A novel method for the sequential removal and separation of multiple heavy metals from wastewater

Effect of organic matter on the environmental behavior of sulfur and heavy metals in mariculture sediments during the aging process

Organic matter (OM) significantly impacts the environmental behavior of sulfur and heavy metals. In this study, the effects of OM on the migration and transformation of sulfur and heavy metals in mariculture sediments were investigated. The results indicated that baiting had a strong impact on the accumulation of acid volatile sulfur (AVS) (P < 0.05) and increased the environmental risk of sulfide in sediments. The addition of bait promoted the generation of chromium (II)-reducible sulfur (CRS); however, the resistance of AVS to CRS conversion increased with increasing bait addition. The addition of bait considerably influenced Cd accumulation. The acid-soluble fractions of Cr and Cu and the oxidizable fraction of Cd were primarily affected by the bait addition (coefficient of variation>15 %). An increase in the reducible fraction promoted the conversion of AVS to CRS, which reduced the degree of sediment aging. Higher OM levels reduced the diversity and abundance of the bacterial communities. The sulfate respiration functional microbiota was particularly affected by OM.

Artificial intelligence and machine learning algorithms in the detection of heavy metals in water and wastewater: Methodological and ethical challenges

Heavy metals (HMs) enter waterbodies through various means, which, when exceeding a threshold limit, cause toxic effects both on the environment and in humans upon entering their systems. Recent times have seen an increase in such HM influx incident rates. This requires an instant response in this regard to review the challenges in the available classical methods for HM detection and removal. As well as provide an opportunity to explore the applications of artificial intelligence (AI) and machine learning (ML) for the identification and further redemption of water and wastewater from the HMs. This review of research focuses on such applications in conjunction with the available in-silico models producing worldwide data for HM levels. Furthermore, the effect of HMs on various disease progressions has been provided, along with a brief account of prediction models analysing the health impact of HM intoxication. Also discussing the ethical and other challenges associated with the use of AI and ML in this field is the futuristic approach intended to follow, opening a wide scope of possibilities for improvement in wastewater treatment methodologies.

Green and effective remediation of heavy metals contaminated water using CaCO3 vaterite synthesized through biomineralization

Heavy metal pollution has attracted significant attention due to its persistent presence in aquatic environments. A novel vaterite-based calcium carbonate adsorbent, named biogenic CaCO3, was synthesized utilizing a microbially induced carbonate precipitation (MICP) method to remediate heavy metal-contaminated water. The maximum Cd2+ removal capacity of biogenic CaCO3 was 1074.04 mg Cd2+/g CaCO3 with a high Cd2+ removal efficiency greater than 90% (initial Cd2+ concentration 400 mg/L). Furthermore, the biogenic CaCO₃ vaterite, induced by microbial-induced calcium carbonate precipitation (MICP) process, demonstrated a prolonged phase transformation to calcite and enhanced stability. This resulted in a sustained high effectiveness (greater than 96%) following six consecutive recycling tests. Additionally, X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses revealed that the semi-stable vaterite type of biogenic CaCO3 spontaneously underwent dissolution and recrystallization to form thermodynamic stable calcite in aquatic environments. However, the presence of Cd2+ leads to the transformation of vaterite into CdCO3 rather than undergoing direct converting to calcite. This transformation is attributed to the relatively low solubility of CdCO3 compared to calcite. Meanwhile, the biogenic CaCO3 proved to be an efficient and viable method for the removal of Pb2+, Cu2+, Zn2+, Co2+, Ni2+ and Mn2+ from water samples, surpassing the performance of previously reported adsorbents. Overall, the efficient and promising adsorbent demonstrates potential for practical in situ remediation of heavy metals-contaminated water.

Retention properties and mechanism of agricultural waste maize whisker on atmospheric mercury

Mercury (Hg) is a global pollutant transmitted mainly through the atmosphere, posing a serious threat to biological survival and human health. Porous materials, with high specific surface area, high porosity, and high adsorption, are particularly suitable for the purification of atmospheric Hg mixtures. However, plant porous materials are rarely directly used for atmospheric Hg purification. In this study, the properties and mechanism of maize whisker in removing atmospheric Hg were analyzed. The results show that the Hg content in the whiskers increases significantly as the initial Hg concentration increases, and 79.38% Hg can be removed by 0.2 g maize whiskers after 1 h exposure when the initial Hg concentration is 0.1 μg m-3, indicating that maize whiskers can accumulate atmospheric Hg rapidly and effectively. The hole diameter of the maize whisker is between 0.83 and 3.06 μm, which is suitable for the adsorption of small substances. Correlation analysis shows that maize whiskers have a significant correlation between atmospheric Hg retention and its specific surface area, pore size, medium pore ratio, and micropore ratio, suggesting that the maize whisker hole feature has a significant influence on its ability to retain atmospheric Hg. Compared with the energy profiles before and after Hg treatment, the peak of Mg decreased after Hg adsorption. Fourier infrared spectrometer analysis suggests that functional groups such as -OH, -COOH, and -O- are involved in the adsorption process. The change in pH value shows an obvious effect on the overall change in zeta potential in the adsorption process. Therefore, a variety of mechanisms, including physical adsorption, electrostatic adsorption, complexation, chelation, and ion exchange, are involved in Hg retention with the maize whisker. This study reveals the important potential value of agricultural waste maize whiskers in the purification of atmospheric heavy metal Hg.

Bi-MOF-Derived Carbon Wrapped Bi Nanoparticles Assembly on Flexible Graphene Paper Electrode for Electrochemical Sensing of Multiple Heavy Metal Ions

The development of nanohybrid with high electrocatalytic activity is of great significance for electrochemical sensing applications. In this work, we develop a novel and facile method to prepare a high-performance flexible nanohybrid paper electrode, based on nitrogen-doped carbon (NC) wrapped Bi nanoparticles (Bi-NPs) assembly derived from Bi-MOF, which are decorated on a flexible and freestanding graphene paper (GP) electrode. The as-obtained Bi-NPs encapsulated by an NC layer are uniform, and the active sites are increased by introducing a nitrogen source while preparing Bi-MOF. Owing to the synergistic effect between the high conductivity of GP electrode and the highly efficient electrocatalytic activity of Bi-NPs, the NC wrapped Bi-NPs (Bi-NPs@NC) modified GP (Bi-NPs@NC/GP) electrode possesses high electrochemically active area, rapid electron-transfer capability, and good electrochemical stability. To demonstrate its outstanding functionality, the Bi-NPs@NC/GP electrode has been integrated into a handheld electrochemical sensor for detecting heavy metal ions. The result shows that Zn²⁺, Cd²⁺, and Pb²⁺ can be detected with extremely low detection limits, wide linear range, high sensitivity, as well as good selectivity. Furthermore, it demonstrates outstanding electrochemical sensing performance in the simultaneous detection of Zn²⁺, Cd²⁺, and Pb²⁺. Finally, the proposed electrochemical sensor has achieved excellent repeatability, reproducibility, stability, and reliability in measuring real water samples, which will have great potential in advanced applications in environmental systems.

Biologically-induced synthetic manganese carbonate precipitate (BISMCP) for potential applications in heavy metal removal

Heavy metal pollution of water is a burning issue of today's world. Among several strategies involved for heavy metal remediation purpose, biomineralization has shown great potential. Of late, research has been focused on developing effective mineral adsorbents with reduced time and cost consumption. In this present paper, the Biologically-Induced Synthetic Manganese Carbonate Precipitate (BISMCP) was produced based on the biologically-induced mineralization method, employing Sporosarcina pasteurii in aqueous solutions containing urea and MnCl₂. The prepared adsorbent was characterized using Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), SEM-energy dispersive X-ray spectroscopy (SEM-EDX), X-ray diffraction (XRD) and BET surface area analyzer. EDX analysis showed the elements in the crystal BISMCP were Mn, C, and O. XRD result of BISMCP determined the crystal structure, which is close to rhodochrosite (MnCO₃). Spectral peaks of FTIR at 1641.79 cm^-1 confirmed the appearance of C[bond, double bond]O binding, with strong stretching of CO₃^2- in Amide I. From the six kinds of BISMCP produced, sample MCP-6 has the higher specific surface area by BET analysis at 109.01 m²/g, with pore size at 8.76 nm and higher pore volume at 0.178 cm³/g. These specifications will be suitable as an adsorbent for heavy metal removal by adsorption process. This study presents a preliminary analysis of the possibility of BISMCP for heavy metals adsorption using ICP multi-element standard solution XIII (As, Cr, Cd, Cu, Ni, and Zn). BISMCP formed from 0.1 MnCl₂ and 30 ml of bacteria volume (MCP-6) produced a better adsorbent material than others concentrations, with the adsorption efficiency of total As at 98.9%, Cr at 97.0%, Cu at 94.7%, Cd at 88.3%, Zn at 48.6%, and Ni at 29.5%. Future work could be examined its efficiency adsorbing individual heavy metals.

Preparation of esterified biomass waste hydrogels and their removal of Pb2+, Cu2+ and Cd2+ from aqueous solution

The treatment of polluted water is a serious environmental problem in the world. Biomass is easily modified and can be prepared into adsorbent materials, which is expected to solve the problem of heavy metal ion adsorption in sewage. In this paper, esterified tobacco straw based hydrogels (ETS-PAA) were synthesized from waste tobacco straw biomass. The structure and thermal stability of these hydrogels were characterized by FTIR, SEM, EDS, XPS and TG. The adsorption of metal ions by the hydrogel was measured by ICP-MS. The effects of initial ion concentration, adsorption time, pH, and temperature on the heavy metal adsorption were investigated. The results showed that ETS-PAA possessed more pores, which led to a better adsorption capacity. The maximum adsorption amounts of Pb²⁺, Cu²⁺ and Cd²⁺ were 2.41 mmol·g^-1, 1.93 mmol·g^-1 and 1.77 mmol·g^-1, respectively. Finally, the adsorption mechanism and kinetics were analyzed. The adsorption was mainly accomplished by ion exchange of -COOK on the monomer chain with heavy metal ions, coordination of -OH and -CONH with heavy metal ions and interaction of ester bond, -COOH with heavy metal ions. The adsorption process was in accordance with the pseudo-second-order kinetic model and Freundlich model. The adsorption process belonged to multilayer chemisorption. This work shows that ETS-PAA was a promising material for the removal of heavy metal pollutants from aqueous solution.

A Review on Graphene (GN) and Graphene Oxide (GO) Based Biodegradable Polymer Composites and Their Usage as Selective Adsorbents for Heavy Metals in Water

Water pollution due to heavy metal ions has become a persistent and increasing problem globally. To combat this, carbonaceous materials have been explored as possible adsorbents of these metal ions from solution. The problem with using these materials on their own is that their lifespan and, therefore, usability is reduced. Hence the need to mask them and an interest in using polymers to do so is picked. This introduces an improvement into other properties as well and opens the way for more applications. This work gives a detailed review of the major carbonaceous materials, graphene and graphene oxide, outlining their origin as well as morphological studies. It also outlines the findings on their effectiveness in removing heavy metal ions from water, as well as their water absorption properties. The section further reports on graphene/polymer and graphene oxide/polymer composites previously studied and their morphological as well as thermal properties. Then the work done in the absorption and adsorption capabilities of these composites is explored, thereby contrasting the two materials. This enables us to choose the optimal material for the desired outcome of advancing further in the utilization of carbonaceous material-based polymer composites to remove heavy metal ions from water.

Electrocoagulation removal of Pb, Cd, and Cu ions from wastewater using a new configuration of electrodes

A new configuration of aluminum electrodes has been performed in an electrocoagulation reactor (ECR) to remove toxic metals from synthetic wastewater. The ECR contains four concentric-cubic electrodes that were connected to the DC power supply with a bipolar mode. The ability of this reactor to eliminate 200 ppm Pb, 200 ppm Cd and 200 ppm Cu from wastewater was investigated under the effect of pH (4-10), applied current (0.2-2.6 A), and the reaction time of (4-60 min). Two grams of NaCl were added to each experiment to enhance the electrical conductivity and minimize the passivation of cathode surfaces. The experiments, analysis, and optimization were conducted using response surface methodology type Box-Behnken design (RSM-BBD) and the Minitab-statistical software program. The highest elimination of heavy metals was: Pb-99.73%, Cd-98.54%, and Cu-98.92% at pH 10, 1.4 A of the applied current, and 60 min of the reaction time. The total real consumption of anodes under these conditions was 0.55 g, and the energy consumption was 12.71 kWh/m3. All reactions of metal removal that occurred in the present EC reactor obey the kinetic of a first-order reaction. Thermodynamics parameters of present electrocoagulation removal of heavy metals indicate an endothermic, spontaneous nature, and random irregularity at the liquid-solid interaction. The highest values of removal efficiencies and the considerably lowest values of energy and electrode consumption proved that the electrocoagulation technology applies in wastewater treatment containing toxic metals.•The anode electrodes were perforated to decrease the amount of electrode consumption, while the cathode electrodes were not perforated.•The new EC reactor eliminated Pb-99.73%, Cd-98.54%, and Cu-98.92% of 200 mg/l of each metal at pH 10, applied current of 1.4 A, and reaction time of 60 min. Moreover, the consumption of energy and electrodes was significantly low.•The performance indicator (R²) of the studied responses was higher than 0.95.

Preparation and Application of Graphene–Based Materials for Heavy Metal Removal in Tobacco Industry: A Review

Heavy metals are nondegradable in the natural environment and harmful to the ecological system and human beings, causing an increased environmental pollution problem. It is required to remove heavy metals from wastewater urgently. Up until now, various methods have been involved in the heavy metal removals, such as chemical precipitation, chemical reduction, electrochemical, membrane separation, ion exchange, biological, and adsorption methods. Among them, adsorption by graphene–based materials has attracted much more attentions for the removal of heavy metals from wastewater systems in recent years, arising due to their large specific surface area, high adsorption capacity, high removal efficiency, and good recyclability. Therefore, it is quite important to review the heavy metal removal with the graphene–based material. In this review, we have summarized the physicochemical property and preparation methods of graphene and their adsorption property to heavy metals. The influencing parameters for the removal of heavy metals by graphene–based materials have been discussed. In addition, the modification of graphene–based materials to enhance their adsorption capability for heavy metal removal is also reviewed. The heavy metal removal by modified graphene–based materials in the tobacco industry has been especially described in detail. Finally, the future trend for graphene–based materials in the field of heavy metal wastewater treatment is proposed. This knowledge will have great impacts on the field and facilitate the researchers to seek the new functionalization method for graphene–based materials with high adsorption capacity to heavy metals in the tobacco industry in the future.

Recent advances in the construction of biocomposites based on fungal mycelia

In recent years, environmental problems have become increasingly serious, significantly effecting the ecosystem and human health. To deal with the problem of environmental pollution in an eco-conscious way, sustainable composite biomaterials are being produced. Mycelium-based composite biomaterials combine biological systems with substrates such as nanomaterials or agricultural and industrial wastes, which can complement each other's advantages or turn waste into a useful resource. Such materials can solve practical wastewater problems as well as replace plastic products, thus reducing plastic pollution and contributing to the green transition of the environment. In this review, we summarized the recent findings of studies on these materials, indicating future research directions.

Polyethyleneimine modified activated carbon for high-efficiency adsorption of copper ion from simulated wastewater

In this study, activated carbon (AC) was chemically activated using sodium hydroxide (NaOH), and polyethyleneimine (PEI) was grafted onto the AC using glutaraldehyde as a cross-linking agent. Then the modified AC was applied to treat water samples containing copper ions (Cu²⁺). Preparation of AC-NaOH@PEI. The grafted AC was characterized, demonstrating that the specific surface area of material decreased from 959.3 to 556.9 m²/g. The ζ-potential changed from -27.2 to 10.4 mV, and the presence of a distinct flocculation on the surface of the AC was observed via scanning electron microscopy. The results demonstrated that PEI was successfully grafted onto the surface of AC. Furthermore, the adsorption results indicated that the Cu²⁺ adsorption capacity of AC-NaOH@PEI was greatly enhanced with increasing PEI loading. The adsorption amount of Cu²⁺ by the grafted AC-NaOH@PEI-200 increased from 20.02 to 47.8 mg/g. In addition, the adsorption of Cu²⁺ by AC-NaOH@PEI was a pH dependent process. At a pH of 6, the maximum removal rate reached 93%. The adsorption process is better described by the Langmuir and quasi-second order adsorption models, signifying that the adsorption of Cu²⁺ on AC@PEI consists of monolayer adsorption and chemisorption. After four adsorption-desorption cycles, AC@PEI exhibited high adsorption capacity for Cu²⁺, indicating that it has good regeneration ability. It is a promising adsorbent material.

Zero-valent bimetallic catalyst/absorbent for simultaneous facilitation of MgSO3 oxidation and arsenic uptake

Cobalt (Co)-based catalysts can efficiently reduce the heat waste from sulfate concentration by enhancing sulfite oxidation during wet flue gas desulfurization system. However, arsenic (As) can poison such catalysts and migrate into the sulfate by-products, resulting in severe secondary pollution. In this study, a zero-valent Co/iron (Fe)-based nanoparticle (NZV-Co₂Fe₁) was fabricated and applied as a bifunctional catalyst/adsorbent. The catalytic stability of the Co-based catalyst was enhanced by the introduction of Fe because the poisonous effect of As was substantially suppressed because of the high adsorption capacity of Fe for As. Compared with the noncatalytic benchmark, the presence of 0.5 g/L NZV-Co₂Fe₁ can increase the rate of MgSO₃ oxidation by approximately 12-fold even at a high concentration of As (2.5 mg/L). The Langmuir model was fitted to the As adsorption isotherms, indicating that As uptake is a single-layer adsorption process. The pseudo-second-order kinetic model indicated that As was removed through chemisorption. The oxidation pathway of As(III) involves reactive radicals (mainly OH, SO₄^- and SO₅^-) and ligand-to-metal charge transfer between SO₃^2- and Co²⁺. The availability of MgSO₃ improved the removal efficiency at high concentrations of As(III) (1 mg/L). These results indicate that using NZV-Co₂Fe₁ as a catalyst to purify the by-products of flue gas desulfurization can effectively prevent secondary pollution.

Comprehensive Ecological Risk Assessment of Heavy Metals Based on Species Sensitivity Distribution in Aquatic of Coastal Areas in Hong Kong

In recent decades, the ecological environment of some coastal areas in China has been seriously affected by terrestrial pollutants, and there is an urgent need for ecological risk assessment of China's coastal environment. The assessment of heavy metal pollution in Hong Kong waters was carried out using different environmental and ecological indicators. The heavy metal contents (Cu, Pb, Zn, Cd, As, Cr, and Hg) in the near coast of Hong Kong were analyzed for two different seasons of the year 2018 (April-spring and September-autumn). We assessed the distribution and enrichment of heavy metals in the near coast of Hong Kong, and the potential biohazardous effects were assessed using the species sensitivity distribution method. The results showed that only Pb, Zn, and Hg in seawater exceeded the Class I standard. Pb, Zn, Cd, and As in organisms exceeded the standard, and no heavy metals exceeded the standard in sediments. The species sensitivity distribution method indicated that the biohazardous factor of heavy metals of the Hong Kong coast is higher in spring than in autumn, and the potential hazard ratio has the characteristics of high northwest and low southeast, which leads to its msPAF also having these characteristics. From the correlational analyses among heavy metals, we found that the pH change in seawater was related to the concentration of heavy metals, the concentration of heavy metals in seawater was proportional to the salinity of seawater, Pb and Cu were likely to have the same source, and Zn and Cd may not have the same emission sources as the other heavy metals. Overall, heavy metal contamination of seawater, sediments, and organisms near the Hong Kong coast was within acceptable limits, but the problem of heavy metal dispersion should be prevented.

Ecological risks assessment of sulfur and heavy metals in sediments in a historic mariculture environment, North Yellow Sea

The environment behaviors of sulfur and heavy metals in sediments are closely related to sediment aging in mariculture area. In this study, the distributions and ecological risks of reduced inorganic sulfur (RIS) and heavy metals were investigated, along with the relationships between different occurrences of RIS and heavy metals. The results indicated that the adequate organic matter in mariculture sediments significantly enhanced the accumulation of acid volatile sulfur (AVS) compared to the control area. In shellfish farming area, biological sedimentation contributed to accumulation of AVS. The chromium (II)-reducible sulfur (CRS) was the main component of RIS in mariculture area. The environmental risks of heavy metals in mariculture area presented low levels. Principal component analysis (PCA) showed that distribution of Cu closely related to mariculture activities compared to other heavy metals. For ecological risks of heavy metals, the ratio of ∑(acid-soluble fraction (F1) + reducible fraction (F2) + oxidizable fraction (F3))/AVS was the appropriate index rather than conventional simultaneous extraction of heavy metals (SEM)/AVS, because SEM/AVS would overestimate the toxicity of heavy metals. AVS/RIS ratios significantly positively correlated with Pb (F2/(F1 + F2 + F3 + residual fraction (F4)), F2/∑F), Pb (F3/∑F), and Zn (F3/∑F), while significantly negatively correlated with Pb (F4/∑F) and Cu (F1/∑F). These results indicated that the accumulation of AVS during the mariculture process was conducive to the formation of F2 and F3 of Pb, and F3 of Zn, conversely to the formation for F4 of Pb and F1 of Cu, because it was opposite to the accumulation of CRS.

Removal of hazardous ions from aqueous solutions: Current methods, with a focus on green ion flotation

Hazardous ions, like those of heavy metals, cause significant health and environmental problems when they are discharged into water resources naturally or through various industrial processes. Removing these ions from water is of significant importance in the provision of high-quality water for drinking and agricultural usage. This work discusses current techniques that are frequently used for the removal of heavy-metal ions from aqueous solutions by absorption, particularly the use of biodegradable surfactants in ion flotation. Certain new surfactants promise high efficiency in their use in the ion-flotation process and in their application in industrial-water treatment to remove heavy metals. As an example, this work demonstrates the high efficiency of surfactants based on an amino-acid (L-cysteine) in removing a range of heavy-metal ions in a simple, single-stage ion-flotation process. High foaming ability, the ability to operate in various temperatures and pHs, decomposing into natural products and high binding affinity for heavy-metal ions make the cysteine-based surfactants a highly suitable compound to replace current commercial surfactants in ion- and froth-flotation processes. Removal of particular ions can also be achieved in ion flotation; a suitable choice of parameters, such as pH and surfactant concentration, favours the surfactant binding to those ions. Further intensive work is required to develop an optimal process to recover valuable elements from waste solutions.

Removal of Zn(II) by magnetic composite adsorbent: synthesis, performance, and mechanism

In this study, L-methionine and nano-Fe₃O₄ were encapsulated and cured on sodium alginate by the ionic cross-linking method to form magnetic composite gel spheres (SML) as an adsorbent for the removal of Zn(II) from water. The influence of adsorbent dosages, pH, reaction time, and initial ion concentration on the ability of the gel spheres to adsorb Zn(II) was investigated, and the adsorption mechanism was identified. The experimental results showed that under the optimum conditions (pH = 5, t = 60 min, dosage of SML is 0.7 g·L^-1), the maximum amount of Zn(II) adsorbed by the adsorbent gel spheres reached 86.84 mgˑg^-1. The reaction process of this adsorbent fits well with the Langmuir and pseudo-second-order kinetic models and is a heat absorption reaction. The adsorbent would preferentially adsorb Pb(II), and the adsorption efficiency of Zn(II) decreased when the concentration of interfering ions increased in the coexistence system. Further mechanistic research showed that this magnetic composite adsorbent is a mesoporous material with superior adsorption performance, and the amino and carboxyl groups on it react with Zn(II) via ligand chelation; the ion exchange effect of Ca(II) also plays a role. The adsorption amount of Zn(II) was maintained at a higher level after 5 cycles, and the loss of Fe was approximately 0.2%. In summary, SML, which is environmentally friendly, efficient, and recyclable, is an ideal adsorbent for Zn(II) removal.

Electro-oxidation of Ni (II)-citrate complexes at BDD electrode and simultaneous recovery of metallic nickel by electrodeposition

The simultaneous electro-oxidation of Ni (II)-citrate and electrodeposition recovery of nickel metal were attempted in a combined electro-oxidation-electrodeposition reactor with a boron-doped diamond (BDD) anode and a polished titanium cathode. Effects of initial nickel citrate concentration, current density, initial pH, electrode spacing, electrolyte type, and initial electrolyte dosage on electrochemical performance were examined. The efficiencies of Ni (II)-citrate removal and nickel metal recovery were determined to be 100% and over 72%, respectively, under the optimized conditions (10 mA/cm², pH 4.09, 80 mmol/L Na₂SO₄, initial Ni (II)-citrate concentration of 75 mg/L, electrode spacing of 1 cm, and 180 min of electrolysis). Energy consumption increased with increased current density, and the energy consumption was 0.032 kWh/L at a current density of 10 mA/cm² (pH 6.58). The deposits at the cathode were characterized by scanning electron microscopy (SEM), energy-dispersive spectrometry (EDS), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). These characterization results indicated that the purity of metallic nickel in cathodic deposition was over 95%. The electrochemical system exhibited a prospective approach to oxidize metal complexes and recover metallic nickel.

Cubic structured SrTiO3 with Ce/Cr Co-doping for photoinduced catalytic oxidation of gaseous mercury

A cubic SrTiO₃ (STO) composite material co-doped with Ce and Cr ions was synthesized by solvothermal method. The fully characterized samples were employed as photocatalysts for the oxidation of Hg⁰. The co-doped samples afforded excellent catalytic removal efficiency of 98.99% using UV irradiation and 89.9% using visible light irradiation for Hg⁰ compared with the single-doped samples. It was found that co-doped samples had a lower electron-hole recombination rate, largest Brunauer-Emmett-Teller specific surface area, and reduced band gap. The electron spin resonance results revealed that ·O₂^- and ·OH were the main active species in the catalytic process. Moreover, the co-doped samples exhibited the best electron transfer rate and the highest photocurrent response intensity. The electron transfer between the elements in the co-doped sample enables it to achieve stable and efficient catalytic performance. In addition, even after five consecutive catalytic runs, the co-doped sample maintained high catalytic activity. This work highlights the potential of the perovskite-type STO materials in the photocatalytic oxidation of gaseous mercury.

Ameliorative effects of plant growth promoting bacteria, zinc oxide nanoparticles and oxalic acid on Luffa acutangula grown on arsenic enriched soil

Arsenic (As) contamination and bioaccumulation are a serious threat to agricultural plants. To address this issue, we checked the efficacy of As tolerant plant growth promoting bacteria (PGPB), zinc oxide nanoparticles (ZnO NPs) and oxalic acid (OA) in Luffa acutangula grown on As rich soil. The selected most As tolerant PGPB i.e Providencia vermicola exhibited plant growth promoting features i.e solubilzation of phosphate, potassium and siderophores production. Innovatively, we observed the synergistic effects of P. vermicola, ZnO NPs (10 ppm) and OA (100 ppm) in L. acutangula grown on As enriched soil (150 ppm). Our treatments both as alone and in combination alleviated As toxicity exhibited by better plant growth and metabolism. Results revealed significantly enhanced photosynthetic pigments, proline, relative water content, total sugars, proteins and indole acetic acid along with As amelioration in L. acutangula. Furthermore, upregulated plant resistance was manifested with marked reduction in the lipid peroxidation and electrolyte leakage and pronounced antagonism of As and zinc content in leaves under toxic conditions. These treatments also improved level of nutrients, abscisic acid and antioxidants to mitigate As toxicity. This marked improvement in plants' defense mechanism of treated plants under As stress is confirmed by less damaged leaves cell structures observed through the scanning electron micrographs. We also found substantial decrease in the As bioaccumulation in the L. acutangula shoots and roots by 40 and 58% respectively under the co-application of P. vermicola, ZnO NPs and OA in comparison with control. Moreover, the better activity of soil phosphatase and invertase was assessed under the effect of our application. These results cast a new light on the application of P. vermicola, ZnO NPs and OA in both separate and combined form as a feasible and ecofriendly tool to alleviate As stress in L. acutangula.

O, N-doped porous biochar by air oxidation for enhancing heavy metal removal: The role of O, N functional groups

Oxygen- and nitrogen-doped porous oxidized biochar (O,N-doped OBC) was fabricated in this study. Biochar (BC) can be enriched in surface functional groups (O and N) and the porosity can be improved by a simple, convenient and green procedure. BC was oxidized at 200 °C in an air atmosphere with quality control via oxidation time changes. As the oxidation time increased, the O and N contents and porosity of the materials improved. After 1.5 h of oxidation, the O and N contents of O,N-doped OBC-1.5 were 54.4% and 3.9%, higher than those of BC, which were 33.4% and 1.8%, respectively. The specific surface area and pore volume of O,N-doped OBC-1.5 were 88.5 m² g^-1 and 0.07 cm³ g^-1, respectively, which were greater than those of BC. The improved surface functionality and porosity resulted in an increased heavy metal removal efficiency. As a result, the maximum adsorption capacity of Cu(II) by O,N-doped OBC was 23.32 mg L^-1, which was twofold higher than that of pristine BC. Additionally, for a multiple ion solution, O,N-doped OBC-1.5 showed a greater adsorption behavior toward Cu(II) than Zn(II) and Ni(II). In a batch experiment, the concentration of Cu(II) decreased 92.3% after 90 min. In a filtration experiment, the O,N-doped OBC-based filter achieved a Cu(II) removal capacity of 12.90 mg g^-1 and breakthrough time after 250 min. Importantly, the chemical mechanism was mainly governed by monolayer adsorption of Cu(II) onto a homogeneous surface of O,N-doped OBC-1.5. Surface complexation and electrostatic attraction were considered to be the chemical mechanisms governing the adsorption process.

Nanocarbon hybrid for simultaneous removal of arsenic, iron and manganese ions from aqueous solutions

Heavy metal contamination is a severe problem with serious ecological and human health effects due to its toxic effect and tendency to accumulate throughout the food chain. Batch experiments were conducted to investigate the simultaneous removal of arsenic, iron and manganese ions from aqueous solutions using Nanocarbon hybrid (NCH). Nanocarbon hybrid (NCH) of carbon xerogel decorated with 1wt% multi-walled carbon nanotubes was prepared by carbonization at 850 °C for 2 h. The TEM, SEM, EDX, FTIR, and N₂ adsorption-desorption measurements were used to characterize the prepared NCH. NCH is enriched with surface oxygen functional groups and micropores as well as it have total surface area of 162 m²/g and total pore volume of 0.129 cm³/g. The adsorption of metal ions onto NCH, which confirmed by EDX, happened quickly, with 30%, 97%, and 41% of As, Fe, and Mn adsorbed in less than 10 min, however the equilibrium time was achieved in less than 30 min. The maximum adsorption capacities for As, Fe, and Mn ions onto NCH were 20, 48, and 21 mg/g, respectively. The experimental adsorption results of the three metal ions showed linearly fitting with Freundlich isotherms. In addition, the computed adsorption energies for Fe, Mn, and As ions were 4.08, 1.95, and 2.42 kJ/mol, indicating physical adsorption. NCH are easily regenerated and reusable sorbent owing to the adsorption–desorption studies. Conclusively, NCH is promising material for removing mixture of metal ions from aqueous media.

Heavy metal pollution and the role of inorganic nanomaterials in environmental remediation

Contamination of water and soil with toxic heavy metals is a major threat to human health. Although extensive work has been performed on reporting heavy metal pollutions globally, there are limited review articles on addressing this pernicious phenomenon. This paper reviews inorganic nanoparticles and provides a framework for their qualities required as good nanoadsorbents for efficient removal of heavy metals from water. Different inorganic nanoparticles including metals, metal oxides and metal sulfides nanoparticles have been applied as nanoadsorbents to successfully treat water with high contaminations of heavy metals at concentrations greater than 100 mg l^-1, achieving high adsorption capacities up to 3449 mg g^-1. It has been identified that the synthesis method, selectivity, stability, regeneration and reusability, and adsorbent separation from solution are critical parameters in deciding on the quality of inorganic nanoadsorbents. Surface functionalized nanoadsorbents were found to possess high selectivity and capacity for heavy metals removal from water even at a very low adsorbent dosage of less than 2 g l^-1, which makes them better than conventional adsorbents in environmental remediation.

Aqueous Adsorption of Heavy Metals on Metal Sulfide Nanomaterials: Synthesis and Application

Heavy metals pollution of aqueous solutions generates considerable concerns as they adversely impact the environment and health of humans. Among the remediation technologies, adsorption with metal sulfide nanomaterials has proven to be a promising strategy due to their cost-effective, environmentally friendly, surface modulational, and amenable properties. Their excellent adsorption characteristics are attributed to the inherently exposed sulfur atoms that interact with heavy metals through various processes. This work presents a comprehensive overview of the sequestration of heavy metals from water using metal sulfide nanomaterials. The common methods of synthesis, the structures, and the supports for metal sulfide nano-adsorbents are accentuated. The adsorption mechanisms and governing conditions and parameters are stressed. Practical heavy metal remediation application in aqueous media using metal sulfide nanomaterials is highlighted, and the existing research gaps are underscored.

Removal of mercury ions from aqueous by functionalized LUS-1 with Bis [3-(triethoxysilyl) propyl] tetrasulfide as an effective nanocomposite using response surface methodology (RSM)

In this study, LUS-1, as a mesoporous silica material, was functionalized using sulfur-containing ligand (Bis [3-(triethoxysilyl) propyl] tetrasulfide, TESPT) and used for mercury removal from the aqueous solution. Different characterizations such as N₂ adsorption-desorption (BET), TGA, XRD, FT-IR, and SEM were used to verify the nanocomposite synthesis. In addition, the effects of several independent parameters like pH, the contact time of reaction, and adsorbent dose on the removal efficiency of mercury from aqueous in a batch system were studied using response surface methodology (RSM). Based on the results and after both theoretical and experimental studies, the optimum conditions using the LUS-1-TESPT were contact time of reaction of 23.16 min, sorbent dose of 51.12 mg, and pH of 4.5. The kinetic and isotherm models for the adsorption process showed a maximum adsorption capacity of adsorbent which was 136.73 mg g^-1 with 99% removal of Hg(II) via the Langmuir model. Meanwhile, the sorbent's reusability and efficiency verified that the sorbent could be used five times after recovery with 99% efficiency.

Functionalized Biomass Carbon-Based Adsorbent for Simultaneous Removal of Pb2+ and MB in Wastewater

It is of great significance to realize the sustainable development of the environment to synthesize functional materials by value-added utilization of waste resources. Herein, a composite material of polyacrylic acid/lignosulfonate sodium/cotton biochar (PAA/LS/BC) was successfully prepared by grafting polyacrylic acid with functionalized waste cotton biochar and lignosulfonate sodium. The obtained absorbent showed prominent capture ability toward Pb²⁺ and methylene blue (MB) with capture characteristics of the pseudo-second-order model and Langmuir isotherm model. This experiment explored the adsorption performance of the adsorbent for pollutants at different conditions, and further revealed the selective adsorption of Pb²⁺ and MB in the mixed system. Analysis confirmed that electrostatic attraction and complexation are the most critical methods to remove contaminants. Additionally, the regeneration and stability experiment showed that the adsorption capacity of PAA/LS/BC for pollutants did not significantly decrease after five runs of adsorption–desorption. Various results can demonstrate that the adsorbent has excellent performance for removing pollutants and can be used as a material with development potential in the field of adsorption.

MnO2 -Based Materials for Environmental Applications

Manganese dioxide (MnO₂ ) is a promising photo-thermo-electric-responsive semiconductor material for environmental applications, owing to its various favorable properties. However, the unsatisfactory environmental purification efficiency of this material has limited its further applications. Fortunately, in the last few years, significant efforts have been undertaken for improving the environmental purification efficiency of this material and understanding its underlying mechanism. Here, the aim is to summarize the recent experimental and computational research progress in the modification of MnO₂ single species by morphology control, structure construction, facet engineering, and element doping. Moreover, the design and fabrication of MnO₂ -based composites via the construction of homojunctions and MnO₂ /semiconductor/conductor binary/ternary heterojunctions is discussed. Their applications in environmental purification systems, either as an adsorbent material for removing heavy metals, dyes, and microwave (MW) pollution, or as a thermal catalyst, photocatalyst, and electrocatalyst for the degradation of pollutants (water and gas, organic and inorganic) are also highlighted. Finally, the research gaps are summarized and a perspective on the challenges and the direction of future research in nanostructured MnO₂ -based materials in the field of environmental applications is presented. Therefore, basic guidance for rational design and fabrication of high-efficiency MnO₂ -based materials for comprehensive environmental applications is provided.

Leaching of iron from copper tailings by sulfuric acid: behavior, kinetics and mechanism

Copper tailing is a widespread and intractable solid waste in copper production. Traditional leaching and recovery technology for copper tailing focuses on copper but neglects the leaching of iron. With the increase in applications and demands of iron-containing materials for environment, understanding the leaching behaviors of iron can promote the utilization of copper tailings. In this study, the kinetics and mechanism of the leaching of iron from copper tailings using sulfuric acid were studied. Under optimal conditions (40 °C, sulfuric acid concentration of 0.53 mol L⁻¹, stirring speed of 400 rpm, solid/liquid ratio of 1 : 10 and leaching time of 120 min), 66.45% of Fe, along with 65.32% of Zn and 59.95% of Cu, were leached from the tailings. The leaching of iron was confirmed to be controlled by solid-film diffusion. The reaction orders for sulfuric acid concentration, solid/liquid ratio, and stirring speed were found to be 0.85, −0.70, and 0.40, respectively. Results from XRF, XRD, and SEM indicated that oxides (including CaO, CuO, and ZnO) were leached first, after which Fe₂SiO₄ was preferentially reacted compared to Fe₃O₄. The accumulation of CaSO₄ and SiO₂ inhibited the further leaching of iron.

Leaching behavior of copper tailings with sulfuric acid was expounded. Leaching was optimized: 1 M H₂SO₄, S/L of 1 : 10, T = 40 °C, 120 min. Fe₂SiO₄ was preferentially leached than Fe₃O₄. Leaching of Fe is controlled by solid-phase diffusion.

Sonochemical synthesis of ZnS nanolayers on the surface of microbial cells and their application in the removal of heavy metals

In order to improve the adsorption performance of microorganisms, we synthesized a novel material - phanerochaete chrysosporium cells covered with a layer of ZnS nanoparticles (ZnS-cells). The preparation of the ZnS-cells is based on the Sonochemical method to synthesize the ZnS nanoparticle layer on the surface of the microbial cells. The ZnS-cells were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS) and Fourier transform infrared spectroscopy (FTIR). Characterization results showed that wurtzite ZnS was coated on the cell surface in the form of nanoclusters by sonochemical reaction, and the formation of ZnS was related to the carboxyl group on the cell surface. Batch experiments showed that the ZnS-cells exhibited high adsorption efficiency for Pb²⁺and Cd²⁺, the removal rate of Pb²⁺ and Cd²⁺ by ZnS-cells was 140 % and 160 % higher than that of pure P. chrysosporium, respectively. Studies on the adsorption mechanism showed that the removal of heavy metals by ZnS-cells mainly depended on the complexation of surface functional groups on the surface of the cells and the ion exchange of ZnS nanofilms.

Nanoscale Pisum sativum pods biochar encapsulated starch hydrogel: A novel nanosorbent for efficient chromium (VI) ions and naproxen drug removal

Assembly of novel ecofriendly and sustainable (N-PSPB/SHGL) nanosorbent was fabricated based on encapsulation of derived nanoscale spherical biochar from Pisum sativum pods (N-PSPB) with starch hydrogel (SHGL). The mass ratio between starch and N-PSPB was examined and 2% (w/w) was selected as the optimum percentage for fabrication of the assembled hydrogel. High swelling capacity was characterized by N-PSPB/SHGL nanosorbent (500.0%) at room temperature (25 °C), excellent stability for ten cycles with respect to regeneration by 0.1 mol L-1 HCl. Additionally, characterizations of N-PSPB/Starch nanosorbent were established by SEM and BET measurement to characterize surface area (226.94 m2/g) and pore volume (9.88 cm3/g). The N-PSPB/SHGL nanosorbent was subjected to extensive investigations to evaluate its efficiency for removal of naproxen drug (NAP) and chromium (VI). The Cr(VI) and NAP drug adsorptions were fitted to pseudo-second kinetic and correlated with Langmuir isotherm. The adsorption processes were spontaneous and endothermic based on thermodynamic study.

Synthesis, Characterization, and Analysis of Hybrid Carbon Nanotubes by Chemical Vapor Deposition: Application for Aluminum Removal

Hybrid carbon nanotubes (CNTs) are grown on biomass powder-activated carbon (bio-PAC) by loading iron nanoparticles (Fe) as catalyst templates using chemical vapor deposition (CVD) and using acetylene as carbon source, under specific conditions as reaction temperature, time, and gas ratio that are 550 °C, 47 min, and 1, respectively. Specifications of hybrid CNTs were analyzed and characterized using field emission scanning electron microscope (FESEM) with energy-dispersive X-ray spectroscopy (EDX), transmission electron microscopic (TEM), Fourier-transform infrared (FTIR), X-ray diffraction (XRD), thermogravimetric analysis (TGA), surface area Brunauer-Emmett-Teller (BET), and zeta potential. The results revealed the high quality and unique morphologies of hybrid CNTs. Furthermore, removal and capacity of Al3+ were optimized by response surface methodology (RSM). However, the results revealed that the pseudo-second-order model well represented adsorption kinetic data, while the isotherm data were effectively fitted using a Freundlich model. The maximum adsorption capacity was 347.88 mg/g. It could be concluded that synthesized hybrid CNTs are a new cost-effective and promising adsorbent for removing Al3+ ion from wastewater.

Encapsulation of starch hydrogel and doping nanomagnetite onto metal-organic frameworks for efficient removal of fluvastatin antibiotic from water

Growing interests and efforts have been recently focused on design and assembly of novel hydrogel nanosorbents for removal of drugs from wastewater. Therefore, this work is aimed to immobilize and encapsulate starch hydrogel matrix onto metal organic frameworks (MOFs) and dope with nanomagnetite. The magnetic MOFs-Starch hydrogel (NFe₃O₄@Zn(GA)/Starch-Hydrogel) was synthesized via microwave irradiation process and characterized with high surface area (528.39 m²/g), mesoporous with pore size 2.90 nm and highly crystalline structure. The maximum swelling ratio (1000.0 %) was optimized at pH 10, 180 min and 25 °C. The validity of NFe₃O₄@Zn(GA)/Starch-Hydrogel for adsorptive removal of Fluvastatin statin drug provided maximum equilibrium adsorption capacity 782.05 mg g^-1. The Langmuir isotherm and pseudo-second kinetics models were correlated well with the computed correlation coefficient values 0.9991 and 0.9997, respectively. The validity of NFe₃O₄@Zn(GA)/Starch-Hydrogel for removal of FLV statin drug from real water matrices was confirmed in the range 96.15-99.99 %.

Optimal conditions for adsorption of zinc from industrial wastewater using groundnut husk ash

Zinc is a toxic metal ion and is of importance in water and wastewater because it causes dizziness as well as lethargy when ingested by man. In the current study, the groundnut husk ash was investigated as a potential adsorbent for adsorption of zinc(II) ions from industrial wastewater. Groundnut husk ash was characterized by Fourier transform infrared spectroscopy, scanning electron microscopy and proximate analysis to identify the presence of the functional groups, surface morphology and the carbon content in the adsorbent respectively. To optimize the process parameters affecting the percentage removal of zinc(II) onto groundnut husk ash, the central composite design was used. The result of the optimization study showed an optimal percentage removal of 80.00%, with the optimal conditions of 1400 μm, 100 min, 25 °C, 40 mg/l and 20 mg for particle size, contact time, temperature, initial zinc concentration and adsorbent dosage respectively. The equilibrium data showed a better fit for Langmuir isotherm, when compared to Freundlich, Temkin and Dubinin-Radushkevich isotherms, with R² of 0.965. The adsorption kinetics was best described by pseudo-second-order kinetics with R² of 0.987. The thermodynamic study, on the other hand, showed a negative value of enthalpy change(∆H = - 27.021), indicating an exothermic as well as a spontaneous reaction, with the degree of spontaneity of the reaction ranging from - 55.487 ≤ ∆G ≤ - 56.427, which showed a corresponding increase in Gibb's free energy (∆G) with an increase in temperature.

A review: Recent advances in ultrasensitive and highly specific recognition aptasensors with various detection strategies

One of the most studied topics in analytical chemistry and physics is to develop bio-sensors. Aptamers are small single-stranded RNA or DNA oligonucleotides (5-25 kDa), which have advantages in comparison to their antibodies such as physicochemical stability and high binding specificity. They are able to integrate with proteins or small molecules, including intact viral particles, plant lectins, gene-regulation factor, growth factors, antibodies and enzymes. The aptamers have reportedly shown some unique characteristics, including long shelf-life, simple modification to provide covalent bonds to material surfaces, minor batch variation, cost-effectiveness and slight denaturation susceptibility. These features led important efforts toward the development of aptamer-based sensors, known as apta-sensors classified into optical, electrical and mass-sensitive based on the signal transduction mode. This review provided a number of current advancements in selecting, development criteria, and aptamers application with the focus on the effect of apta-sensors, specifically for disease-associated analyses. The review concentrated on the current reports of apta-sensors that are used for evaluating different food and environmental pollutants.

Highly efficient and irreversible removal of cadmium through the formation of a solid solution

Sulfur-containing materials are very attractive for the efficient decontamination of some heavy metals. However, the effective and irreversible removal of Cd²⁺, coupled with a high uptake efficiency, remains a great challenge due to the relatively low bond dissociation energy of CdS. Herein, we propose a new strategy to overcome this challenge, by the incorporation of Cd²⁺ into a stable Zn_xCd_1-xS solid solution, rather than into CdS. This can be realised through the adsorption of Cd²⁺ by ZnS nanoparticles, which have exhibited a Cd²⁺ uptake capacity of approximate 400 mg g^-1. Through this adsorption mechanism, the Cd²⁺ concentration in a contaminated solution could effectively be reduced from 50 ppb to <3 ppb, a WHO limit acceptable for drinking water. In addition, ZnS continued to exhibit this noteworthy uptake capacity even in the presence of Cu²⁺, Pb²⁺, and Hg²⁺. ZnS displayed high chemical stability. Particles aged in air for 3 months still retained a> 80% uptake capacity for Cd²⁺, compared with only 9% uptake capacity for similarly-aged FeS particles. This work reveals a new mechanism for Cd²⁺ removal with ZnS and establishes a valuable starting point for further studies into the formation of solid solutions for hazardous heavy metal removal applications.

Removal of heavy metals in aquatic environment by graphene oxide composites: a review

As the most important graphene derivate, graphene oxide (GO) is a high-efficient adsorbent for the removal of heavy metals in aquatic environment due to its abundant oxygen functional groups, enormous specific area, and strong hydrophilia. However, there are some drawbacks, such as easily aggregating and difficult separation, restricting the environmental application of GO. GO is not a suitable adsorbent by itself. Hence, some materials were used to synthesize GO composites, and GO composites are commonly characterized by high adsorption capacity to overcome the above drawbacks. This review discusses five main GO composites-GO-chitosan, GO-alginate, GO-SiO₂, NZVI-rGO, and magnetic GO composites-and summarizes the synthesis methods of GO composites and its application for the removal of heavy metals in aquatic environments. The influencing factors, adsorption capacities, and mechanisms related to the removal of heavy metals by GO composites are highlighted. Lastly, the application potentials and challenges of GO composites for aqueous environmental remediation are discussed. Graphical abstract.