Hydrogel biochar composite for arsenic removal from wastewater

Chitosan-sunflower meal biochar hydrogel incorporated with green synthesized NiO nanoparticles for enhanced catalytic reduction of anthropogenic water pollutants

Anthropogenic activities have been one of the crucial driving factors for water pollution globally, thereby warranting a sustainable strategy for its redressal. In this study, we have developed a hydrogel-biochar nanocomposite for catalytic reduction of water pollutants. To begin with, green synthesis of nickel oxide nanoparticles (NiO NPs) was accomplished from waste kinnow peel extract via the environmentally benign microwave method. The formation of NiO NPs was affirmed from different analytical techniques namely ultraviolet-visible (UV-Vis), Fourier transform infrared (FTIR), X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) and energy-dispersive spectroscopy (EDS). The FESEM images revealed spherical nature of NiO NPs. The average particle size was found to be 15.61 nm from XRD data. A novel hydrogel-biochar nanocomposite comprising the green NiO NPs, sunflower meal biochar and chitosan was prepared (Cs-biochar@ NiO) and explored as a nanocatalyst towards catalytic reduction of pollutants such as 4-nitrophenol, potassium hexacyanoferrate (III) and organic dyes methyl orange (MO), Congo red (CR), methylene blue (MB) in the presence of a reducing agent, i.e. NaBH4. Under optimized conditions, the reduction reactions were completed by 120 s and 60 s for 4-NP and potassium hexacyanoferrate (III) respectively and the rate constants were estimated to be 0.044 s-1 and 0.110 s-1. The rate of reduction was found to be faster for the dyes and the respective rate constants were 0.213 s-1 for MO, 0.213 s-1 for CR and 0.135 s-1 for MB. The assessment of the nanocatalyst in the reduction of binary dye systems depicted its selectivity towards the anionic dyes CR and MO. The nanocatalyst displayed effective reduction of dyes in real-water samples collected from different sources. Taken altogether, this study validates the design of sustainable hydrogel-biochar nanocatalyst for the efficient reduction of hazardous anthropogenic water pollutants.

Machine learning based prediction and experimental validation of arsenite and arsenate sorption on biochars

Arsenic (As) contamination in water is a significant environmental concern with profound implications for human health. Accurate prediction of the adsorption capacity of arsenite [As(III)] and arsenate [As(V)] on biochar is vital for the reclamation and recycling of polluted water resources. However, comprehending the intricate mechanisms that govern arsenic accumulation on biochar remains a formidable challenge. Data from the literature on As adsorption to biochar was compiled and fed into machine learning (ML) based modelling algorithms, including AdaBoost, LGBoost, and XGBoost, in order to build models to predict the adsorption efficiency of As(III) and As(V) to biochar, based on the compositional and structural properties. The XGBoost model showed superior accuracy and performance for prediction of As adsorption efficiency (for As(III): coefficient of determination (R2) = 0.93 and root mean square error (RMSE) = 1.29; for As(V), R2 = 0.99, RMSE = 0.62). The initial concentrations of As(III) and As(V) as well as the dosage of the adsorbent were the most significant factors influencing adsorption, explaining 48 % and 66 % of the variability for As(III) and As(V), respectively. The structural properties and composition of the biochar explained 12 % and 40 %, respectively, of the variability of As(III) adsorption, and 13 % and 21 % of that of As(V). The XGBoost models were validated using experimental data. R2 values were 0.9 and 0.84, and RMSE values 6.5 and 8.90 for As(III) and As(V), respectively. The ML approach can be a valuable tool for improving the treatment of inorganic As in aqueous environments as it can help estimate the optimal adsorption conditions of As in biochar-amended water, and serve as an early warning for As-contaminated water.

Biochar modification to enhance arsenic removal from water: a review

Arsenic (As) contamination is a major threat to drinking water quality throughout the world, and the development of appropriate remediation methods is critical. Adsorption is considered the most effective method for remediation of As-contaminated water. Biochar is a promising adsorbent and widely discussed for As removal due to its potential low cost and environmental friendliness. However, pristine biochar generally exhibited relatively low adsorption capacity for As mainly due to the electrostatic repulsion between the negatively charged biochar and As. Biochar modification, especially metal modification, was developed to boost the adsorption capacity for As. A systematic analysis of As removal as affected by biochar properties and modification will be of great help for As removal. This paper presents a comprehensive review on As removal by biochars from different feedstock, preparation procedures, and modification methods, with a major focus on the possible mechanisms of interaction between As and biochar. Biochar derived from sewage sludge exhibited relatively high adsorption capacity for As. Considering energy conservation, biochars prepared at 401-500 °C were more favorable in adsorbing As. Fe-modified biochar was the most popular modified biochar for As remediation due to its low cost and high efficiency. In addition, the limitations of the current studies and future perspectives are presented. The aim of this review is to provide guidance for the preparation of low-cost, environmentally friendly, and high efficiency biochar for the remediation of As-contaminated water.

Remediation of cadmium or arsenic contaminated water and soil by modified biochar: A review

Biochar has a high specific surface area with abundant pore structure and functional groups, which has been widely used in remediation of cadmium or arsenic contaminated water and soil. However, the bottleneck problem of low-efficiency of pristine biochar in remediation of contaminated environments always occurs. Nowadays, the modification of biochar is a feasible way to enhance the performance of biochar. Based on the Web of science™, the research progress of modified biochar and its application in remediation of cadmium or arsenic contaminated water and soil have been systematically summarized in this paper. The main modification strategies of biochar were summarized, and the variation of physicochemical properties of biochar before and after modification were illustrated. The efficiency and key mechanisms of modified biochar for remediation of cadmium or arsenic contaminated water and soil were expounded in detail. Finally, some constructive suggestions were given for the future direction and challenges of modified biochar research.

Research Progress on Adsorption of Arsenic from Water by Modified Biochar and Its Mechanism: A Review

Arsenic (As) is a non-metallic element, which is widely distributed in nature. Due to its toxicity, arsenic is seriously harmful to human health and the environment. Therefore, it is particularly important to effectively remove arsenic from water. Biochar is a carbon-rich adsorption material with advantages such as large specific surface area, high porosity, and abundant functional groups, but the original biochar has limitations in application, such as limited adsorption capacity and adsorption range. The modified biochar materials have largely enhanced the adsorption capacity of As in water due to their improved physicochemical properties. In this review, the changes in the physicochemical properties of biochar before and after modification were compared by SEM, XRD, XPS, FT-IR, TG, and other characterization techniques. Through the analysis, it was found that the adsorbent dosage and pH are the major factors that influence the As adsorption capacity of the modified biochar. The adsorption process of As by biochar is endothermic, and increasing the reaction temperature is conducive to the progress of adsorption. Results showed that the main mechanisms include complexation, electrostatic interaction, and precipitation for the As removal by the modified biochar. Research in the field of biochar is progressing rapidly, with numerous achievements and new types of biochar-based materials prepared with super-strong adsorption capacity for As. There is still much space for in-depth research in this field. Therefore, the future research interests and applications are put forward in this review.

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.

Green Hydrogel-Biochar Composite for Enhanced Adsorption of Uranium

Uranium is the backbone of the nuclear fuel used for energy production but is still a hazardous environmental contaminant; thus, its removal and recovery are important for energy security and environmental protection. So far, the development of biocompatible, efficient, economical, and reusable adsorbents for uranium is still a challenge. In this work, a new orange peel biochar-based hydrogel composite was prepared by graft polymerization using guar gum and acrylamide. The composite's structural, morphological, and thermal characteristics were investigated via Fourier transform infrared (FTIR), scanning electron microscope (SEM), X-ray diffraction (XRD), and thermogravimetric analysis (TGA) methods. The composite's water absorption properties were investigated in different media. The performance of the prepared composite in adsorbing uranium (VI) ions from aqueous media was systematically investigated under varying conditions including solution's acidity and temperature, composite dose, contact time, and starting amount of uranium. The adsorption efficiency increased with solution pH from 2 to 5.5 and composite dose from 15 to 50 mg. The adsorption kinetics, isotherms, and thermodynamics parameters were analyzed to get insights into the process's feasibility and viability. The equilibrium data were better described through a pseudo-second-order mechanism and a Langmuir isotherm model, indicating a homogeneous composite surface with the maximum uranium (VI) adsorption capacity of 263.2 mg/g. The calculated thermodynamic parameters suggested that a spontaneous and endothermic process prevailed. Interference studies showed high selectivity toward uranium (VI) against other competing cations. Desorption and recyclability studies indicated the good recycling performance of the prepared composite. The adsorption mechanism was discussed in view of the kinetics and thermodynamics data. Based on the results, the prepared hydrogel composite can be applied as a promising, cost-effective, eco-friendly, and efficient material for uranium (VI) decontamination.

Advances in decontamination of wastewater using biomass-basedcomposites: A critical review

Contaminant removal from wastewater using natural biosorbents has been widely studied as a suitable and environmentally benign alternative for conventional techniques. Currently, researchers are working on various biomass-based composites for wastewater remediation to improve the performance of natural biosorbents. This review takes into focus a wide range of biomass-based composites like hydrogel composites, metal oxide composites, magnetic composites, polymer composites, carbon nanotubes (CNTs) and graphene composites, metal organic framework composites (MOFs) and clay composites for the removal of various contaminants from wastewater. It is evident from the literature survey that the composite fabrication involves the modification of morphological and textural features of the biomass which results in significant enhancement of adsorption capacity. Apart from this, regeneration of the used biomass-based composite is also studied in depth in order to overcome the problem of solid waste generation. This review would prove to be beneficial for researchers who are currently focusing on the development of cost-effective, easily available, recyclable biomass-based composites with enhanced adsorption capacities for wastewater treatment.

Biochar versus bone char for a sustainable inorganic arsenic mitigation in water: What needs to be done in future research?

Arsenic (As) is an emerging contaminant on a global scale posing threat to environmental and human health. The relatively brief history of the applications of biochar and bone char has mapped the endeavors to remove As from water to a considerable extent. This critical review attempts to provide a comprehensive overview for the first time on the potential of bio- and bone-char in the immobilization of inorganic As in water. It seeks to offer a rational assessment of what is existing and what needs to be done in future research as an implication for As toxicity of human health risks through acute and chronic exposure to As contaminated water. Bio- and bone-char are recognized as promising alternatives to activated carbon due to their lower production and activation cost. The surface modification via chemical methods has been adopted to improve the adsorption capacity for anionic As species. Surface complexation, ion exchange, precipitation and electrostatic interactions are the main mechanisms involved in the adsorption of As onto the char surface. However, arsenic-bio-bone char interactions along with their chemical bonding for the removal of As in aqueous solution is still a subject of debate. Hence, the proposed mechanisms need to be scrutinized further using advanced analytical techniques such as synchrotron-based X-ray. Moving this technology from laboratory phase to field scale applications is an urgent necessity in order to establish a sustainable As mitigation in drinking water on a global scale.

Development and regeneration of composite of cationic gel and iron hydroxide for adsorbing arsenic from ground water

Globally, arsenic contaminated groundwater is a serious concern for human health. Previous studies have developed various methods to remove arsenic. But, most of them fail to selectively adsorb arsenic and regenerate. In this study, we developed an adsorbent, a cationic polymer gel loaded with iron hydroxide, which can adsorb arsenic from groundwater more effectively than the other adsorbents. The cationic polymer gel is N,N-dimethylamino propylacrylamide, methyl chloride quaternary (DMAPAAQ). The preparation of the gel is different from the other polymer gels used for adsorption of arsenic and other metals, and it ensures that the gel contains 53.7% FeOOH particles. It should also provide good selectivity, be simple to use and be cost-effective in terms of reusability. The study showed that the gel selectively adsorbed arsenic effectively at neutral pH levels. The results demonstrate that the maximum amount of As(V) adsorption was 123.4 mg/g, which is higher than the other adsorbents. In addition, the gel adsorbed As(V) selectively in the presence of Sulphate. Also, regeneration of the gel was performed for eight consecutive days with 87.6% effectiveness. Additionally, the adsorption mechanism of this gel composite and time required for reaching the equilibrium adsorption is discussed in this paper.

Arsenic Removal Using "Green" Renewable Feedstock-Based Hydrogels: Current and Future Perspectives

In the recent times, scanty access to clean water has been one of the most prevalent problems, affecting humankind throughout the world. This calls for a tremendous amount of research to recognize new methods of purifying water at lower cost, minimizing the use of hazardous chemicals and impact on the environment. The interest of the scientific community in the potential applications of renewable feedstock-based hydrogels for heavy-metal adsorption for water remediation has been continuously increasing during the last few decades. This study is an effort to highlight the application of hydrogels for revolutionizing the present research on heavy-metal adsorption, particularly arsenic. Besides, the arsenic chemistry, health hazards of arsenic to human health, and adsorption of arsenic by natural polymer-based hydrogels have been reviewed in detail. In addition, challenges in taking the hydrogel technology forward and future prospectives like cost, handling, and disposal of the adsorbent have been discussed systematically.

Synthesis and characterization of an iron-impregnated biochar for aqueous arsenic removal

The iron (Fe)-impregnated biochar (FBC), fabricated via thermal pyrolysis of corn straw treated with FeCl₃, was investigated for the sorption characteristics and mechanisms of aqueous arsenate removal. Structural and morphological analysis showed that large quantity of iron oxide particles tightly grew within the porous matrix of biochar (BC) through iron-impregnation. Batch sorption experimental results showed that the composite, with larger surface area, more functional groups, and greater thermal stability, exhibited excellent As(V) adsorption efficiency of 6.80mg/g compared to 0.017mg/g for unmodified BC (a 400-fold increase). The adsorption kinetics data were fitted well by pseudo second-order model, and sorption isotherms of As(V) were simulated well by both Freundlich and Langmuir models. XRD and FTIR analysis suggested that electrostatic attraction and precipitation were dominant mechanisms for As(V) sorption. The As(V)-loaded FBC could be easily separated from the solution by a magnet at the end of the sorption experiment. The FBC showed excellent re-sorption capacity, which account for about 70% removal efficiency for the second and third reuse in As(V) sorption. Results from this study demonstrated the promise of FBC composite as an efficient, low-cost, environmentally friendly, and regenerable adsorbent for As(V) remediation.

CAPSULE

FBC showed enhanced As(V) sorption capacity, excellent re-sorption capacity, and could be easily separated by a magnet.

Enhanced As (V) Removal from Aqueous Solution by Biochar Prepared from Iron-Impregnated Corn Straw

Fe-loaded adsorbents have received increasing attention for the removal of arsenic in contaminated water or soil. In this study, Fe-loaded biochar was prepared from iron-impregnated corn straw under a pyrolysis temperature of 600°C. The ratio of crystalline Fe oxides including magnetite and natrojarosite to amorphous iron oxyhydroxide in the composite was approximately 2 : 3. Consisting of 24.17% Fe and 27.76% O, the composite exhibited a high adsorption capacity of 14.77 mg g−1 despite low surface areas (4.81 m2 g−1). The pH range of 2.0–8.0 was optimal for arsenate removal and the adsorption process followed the Langmuir isotherms closely. In addition, pseudo-second-order kinetics best fit the As removal data. Fe oxide constituted a major As-adsorbing sink. Based on the X-ray diffraction spectra, saturation indices, and selective chemical extraction, the data suggested three main mechanisms for arsenate removal: sorption of arsenate, strong inner-sphere surface complexes with amorphous iron oxyhydroxide, and partial occlusion of arsenate into the crystalline Fe oxides or carbonized phase. The results indicated that the application of biochar prepared from iron-impregnated corn straw can be an efficient method for the remediation of arsenic contaminated water or soil.

A holistic review of hydrogel applications in the adsorptive removal of aqueous pollutants: Recent progress, challenges, and perspectives

Due to the unique physical and chemical characteristics of hydrogels, such as hydrophilicity, swellability, and modifiability, there is increasing research interest in the development and application of novel hydrogels in water and wastewater treatment. Hydrogels have exhibited superior performance in the adsorptive removal of a wide range of aqueous pollutants including heavy metals, nutrients, and toxic dyes. However, there remain certain challenges which need to be addressed in order to evolve hydrogel based treatment systems from the lab-scale to practical applications. This review provides a coverage of the latest developments in the application of hydrogels for the adsorptive removal of aqueous pollutants. A holistic overview of different steps involved in the hydrogel based treatment systems is provided, and the influencing factors and mechanisms of pollutants removal are reviewed. Major challenges pertaining to adsorption kinetics, operational pH range, interference, and hydrogel recovery are examined. Important considerations such as stability and reusability of hydrogels and resource recovery are also discussed, for economic and sustainability concerns.