase in fiber diameter through the decrease in pore size and increase in the wettability of the ENMs produced. The adsorption experiment showed that ENM blend 2, M2, containing 1.0 wt.% Fe-MNZ had the highest removal efficiency for Ni2+ ions at around 79.20%, while ENM blend 3, M3, which contained 1.2 wt.% Fe-MNZ had adsorbed most of the Co2+ and Cu2+ ions with removal efficiencies equal to 54.04% and 100%, respectively. Adsorption of Ni2+ and Co2+ ions was governed by Freundlich isotherm and pseudo-first order kinetics, whereas the adsorption data of Cu2+ ions best fitted both Langmuir and Freundlich isotherms, and pseudo-second order kinetics. Lastly, the adsorption of the heavy metal ions was verified by the results of SEM-EDX.

Hydrogel-Based Adsorbent Material for the Effective Removal of Heavy Metals from Wastewater: A Comprehensive Review.

Water is a vital resource that is required for social and economic development. A rapid increase in industrialization and numerous anthropogenic activities have resulted in severe water contamination. In particular, the contamination caused by heavy metal discharge has a negative impact on human health and the aquatic environment due to the non-biodegradability, toxicity, and carcinogenic effects of heavy metals. Thus, there is an immediate need to recycle wastewater before releasing heavy metals into water bodies. Hydrogels, as potent adsorbent materials, are a good contenders for treating toxic heavy metals in wastewater. Hydrogels are a soft matter formed via the cross-linking of natural or synthetic polymers to develop a three-dimensional mesh structure. The inherent properties of hydrogels, such as biodegradability, swell-ability, and functionalization, have made them superior applications for heavy metal removal. In this review, we have emphasized the recent development in the synthesis of hydrogel-based adsorbent materials. The review starts with a discussion on the methods used for recycling wastewater. The discussion then shifts to properties, classification based on various criteria, and surface functionality. In addition, the synthesis and adsorption mechanisms are explained in detail with the understanding of the regeneration, recovery, and reuse of hydrogel-based adsorbent materials. Therefore, the cost-effective, facile, easy to modify and biodegradable hydrogel may provide a long-term solution for heavy metal removal.

Insight into different removal behaviors and mechanism of heavy metals via sulfate-loaded layered double oxide

Sulfur-containing materials with high affinity toward heavy metals have recently aroused great research enthusiasm. Yet the removal differences and mechanisms for various heavy metals remain unclear in view of rarely systematical reports. In this regard, a sulfate-loaded adsorbent (MgAl-LDO@S) synthesized by precursor-calcination strategy was employed for investigating the removal differences of heavy metals (i.e., Pb(Ⅱ), Cd(Ⅱ) and Ni(Ⅱ), typical heavy metals in wastewater). As predicted, a diverse adsorption capability for metals on MgAl-LDO@S was observed, following the order of Pb(Ⅱ) (631.90 mg g−1) > Cd(Ⅱ) (208.27 mg g−1) > Ni(Ⅱ) (119.02 mg g−1). Both adsorption isotherm and kinetic demonstrated that the monolayer, heterogeneous-homogeneous, and multilayer adsorption occurred in Pb(Ⅱ), Cd(Ⅱ) and Ni(Ⅱ), respectively, which were controlled by chemisorption. Furthermore, the removal differences were deeply exploded via the combination of XRD, FT-IR and XPS analyses and contribution calculation. The ion exchange was the main adsorption mechanisms for Pb (II) with 44.77%, while complexation dominated in Cd(II) and Ni(II) removal, with the proportion of 53.36% and 55.89%, respectively. Besides, the solubility constant (Ksp) of precipitate, affinity of sulfate group, as well as the self-characteristic properties contributed to various removal ability for heavy metal ions (e.g., ionic radius, electronegativity, and hard-soft-acid-base attribute) as well.