Hydrochar (HC) serves as a promising adsorbent to remove the cadmium from aqueous solution due to porous structure. The chemical aging method is an efficient and easy-operated approach to improve the adsorption capacity of HC. In this study, four chemical aging hydrochars (CAHCs) were obtained by using nitric acid (HNO₃) with mass fractions of 5% (N5-HC), 10% (N10-HC), and 15% (N15-HC) to age the pristine HC (N0-HC) and remove the Cd²⁺ from the aqueous solution. The results displayed that the N15-HC adsorption capacity was 19.99 mg g⁻¹ (initial Cd²⁺ concentration was 50 mg L⁻¹), which increased by 7.4 folds compared to N0-HC. After chemical aging, the specific surface area and oxygen-containing functional groups of CAHCs were increased, which contributed to combination with Cd²⁺ by physical adsorption and surface complexation. Moreover, ion exchange also occurred during the adsorption process of Cd²⁺. These findings have important implications for wastewater treatment to transform the forestry waste into a valuable adsorbent for Cd²⁺ removal from water.

Excessive copper (Cu) in contaminated soil and groundwater has attracted continuous attentions due to the bioaccumulation and durability. In this study, the feasibility of remediation of heavy metal pollution in soil and groundwater was investigated using hydroxyapatite/calcium silicate hydrate (HAP/C–S–H) recovered from phosphorus-rich wastewater in farmland. The results show that the pH has a strong effect on copper removal from Cu-contaminated groundwater but the impact of ion strength on the removal is weak. In general, high pH and low ion strength give better results in copper removal. Kinetic and isotherm data from the study fit well with Pseudo-second-order kinetic model and Langmuir isotherm model, respectively. The maximum adsorption capacity of HAP/C–S–H (138 mg/g) was higher than that of C–S–H (90.3 mg/g) when pH value, temperature, and ionic strength were 5, 308 K, and 0.01 M, respectively. Thermodynamics results indicate that Cu removal is a spontaneous and endothermic process. X-ray diffraction (XRD) results show that the mechanism of copper removal involves physical adsorption, chemical precipitation and ion exchange. For the remediation of Cu-contaminated soil, 76.3% of leachable copper was immobilized by HAP/C–S–H after 28 d. Acid soluble Cu, the main contributor to biotoxicity, decreased significantly while reducible and residual Cu increased. After immobilization, the acid neutralization capacity of the soil increased and the dissolution of copper was substantially reduced in near-neutral pH. It can be concluded that HAP/C–S–H is an effective, low-cost and eco-friendly reagent for in-situ remediation of heavy metal polluted soil and groundwater.

Selenium (Se) radioactive wastes can be disposed through stabilization/solidification (S/S) based on the cementitious matrix on hydration products, where hydrocalumite (Ca₂Al-LDH) is expected to play an important role in the retention of SeO₄²⁻. Natural organic matters (NOMs) are known to be a risk to affect the transportation and mobility of undesirable chemical species in the pedosphere which receives the low level radioactive wastes (LLW). In the present work, five amino acids were selected as the simplified models of NOMs in the pedosphere to explore their effects on the stability of Ca₂Al-LDH after immobilized SeO₄²⁻ under alkaline conditions. As the loading amount of amino acids on Ca₂Al-LDH increasing, release of SeO₄²⁻ was enhanced in HGly, H₂Asp, and H₂Cys series, while no enhancement was observed in HPhe and HTrp series. Density functional theory (DFT) calculation predicted ion-exchange of amino acids and CO₃²⁻ with SeO₄²⁻ in a unit cell of LDH model. The intercalation of Asp²⁻ and CO₃²⁻ caused 003 peaks in XRD sharper and d₀₀₃ decreased from 8.15 Å to 7.70 Å which is assigned to Ca₂Al-LDH(Asp, CO₃). In H₂Cys series, the 003 peaks were kept broad and SeO₄²⁻ was still relatively maintained in LDH which was caused by the lower amounts of intercalated CO₃²⁻ in the presence of H₂Cys. Amino acids in the interlayer of Ca₂Al-LDH have several possible configurations, where the most stable one is prone to be in a horizontal direction through hydrogen bonds and Ca–O chemical bonds. This provides an insight on the stability of selenate immobilized in hydrocalumite, which can be produced in cement disposing in the pedosphere for a long term of burying. Not only carbonate but also small molecular organic matters like amino acids possibly give environmental impact on the mobility of low level anionic radionuclides in LDH.

The effectiveness of biochar as a sorptive material to remove contaminants, particularly heavy metals, from water is dependent on biomass type and pyrolysis condition. Biochars were produced from pulp mill sludge (PMS) and rice straw (RS) with nitrogen (N₂) or carbon dioxide (CO₂) as the purging gas. The sorptive capacity of the biochars for cadmium(II), copper(II), nickel(II) and lead(II) was studied. The heavy metal adsorption capacity was mainly affected by biomass type, with biochars adsorption capacities higher for lead(II) (109.9–256.4 mg g⁻¹) than for nickel(II) (40.2–64.1 mg g⁻¹), cadmium(II) (29.5–42.7 mg g⁻¹) and copper(II) (18.5–39.4 mg g⁻¹) based on the Langmuir adsorption model. The highest lead(II) adsorption capacities for PMS and RS biochars were 256.4 and 133.3 mg g⁻¹, respectively, when generated using N₂ as the purging gas. The corresponding lead(II) adsorption capacities were 250.0 and 109.9 mg g⁻¹, respectively, when generated using CO₂ as the purging gas. According to the intraparticle diffusion model, 30–62% of heavy metal adsorption was achieved in 1 h; film diffusion was the rate-dominating step, whereas pore diffusion was a rate-limiting step. Ion exchange and complexation between heavy metals and biochar surface functional groups such as carbonyl and hydroxyl groups were effective mechanisms for heavy metal sorption from the aqueous solution. We conclude that proper selection of both the feedstock type and the purging gas is important in designing biochars for the effective removal of potentially toxic metals from wastewater.

Selective removal of arsenic (As) is the key challenge for any of As removal mechanisms as this not only increases the efficiency of removal of the main As species (neutral As(III) and As(V) hydroxyl-anions) but also allows for a significant reduction of waste as it does not co-remove other solutes. Selective removal has a number of benefits: it increases the capacity and lifetime of units while lowering the cost of the process. Therefore, a sustainable selective mitigation method should be considered concerning the economic resources available, the ability of infrastructure to sustain water treatment, and the options for reuse and/or safe disposal of treatment residuals. Several methods of selective As removal have been developed, such as precipitation, adsorption and modified iron and ligand exchange. The biggest challenge in selective removal of As is the presence of phosphate in water which is chemically comparable with As(V). There are two types of mechanisms involved with As removal: Coulombic or ion exchange; and Lewis acid-base interaction. Solution pH is one of the major controlling factors limiting removal efficiency since most of the above-mentioned methods depend on complexation through electrostatic effects. The different features of two different As species make the selective removal process more difficult, especially under natural conditions. Most of the selective As removal methods involve hydrated Fe(III) oxides through Lewis acid-base interaction. Microbiological methods have been studied recently for selective removal of As, and although there have been only a small number of studies, the method shows remarkable results and indicates positive prospects for the future.

Biochar has been widely used in the mitigation of soil potentially toxic metals due to its high efficiency and low cost. Crayfish shell biochar (CSBC) was prepared at 300, 500, and 700 °C (referred to as CS300, CS500, and CS700, respectively) and the performance and mechanism of CSBC for mitigating Pb polluted water and soil was investigated. The results indicated that CSBC prepared at higher temperatures possessed higher pH value and ash content, more abundant pore structure, and higher stability. Pb²⁺ adsorption onto CSBC fitted well with the pseudo second order and intraparticle diffusion models. The maximum adsorption capacity of Pb²⁺ increased with the pyrolysis temperature, being 599.70, 1114.53, and 1166.44 mg·g⁻¹ for CS300, CS500 and CS700, respectively. Compared with the control soil samples, the content of available Pb after applying 0.05%–5% CSBC was reduced by 1.87%–16.48% in acidic soils and 1.00%–11.09% in alkaline soils. Moreover, the fractionation of exchangeable Pb was converted to stable organic matter bound, Fe-Mn oxide bound, and residue fractions. XRD, SEM-EDS, and FTIR analysis showed that ion exchange, complexation, precipitation, and C−π interaction are the dominant interaction mechanisms. Therefore, CSBC can employ as an effective immobilizing agent for the mitigation of Pb contaminated water and soil.

Exploring effective uses of waste concrete powder (WCP), produced from recycling of construction & demolition waste is beneficial to the environment and sustainable development. In this study, WCP was first treated thermally to enhance the ability to remove Pb (II) from aqueous solutions. The experimental results revealed that the thermal treatment could enhance adsorption capacity due to modification of calcium bonding and pore structure of WCP. Preparation parameters such as temperature, particle size, and water-cement ratio were investigated to obtain the optimal operational conditions. Batch adsorption experiments were performed to explore influence factors of pH (1.00–6.00), ionic strength (0.05–2 mol/L), dosage (2–50 g/L), and temperature (25–45 °C). The pseudo-second-order kinetics model could adequately describe the adsorption process, and the Langmuir model was capable to predict the isotherm data well in the low concentration region (C₀ < 500 mg/L). The maximum uptake capacity for Pb (II) calculated by Langmuir model at 25, 35 and 45 °C were 46.02, 38.58 and 30.01 mg/g respectively, and the removal rate of Pb (II) was 92.96% at a dosage of 50 g/L (C₀ = 1000 mg/L). Precipitation, ion exchange, and surface complexation were identified to be the main mechanisms of Pb (II) adsorption through microscopic investigation by SEM-EDX, XRD, FTIR, XPS, and BET inspections. The study confirms that the WCP after thermal modification, can be selected as a promising adsorbent for the high performance and eco-friendliness.

The present study aimed at exploring the sources of salinity and the link it shares with the enrichment of As (arsenic) and F- (fluoride) in the groundwater of Rachna Doab. Total Dissolved Solids (TDS) were used as the measure of salinity to classify samples into three groups: TDS <1000 mg/L (freshwater), 1000–3000 mg/L (slightly saline) and 3000–10,000 mg/L (moderately saline). The stable isotope analysis (δ²H and δ¹⁸O relative to VSMOW) were used to explore the sources of salinity and a conceptual model, based on secondary data was used for comparing the current and past scenarios of groundwater salinization sources. Groundwater ion chemistry and geochemical modeling (PHREEQC) were used to develop a link between the occurrence of salinity and enrichment patterns of As and F- in the groundwater of study area. TDS, As and F- concentrations in groundwater ranged from 234 to 4557 mg/L, below detection limit to 240 μg/L and below detection limit to 3.9 mg/L, respectively. Mineral dissolution, ion exchange processes, and partial input of evaporation were identified as the factors affecting groundwater salinity in the region in accordance with the conceptual model developed based on secondary data. Groundwater salinity accounts as one of the factors that positively influence the enrichment of F- in groundwater, whereas As shows no clear relationship with saline groundwaters.

In this study, an electrokinetic technique for remediation of Pb²⁺, Zn²⁺ and Cu²⁺ contaminated soil was explored using sodium alginate (SA) and chitosan (CTS) as promising biodegradable complexing agents. The highest Cu²⁺ (95.69%) and Zn²⁺ (95.05%) removal rates were obtained at a 2 wt% SA dosage, which demonstrated that SA significantly improved the Cu²⁺ and Zn²⁺ removal efficiency during electrokinetic process. The abundant functional groups of SA allowed metal ions desorption from soil via ion-exchange, complexation, and electrolysis. Pb²⁺ ions were difficult to remove from soil by SA due to the higher gelation affinity with Pb²⁺ than Cu²⁺ and Zn²⁺, despite the Pb²⁺ exchangeable fraction partially transforming to the reducible and oxidizable fractions. CTS could complex metal ions and migrate into the catholyte under the electric field to form crosslinked CTS gelations. Consequently, this study proved the suitability of biodegradable complexing agents for treating soil contaminated with heavy metals using electrokinetic remediation.

PURPOSE OF REVIEW: Untreated wastewater discharge can significantly and negatively impact the state of the environment. Rapid industrialization and economic development have directly contributed to land and water pollution resulting from the application of many chemicals such as organic dyes, pharmaceuticals, and industrial reagents. The removal of these chemicals before effluent discharge is crucial for environmental protection. This review aims to explore the importance of functionalized materials in the preparation of biocatalytic systems and consider their application in eliminating water pollutants. RECENT FINDINGS: Wastewater treatment methods can be classified into three groups: (i) chemical (e.g., chemical oxidation and ozonation), (ii) physical (e.g., membrane separation and ion exchange), and (iii) biological processes. Biological treatment is the most widely used method due to its cost-effectiveness and eco-friendliness. In particular, the use of immobilized enzymes has recently become more attractive as a result of scientific progress in advanced material synthesis. The selection of an appropriate support plays an important role in the preparation of such biologically active systems. Recent studies have demonstrated the use of various materials for enzyme immobilization in the purification of water. This review identifies and discusses different biocatalytic systems used in the enzymatic degradation of various water pollutants. Materials functionalized by specific groups can serve as good support matrices for enzyme immobilization, providing chemical and thermal stability to support catalytic reactions. Enzymatic biocatalysis converts the pollutants into simpler products, which are usually less toxic than their parents. Due to immobilization, the enzyme can be used over multiple cycles to reduce the cost of wastewater treatment. Future studies in this field should focus on developing new platforms for enzyme immobilization in order to improve degradation efficiency.