Investigation of 3-aminopropyltriethoxysilane modifying attapulgite for Congo red removal: Mechanisms and site energy distribution

Colloid Mill-Assisted Ultrasonic-Fractional Centrifugal Purification of Low-Grade Attapulgite and Its Modification for Adsorption of Congo Red

Attapulgite (APT) is widely used in wastewater treatment due to its exceptional adsorption and colloidal properties, as well as its cost-effectiveness and eco-friendliness. However, low-grade APT generally limits its performance. Here, a colloid mill-assisted ultrasonic-fractional centrifugal purification method was developed to refine low-grade APT. This process successfully separated and removed impurity minerals such as quartz and dolomite from the raw ore, resulting in a refined APT purity increase from 16.9% to 60% with a specific surface area of 135.5 m2∙g-1. Further modifying of the refined APT was carried out through the hydrothermal method using varying dosages of cetyltrimethylammonium chloride (CTAC), resulting in the production of four different APT adsorbents denoted as QAPT-n (n = CTAC mole number) ranging from 0.5 to 5 mmol. Using Congo red (CR) as the target pollutant, the QAPT-5 sample exhibited the best adsorption capacity with the maximum quantity of 1652.2 mg∙g-1 in a neutral solution at 30 °C due to the highest surface charge (zeta potential = 8.25 mV). Moreover, the QAPT-5 pellets (~2.0 g adsorbent) shaped by the alginate-assisted molding method removed more than 96% of 200 mL aqueous solution containing 200 mg∙L-1 CR and maintained this efficiency in 10 adsorption-elution cycles, which exhibited the promising practical application.

Biochar supported nanoscale zerovalent iron-calcium alginate composite for simultaneous removal of Mn(II) and Cr(VI) from wastewater: Sorption performance and mechanisms

Heavy metal pollution in water caused by industrial activities has become a global environmental issue. Among them, manganese mining and smelting activities have caused the combined pollution of Cr(VI) and Mn(II) in water, posing a serious ecotoxicological risk to ecological environments and human health. To efficiently remove Cr(VI) and Mn(II) from wastewater, a novel biochar supported nanoscale zerovalent iron-calcium alginate composite (CA/nZVI/RSBC) was synthesized by liquid-phase reduction and calcium alginate embedding methods. The adsorption performance and mechanisms of Cr(VI) and Mn(II) by CA/nZVI/RSBC were investigated. The maximum adsorption capacities of Cr(VI) and Mn(II) onto CA/nZVI/RSBC fitted by the Langmuir model were 5.38 and 39.78 mg/g, respectively, which were much higher than the pristine biochar. The iron release from CA/nZVI/RSBC was comparatively lower than that of nZVI/RSBC. Mn(II) presence enhanced the reduction of Cr(VI) by CA/nZVI/RSBC. The results of XRD, XPS, and site energy distribution analysis indicated that redox was the predominant mechanism of Cr(VI) adsorption, while electrostatic attraction dominated Mn(II) adsorption. This study provides a novel alternative way for the simultaneous removal of Cr(VI) and Mn(II) in wastewater.

A Novel Vermiculite/TiO2 Composite: Synergistic Mechanism of Enhanced Photocatalysis towards Organic Pollutant Removal

There has been increasing concern over water pollution, which poses a threat to human life and health. Absorption by low-cost absorbents is considered to be a cost-effective and efficient route. However, the non-reusability of absorbents greatly limits their applications. In this study, a novel vermiculite/TiO2 composite combining the inexpensive absorbent with the commonly used photocatalyst was firstly synthesized via the sol-gel method. On the one hand, the organic pollutants are absorbed by vermiculite and then decomposed through the photocatalysis process, enabling the next round of absorption and creating an absorption-decomposition reusable cycle. On the other hand, the modulation effect of optical and electronic structure on the prepared TiO2 photocatalyst by the vermiculite incorporation could significantly improve the photocatalytic activity and eventually enhance the aforementioned cyclic degradation capacity. The layer-structured vermiculite (Vt) supports a uniform coverage of TiO2 at an optimized ratio, providing an optimal adsorption environment and contact area between the photocatalyst and methylene blue (MB) molecules. Vt/TiO2 heterojunction is formed with Si-O-Ti bonding, at which electrons transfer from Vt to TiO2, enriching electron density in TiO2 and favoring its photocatalytic activity. Furthermore, the incorporation of Vt increases the light absorption of TiO2 in the visible range by narrowing the optical band gap to 1.98 eV, which could promote the generation of photo-excited carriers. In addition, PL measurements revealed that the carrier recombination is substantially suppressed, and the charge separation and migration are greatly enhanced by a factor of 3. As a result, the decomposition rate of MB is substantially increased 5.3-fold, which is ascribed to the synergistic effects of the elevated photocatalysis and the large absorption capacity governed by the chemisorption mechanism of the intra-particle diffusion. These results pave the way for composite design towards efficient, economical, and pragmatic water pollution treatment.

Surface modification of quartz sand: A review of its progress and its effect on heavy metal adsorption

Quartz sand (SiO₂) is a prevalent filtration medium, boasting wide accessibility, superior stability, and cost-effectiveness. However, its utility is often curtailed by its sleek surface, limited active sites, and swift saturation of adsorption sites. This review outlines the prevalent strategies and agents for quartz sand surface modification and provides a comprehensive analysis of the various modification reagents and their operative mechanisms. It delves into the mechanism and utility of surface-modified quartz sand for adsorbing heavy metal ions (HMIs). It is found that the reported modifiers usually form connections with the surface of quartz sand through electrostatic forces, van der Waals forces, pore filling, chemical bonding, and/or molecular entanglement. The literature suggests that these modifications effectively address issues inherent to natural quartz sand, such as its low superficial coarseness, rapid adsorption site saturation, and limited adsorption capacity. Regrettably, comprehensive investigations into the particle size, regenerative capabilities, and application costs of surface-modified quartz sand and the critical factors for its wider adoption are lacking in most reports. The adsorption mechanisms indicate that surface-modified quartz sand primarily removes HMIs from aqueous solutions through surface complexation, ion exchange, and electrostatic and gravitational forces. However, these findings were derived under controlled laboratory conditions, and practical applications for treating real wastewater necessitate overcoming further laboratory-scale obstacles. Finally, this review outlines the limitations of partially surface modified quartz sand and suggests potential venues for future developments, providing a valuable reference for the advancement of cost-effective, HMI-absorbing, surface-modified quartz sand filter media.

Adsorption of Brilliant Green Dye onto a Mercerized Biosorbent: Kinetic, Thermodynamic, and Molecular Docking Studies

This study reports the valorization of pistachio shell agricultural waste, aiming to develop an eco-friendly and cost-effective biosorbent for cationic brilliant green (BG) dye adsorption from aqueous media. Pistachio shells were mercerized in an alkaline environment, resulting in the treated adsorbent (PS_NaOH). The morphological and structural features of the adsorbent were analyzed using scanning electron microscopy, Fourier transform infrared spectroscopy, and polarized light microscopy. The pseudo-first-order (PFO) kinetic model best described the adsorption kinetics of the BG cationic dye onto PS_NaOH biosorbents. In turn, the equilibrium data were best fitted to the Sips isotherm model. The maximum adsorption capacity decreased with temperature (from 52.42 mg/g at 300 K to 46.42 mg/g at 330 K). The isotherm parameters indicated improved affinity between the biosorbent surface and BG molecules at lower temperatures (300 K). The thermodynamic parameters estimated on the basis of the two approaches indicated a spontaneous (ΔG < 0) and exothermic (ΔH < 0) adsorption process. The design of experiments (DoE) and the response surface methodology (RSM) were employed to establish optimal conditions (sorbent dose (SD) = 4.0 g/L and initial concentration (C₀) = 10.1 mg/L), yielding removal efficiency of 98.78%. Molecular docking simulations were performed to disclose the intermolecular interactions between the BG dye and lignocellulose-based adsorbent.

Gentle one-step co-precipitation to synthesize bimetallic CoCu-MOF immobilized laccase for boosting enzyme stability and Congo red removal

Laccase has received extensive attention in pollutant degradation due to its high efficiency and environmental friendliness, but free laccase has poor stability, easy inactivation, and difficulty in recycling, which limited its application. It was a smart strategy to construct a synergistic system for the efficient adsorption and degradation of pollutants by enzyme immobilization to improve the stability and recyclability of the enzyme. In this study, the materials were synthesized by a one-step co-precipitation method. With Cu-MOF as the main body, Co²⁺ was introduced to construct bimetallic CoCu-MOF as the protective carrier of the enzyme. The enzyme-carrying capacity and enzyme activity of Lac@CoCu-MOF were 2-fold and 3.5-fold higher than those of Lac@Cu-MOF, respectively. Lac@MOF composites had a good protective effect on enzyme in various interfering environments. At pH = 7, free laccase was completely inactivated and Lac@CoCu-MOF maintained 51.76% enzyme activity. In addition, the removal rate of Congo red by Lac@CoCu-MOF reached 90 % in 1 h at pH = 4 % and 95 % in 5 h at pH = 7, and the final TOC mineralization rate reached 86.05 %. After six cycles, the degradation rate of Lac@CoCu-MOF remained above 75 %. Therefore, Lac@CoCu-MOF was constructed with the advantages of enzyme immobilization (enhanced stability and easy operation), material adsorption, and biocatalysis (fast diffusion and high activity), which has great guiding significance for the industrial application of enzyme.

Adsorption of Tannic Acid and Macromolecular Humic/Fulvic Acid onto Polystyrene Microplastics: A Comparison Study

Dissolved organic matter (DOM) has been widely reported to influence the environmental behavior of microplastics (MPs), but little is known about the properties and mechanisms of interaction between specific DOM components and MPs. Here, we studied the adsorption of three representative DOM components (humic acid, HA; fulvic acid, FA; and tannic acid, TA) on polystyrene (PS) MPs in batch adsorption experiments. Results revealed that HA/FA adsorption was greater under acidic conditions, while higher TA adsorption on PS was found at pH 4 and 6. The divalent cation (Ca2+) exerted a more prominent role in enhancing HA, FA, and TA adsorption on PS than did monovalent ones (K+ and Na+). The adsorption process fitted well with the Freundlich isotherm model and the pseudo-second-order kinetics model. The adsorption site heterogeneity was evaluated using the site energy distribution analysis based on the Freundlich model. The greater binding ability of HA on the PS surface caused a more negatively charged surface than FA/TA, as reflected by Zeta potential values. The findings of this study not only provide valuable information about the adsorption behavior and interaction processes of various DOM components on PS MPs, but also aid our efforts to evaluate the environmental behaviors of MPs.

Organic Dyes versus Adsorption Processing

Even in the first quarter of the XXI century, the presence of organic dyes in wastewaters was a normal occurrence in a series of countries. As these compounds are toxic, their removal from these waters is a necessity. Among the separation technologies, adsorption processing appeared as one of the most widely used to reach this goal. The present work reviewed the most recent approaches (first half of the 2021 year) regarding the use of a variety of adsorbents in the removal of a variety of organic dyes of different natures.