Comparative study of Pb2+, Ni2+, and methylene blue adsorption on spherical waste solid-based geopolymer adsorbents enhanced with carbon nanotubes

Effective acid mine drainage remediation in fixed bed column using porous red mud/fly ash-containing geopolymer spheres

Acid mine drainage (AMD) poses a significant threat to water quality worldwide, being amongst the most problematic environmental concerns of the millennium. This work reports for the first time the remediation of real AMD, from a Portuguese abandoned mine, in fixed bed column using porous red mud/fly ash-based geopolymeric spheres. Porous waste-based spheres (2.6 ± 0.2 mm) were obtained by a suspension-solidification method through the addition of optimum foaming agent dosage. The sorbent capacity in removing cations from AMD was evaluated by targeting selected hazardous elements: Zn, Cu, Co, Pb and Ni, based on their occurrence in the effluent and potential hazard. The spheres exhibited a dual mechanism of action, simultaneously neutralizing the acidic sample while removing cations through adsorption achieving removal efficiencies between 51 % and 80 %. Other elements present in high levels, such as iron were efficiently removed (>96 %). The role of precipitation, due to the pH neutralization, and adsorption was determined. The sorbent regeneration and reusability were evaluated for up to five cycles. Moreover, the effectiveness of waste-based geopolymers treating distinct AMD waters due to seasonal variations was also evaluated, further demonstrating the effectiveness of the proposed strategy to address environmental concerns stemming from mining activities.

Adsorptive removal of heavy metals from aqueous solutions: Progress of adsorbents development and their effectiveness

Increased levels of heavy metals (HMs) in aquatic environments poses serious health and ecological concerns. Hence, several approaches have been proposed to eliminate/reduce the levels of HMs before the discharge/reuse of HMs-contaminated waters. Adsorption is one of the most attractive processes for water decontamination; however, the efficiency of this process greatly depends on the choice of adsorbent. Therefore, the key aim of this article is to review the progress in the development and application of different classes of conventional and emerging adsorbents for the abatement of HMs from contaminated waters. Adsorbents that are based on activated carbon, natural materials, microbial, clay minerals, layered double hydroxides (LDHs), nano-zerovalent iron (nZVI), graphene, carbon nanotubes (CNTs), metal organic frameworks (MOFs), and zeolitic imidazolate frameworks (ZIFs) are critically reviewed, with more emphasis on the last four adsorbents and their nanocomposites since they have the potential to significantly boost the HMs removal efficiency from contaminated waters. Furthermore, the optimal process conditions to achieve efficient performance are discussed. Additionally, adsorption isotherm, kinetics, thermodynamics, mechanisms, and effects of varying adsorption process parameters have been introduced. Moreover, heavy metal removal driven by other processes such as oxidation, reduction, and precipitation that might concurrently occur in parallel with adsorption have been reviewed. The application of adsorption for the treatment of real wastewater has been also reviewed. Finally, challenges, limitations and potential areas for improvements in the adsorptive removal of HMs from contaminated waters are identified and discussed. Thus, this article serves as a comprehensive reference for the recent developments in the field of adsorptive removal of heavy metals from wastewater. The proposed future research work at the end of this review could help in addressing some of the key limitations facing this technology, and create a platform for boosting the efficiency of the adsorptive removal of heavy metals.

Simultaneous removal of multiple metal(loid)s and neutralization of acid mine drainage using 3D-printed bauxite-containing geopolymers

The mining industry is one of the largest sources of environmental concern globally. Herein we report for the first time the application of highly porous 3D-printed sorbents containing high amounts (50 wt%) of red mud, a hazardous waste derived from the alumina industry, for the remediation of acid mine drainage (AMD). The sorption capacity of the inorganic polymers was initially evaluated for the simultaneous removal of five metal(loid) elements, namely Cu(II), Ni(II), Zn(II), Cd(II) and As(V) in synthetic wastewater. The effect of the initial concentration, pH and contact time were assessed, reaching removal efficiencies between 64% and 98%, at pH 4 and initial concentration of 50 mg L-1 of each cation, after 24 h of contact time. The 3D-printed lattices were then used for the remediation of the real AMD water samples, and the role of adsorption and acidic neutralization was investigated. Lattices were also successfully regenerated and reused up to five cycles without compromising their performance. This work paves the way for the use of an industrial waste derived from the production of alumina as raw material for the management of the hazardous AMD.

Carbon Nanotubes-Based Fuel Cell for Cr(VI) Removal and Electricity Generation

A fuel cell, an energy transducer, can convert chemical energy into electrical energy. In this work, graphite felt (GF) loaded with polypyrrole (PPy) and carboxylic carbon nanotubes (CNTs-COOH) was used as a cathode (GF/PPy/CNTs-COOH) in a double-chamber nonbiofuel cell (D-nBFC) to remove Cr(VI) efficiently. Therein, Na₂S₂O₃ in an alkaline solution and Cr(VI) in a strongly acidic solution were employed as anode and cathode solutions, respectively. An agar salt bridge, consisting of saturated KCl solution, was used to transport ions between the anode and cathode. This system suggested that the removal efficiency of Cr(VI) could reach 99.6%. The maximum current, power, and power density could achieve 136.8 μA, 18.7 μW, and 20.8 mW/m² at 90 min, respectively. Additionally, GF/PPy/CNTs-COOH also had good electrocatalytic stability and reusability after four cycles, which played an important role in the development of the D-nBFC system. Therefore, this study provides an environmentally friendly and efficient method to remove Cr(VI) and generate electricity simultaneously.

Preparation of a surface modified fly ash-based geopolymer for removal of an anionic dye: Parameters and adsorption mechanism

Geopolymers have been recently studied as environmentally friendly and low-cost adsorbents especially for the removal of cationic species in wastewater treatment mainly because of their negative surface charge at spontaneous pH conditions. Although there are very few recent studies conducted with different geopolymer composites on anionic dyes, high cost, difficulty of the composite preparation and most importantly the necessity of very low pH values limit their usage. Hence, in this study, a simple and low-cost surface modification with CTAB was applied to a previously prepared fly ash-based geopolymer (GEO) for the removal of anionic Acid Blue 185 (AB185) without the need of strongly acidic conditions. Within this scope, the effects of CTAB dosage (1-5% by weight of GEO), adsorbent dosage (0.5-3.0 g L^-1) and initial dye concentration (10-50 mg L^-1) were studied as a function of retention time (5-300 min). For 40 min, the removal efficiency of AB185 substantially increased from 0.29 up to 79.36% for the respective GEO and its modified product with 4% CTAB (MGEO4). The efficiency increased with the adsorbent (MGEO4) dosage of up to 2.0 g L^-1 at which 89.20% was obtained for 300 min. However, a little decrease was observed down to 81.10% for 3.0 g L^-1. The efficiency values of 98.19 and 89.20% were obtained for the initial AB185 concentrations of 10 and 50 mg L^-1, respectively. The Langmuir-Hinshelwood kinetic model is highly correlated with the experimental results. The high adsorption capacity attained in a very short time suggests that the main mechanism is based on physical adsorption via the electrostatic attraction between MGEO4 and AB185. Overall results have indicated that the CTAB-modified fly ash-based geopolymer can be effectively used for the adsorption of AB185.

Fabrication of Polyethyleneimine-Functionalized Magnetic Cellulose Nanocrystals for the Adsorption of Diclofenac Sodium from Aqueous Solutions

Diclofenac sodium (DS), one of the most used non-steroidal anti-inflammatory drugs worldwide, is often detected in wastewater and natural water. This drug is ecotoxic, even at low concentrations. Therefore, it is essential to fabricate low-cost adsorbents that can easily and effectively remove DS from contaminated water bodies. In this study, a polyethyleneimine (PEI)-modified magnetic cellulose nanocrystal (MCNC) was prepared with a silane coupling agent as a bridge. TEM, FTIR, XRD, and VSM were used to demonstrate the successful preparation of MCNC-PEI. This composite adsorbent exhibited efficient DS removal. Furthermore, the adsorption performance of MCNC-PEI on DS was optimal under mildly acidic conditions (pH = 4.5). Adsorption kinetics showed that the adsorption process involves mainly electrostatic interactions. Moreover, the maximum adsorption capacity reached 299.93 mg/g at 25 °C, and the adsorption capacity only decreased by 9.9% after being reused five times. Considering its low cost, low toxicity, and high DS removal capacity, MCNC-PEI could be a promising adsorbent for treating DS-contaminated water.