Modified Water Hyacinth Functionalized with Citric Acid as an Effective and Inexpensive Adsorbent for Heavy Metal-Ion Removal

Adsorptive removal of toxic heavy metals from wastewater using water hyacinth and its biochar: A review

Heavy metal contamination in aquatic ecosystems worsens due to rapid industrial expansion. Biochar, an efficient and economical adsorbent, has attracted much interest in environmental science, particularly in removing heavy metals (HMs). The paper covers basic details on biochar, its preparation, and potential chemical and inorganic modifications. Possible adsorption mechanisms of HMs on biochar, which include electrostatic attraction, ion exchange, surface complexation, chemical precipitation, and hydrogen bonding, are also discussed. These mechanisms are affected by the type of biochar used and the species of HMs present. Research findings suggest that while biochar effectively removes HMs, modifications to the carbon-rich hybrid can enhance surface properties such as surface area, pore size, functional groups, etc., and magnetic properties in a few cases, making them more efficient in HM removal. The choice of feedstock materials is one of the key parameters influencing the sorption capacity of biochars. This review aims to investigate the use of various forms of water hyacinth (WH), including aquatic plants, biomass, biochar, and modified biochar, as effective adsorbents for removing HMs from aqueous solutions and industrial effluents through a comparative analysis of their adsorption processes. However, further studies on the diverse effects of functional groups of modified biochar on HMs adsorption are necessary for future research.

An integrated biorefinery approach for the valorization of water hyacinth towards circular bioeconomy: a review

Water hyacinth (WH) has become a considerable concern for people across the globe due to its environmental and socio-economic hazards. Researchers are still trying to control this aquatic weed effectively without other environmental or economic losses. Research on WH focuses on converting this omnipresent excessive biomass into value-added products. The potential use of WH for phytoremediation and utilizing waste biomass in various industries, including agriculture, pharmaceuticals, and bioenergy, has piqued interest. The use of waste WH biomass as a feedstock for producing bioenergy and value-added chemicals has emerged as an eco-friendly step towards the circular economy concept. Here, we have discussed the extraction of bio-actives and cellulose as primary bioproducts, followed by a detailed discussion on different biomass conversion routes to obtain secondary bioproducts. The suggested multi-objective approach will lead to cost-effective and efficient utilization of waste WH biomass. Additionally, the present review includes a discussion of the SWOT analysis for WH biomass and the scope for future studies. An integrated biorefinery scheme is proposed for the holistic utilization of this feedstock in a cascading manner to promote the sustainable and zero-waste circular bio-economy concept.

Insights into the Superstable Mineralization of Chromium(III) from Wastewater by CuO

The removal of Cr^III ions from contaminated wastewater is of great urgency from both environmental protection and resource utilization perspectives. Herein, we developed a superstable mineralization method to immobilize Cr³⁺ ions from wastewater using CuO as a stabilizer, leading to the formation of a CuCr layered double hydroxide (denoted as CuCr-LDH). CuO showed a superior Cr³⁺ removal performance with a removal efficiency of 97.97% and a maximum adsorption capacity of 207.6 mg/g in a 13000 mg/L Cr³⁺ ion solution. In situ and ex situ X-ray absorption fine structure characterizations were carried out to elucidate the superstable mineralization mechanism. Two reaction pathways were proposed including coprecipitation-dissolution and topological transformation. The mineralized product of CuCr-LDH can be reused for the efficient removal of organic dyes, and the adsorption capacities were up to 248.0 mg/g for Congo red and 240.1 mg/g for Evans blue, respectively. Moreover, CuCr-LDH exhibited a good performance for photocatalytic CO₂ reduction to syngas (H₂/CO = 2.66) with evolution rates of 54.03 μmol/g·h for CO and of 143.94 μmol/g·h for H₂ under λ > 400 nm, respectively. More encouragingly, the actual tanning leather Cr³⁺ wastewater treated by CuO showed that Cr³⁺ can reduce from 3438 to 0.06 mg/L, which was much below discharge standards (1.5 mg/L). This work provides a new approach to the mineralization of Cr³⁺ ions through the "salt-oxide" route, and the findings reported herein may guide the future design of highly efficient mineralization agents for heavy metals.

Remediation of potentially toxic elements -containing wastewaters using water hyacinth - a review

For a long time, water hyacinth has been considered a very stubborn and troublesome weed. However, research has shown that it can be used to remove many pollutants from water. Among the different pollutants, potentially toxic elements (PTE) or their ions have been found to be very toxic for humans, animals, and plants. Among the many conventional methods for removing PTE from wastewaters, phytoremediation has several advantages. This method is highly eco-friendly, cost-effective, and can remove a wide range of metal pollutants and organic pollutants. Both, living and non-living water hyacinth plants, can be used for remediation - either entirely or their parts. Study on mechanisms and different factors involved in the process would help to effectively use water hyacinth for remediation. This review presents different studies conducted in the past thirty years for the removal of PTEs. Detailed analysis of the work done in this field showed that in spite of the main advantages provided by the plant, not much has been done to increase the efficiency of the remediation process and for reusing the water hyacinth biomass for other applications after desorption of the PTE. Hence, the section on scope for future work highlights these prospective ideas. Novelty statement: Water hyacinth, which is a very stubborn weed and has a negative impact on the environment, can be constructively used to remove potentially toxic elements (PTEs) along with other pollutants from wastewaters. Different parts of the water hyacinth plant like roots, leaves, and stems or the entire plant can be used. Further, either the live plant or its other forms, such as dried powder, biochar, or activated carbon can be used. This review focuses on different forms of water hyacinth plant used, the advantages and limitations associated with these methods and the scope for future work.

A novel graphene oxide-dicationic ionic liquid composite for Cr(VI) adsorption from aqueous solutions

A novel graphene oxide-dicationic ionic liquid composite (GO-DIL) was prepared by modifying graphene oxide (GO) with a dicationic ionic liquid (DIL), 3,3'-(butane-1,4-diyl) bis (1-methyl-1H-imidazol-3-ium) chloride ([C₄(MIM)₂]Cl₂). GO and GO-DIL were characterized by SEM, BET, FTIR, and XPS, and the materials were used for Cr(VI) adsorption. Batch adsorption studies showed that adsorption reached equilibrium within 40 min, and the optimal pH was 3, where the electrostatic attraction between GO-DIL and Cr(VI) was maximized. The maximum theoretical Cr(VI) adsorption capacity (q_m) was 271.08 mg g^-1, and q_m remained above 228.00 mg g^-1 after five cycles. The adsorption data were fitted well by both the pseudo-first-order kinetic model and the Langmuir model. Furthermore, thermodynamics calculations revealed that adsorption was a spontaneous endothermic process. Importantly, electrostatic attraction between Cr(VI) and the protonated imidazole N⁺ of GO-DIL played a critical role in Cr(VI) adsorption, and Cr(VI) was reduced to Cr(III). Thus, GO-DIL is predicted to be an effective adsorbent for Cr(VI) and other heavy metal ions in wastewater.

A facile approach to synthesize CdS-attapulgite as a photocatalyst for reduction reactions in water

At room temperature, a facile approach has been utilized for preparing novel CdS–attapulgite (CdS–ATP) composites and the composites were applied in photocatalytic reduction of p-nitrophenol and Cr(vi). The effect of ATP on the photocatalytic activity of the CdS–ATP composites were studied by controlling the mass ratio of attapulgite. The results showed that the CdS–20%ATP composite has an excellent photocatalytic activity. In order to figure out the key to improve the photocatalytic efficiency, the prepared composites were characterized by Brunauer–Emmett–Teller (BET) specific surface area, UV-vis diffuse reflectance spectroscopy (DRS) and electrochemical impedance spectroscopy (EIS). The superior photocatalytic performance of the CdS–20%ATP composite can be ascribed to the existence of the ATP which can fix the CdS and prevent agglomeration. The interaction between ATP and CdS in the composites facilitates the electron transfer and also promoted their photocatalytic performance. This work provides us with some significant guidance in the development of CdS–ATP composite photocatalysts.

The application of CdS–attapulgite composites in photocatalytic reduction of p-nitrophenol and Cr(vi) demonstrated that the attapulgite could overcome the limitations of CdS.

Water hyacinth for energy and environmental applications: A review

This review is focused on the sustainable management of harvested water hyacinth (WH) via thermochemical conversion to carbonaceous materials (CMs), biofuels, and chemicals for energy and environmental applications. One of the major challenges in thermochemical conversion is to guarantee the phytoremediation performance of biochar and the energy conversion efficiency in biowaste-to-energy processes. Thus, a circular sustainable approach is proposed to improve the biochar and energy production. The co-conversion process can enhance the syngas, heat, and energy productions with high-quality products. The produced biochar should be economically feasible and comparable to available commercial carbon products. The removal and control of heavy and transition metals are essential for the safe implementation and management of WH biochar. CMs derived from biochar are of interest in wastewater treatment, air purification, and construction. It is important to control the size, shape, and chemical compositions of the CM particles for higher-value products like catalyst, adsorbent or conductor.

Discoloration of azo dye Brown HT using different advanced oxidation processes

In this study, known combinations of Advanced Oxidation Processes (AOPs, namely Electro-Fenton (EF), Photo-Electro-Fenton (PEF), Electro-Oxidation (EO), and EO/Ozone (O₃) were compared for the discoloration of tannery industry azo dye Brown HT (BHT). The different AOPs were tested in a 0.160 L batch electrochemical stirred thank reactor using Boron Doped Diamond (BDD) electrodes. The influence of parameters such as the current density (j) and the initial BHT concentration were to exanimated on the efficiency of all the tested processes. The oxidation tendency of EF, and PEF were compared with those of EO and O₃, based on their efficiency for BHT discoloration, which resulted as PEF > EF > EO > O₃. The AOPs showing the best oxidation performance was PEF which, using Na₂SO₄ (0.05 M) electrolyte solution and Fe²⁺ (0.5 mM), pH 3.0, j = 71 mA cm^-2, and 500 rpm process, achieved 100% discoloration and 80% chemical oxygen demand (COD) abatement after 60 min of treatment for two initial BHT concentrations (50 and 80 mg L^-1). The process accounted for a current efficiency of 30% and energy consumption 2.25 kWh (g COD)^-1 through the discoloration test. The azo dye gradually degraded, yielding non-toxic oxalic, oxamic, and glyoxylic acid, whose Fe(III) complexes were quickly photolyzed.

Electrochemical oxidative degradation of X-6G dye by boron-doped diamond anodes: Effect of operating parameters

Boron-doped diamond (BDD) is an excellent electrode material. As the anode in an electrochemical degradation tank, BDD has been receiving widespread attention for the treatment of azo dye wastewater. In this study, electrochemical oxidation (EO) was applied to electrolyze reactive brilliant yellow X-6G (X-6G) using BDD as the anode and Pt as the cathode. To balance the degradative effects and power consumption in the electrolysis process, the effects of a series of operating parameters, including current density, supporting electrolyte, initial pH, reaction temperature and initial dye concentration, were systematically studied. The oxidative process was analyzed by color removal rate, and the degree of mineralization was evaluated by TOC. The optimal experimental parameters were finally determined: 100 mA cm^-2, 0.05 M Na₂SO₄ electrolyte, pH 3.03, 60 °C, and an initial X-6G concentration of 100 mg L^-1. As a result, color completely disappeared after 0.75 h of electrolysis, and TOC was removed by 72.8% after 2 h of electrolysis. In conclusion, the EO of a BDD electrode as an anode can be a potent treatment method for X-6G synthetic wastewater.