Site-imprinted hollow composites with integrated functions for ultra-efficient capture of hexavalent chromium from water

Amino-Functionalized Lotus Stem Hydrochar for Rapid Adsorption and In Situ Detoxification of Cr(VI) from Water

The development of low-cost, efficient, and environmentally friendly adsorbents is the key to highly toxic hexavalent chromium [Cr(VI)] removal by adsorption. In this paper, amino-functionalized lotus stem hydrochar (ALSHC) was prepared from an agricultural waste lotus stem (LS) for the adsorption removal of Cr(VI) from water. The effects of the initial Cr(VI) concentration, contact time, temperature, coexisting anions, and reusability of ALSHC on Cr(VI) removal were examined in detail. The adsorption mechanism was further discussed by investigating the impact of the solution's initial pH, the relation between the pH change in solution and Cr(VI) removal during the process, the changes of chromium (Cr) species in solution and on ALSHC during adsorption, and the XPS characterization. The results demonstrated that ALSHC effectively removed Cr(VI) from water with rapid adsorption (the removal rate reached 80.90% in only 10 min) and in situ detoxification. Most importantly, ALSHC still had better adsorption performance (adsorption capacity of 30.95 mg g-1) than commercially activated carbon, even at pH = 9.00. The adsorption of Cr(VI) by ALSHC accorded with the pseudo-second-order kinetic model and Langmuir isotherm model, indicating a monolayer chemisorption process. The adsorption process was shown to be spontaneous and endothermic based on the thermodynamic characteristics (ΔG0 < 0, ΔH0 > 0, and ΔS0 > 0). The mechanism of Cr(VI) removal was mainly composed of three parts in sequence: Firstly, Cr(VI) in solution was quickly adsorbed onto ALSHC with protonated -NH2 through electrostatic attraction; subsequently, the adsorbed Cr(VI) on ALSHC was mostly detoxicated by in situ reduction; and finally, the reduced Cr(III) and the remaining Cr(VI) were fixed on the ALSHC surface by complexation. The prepared ALSHC displayed a certain superiority in Cr(VI) adsorption and had the prospect of further development.

Using NH2-MIL-125(Ti) for efficient removal of Cr(VI) and RhB from aqueous solutions: Competitive and cooperative behavior in the binary system

The coexistence of inorganic and organic contaminants is a challenge for real-life water treatment applications. Therefore, in this research, we used NH2-MIL-125(Ti) to evaluate the single adsorption of hexavalent chromium (Cr(VI)) or Rhodamine B (RhB) in an aqueous solution and further investigate simultaneous adsorption experiments to compare the adsorption behavior changes. The main influencing factors, for example, reaction time, initial concentration, reaction temperature, and pH were studied in detail. In all reaction systems, the pseudo-second-order kinetic and Langmuir isotherm models were well illuminated the adsorption progress of Cr(VI) and RhB. Thermodynamic studies showed that the adsorption process was spontaneous and endothermic. As compared to the single system, the adsorption capacity of Cr(VI) in the binary system gradually decreased as the additive amount of RhB increased, whereas the adsorption capacity of RhB in the binary system was expanded brilliantly. When the binary reaction system contained 100 mg/L Cr(VI), the removal rate of RhB increased to 97.58%. The formation of Cr(VI)-RhB and Cr(III)-RhB complexes was the cause that provided facilitation for the adsorption of RhB. These findings prove that the interactions during the water treatment process between contaminants may obtain additional benefits, contributing to a better adsorption capacity of co-existing contaminant.

Preparation, characterization, and adsorption kinetics of graphene oxide/chitosan/carboxymethyl cellulose composites for the removal of environmentally relevant toxic metals

This study attempted to develop a low-cost and eco-friendly bio-based composite adsorbent that is highly efficient in capturing potential toxic metals. The bio-composite adsorbent was prepared using graphene oxide (GO), carboxymethyl cellulose (CMC) and chitosan (CS); and characterized using FTIR, SEM-EDX and WAXD techniques. Metal-ion concentration in an aqueous solution was measured by ICP-OES. This article reveals that the adsorption of heavy metal ions varied according to the adsorbent quantity, initial metal concentration, pH, and interaction time. The metal ions' adsorption capacity (mg/g) was observed to increase when the interaction time and metal concentration increased. Conversely, metal ions adsorption was decreased with an increase in adsorbent dosages. The effect of pH on metal ions' adsorption was ion-specific. The substantial adsorption by GO/CMC/CS composite for Co2+, CrO42-, Mn2+ and Cd2+, had the respective values of 43.55, 77.70, 57.78, and 91.38 mg/g under acidic conditions. The metal ions experimental data were best fitted with pseudo-second-order (PSO) kinetics, and Freundlich isotherm model (except Co2+). The separation factors (RL) value in the present investigation were found between 0 and 1, meaning that the metal ions adsorption onto GO/CS/CMC composite is favorable. The RL and sorption intensity (1/n) values fitted well to the adsorption isotherm.

Facile synthesis of sodium lignosulfonate/polyethyleneimine/sodium alginate beads with ultra-high adsorption capacity for Cr(VI) removal from water

Chromium (VI) is a widely occurring toxic heavy metal ion in industrial wastewater that seriously impacts the environment. In this study, we used environmentally friendly sodium lignosulfonate (SL), polyethyleneimine (PEI), and sodium alginate (SA) to synthesize SL/PEI/SA beads by employing a simple crosslinking method with to develop a novel absorbent with excellent adsorption capacity and practical application in wastewater treatment. We studied the adsorption performance of SL/PEI/SA through batch adsorption and continuous dynamic adsorption experiments. SL/PEI/SA has ultra-high adsorption capacity (2500 mg·g^-1) at 25 ℃, which is much higher than that of existing adsorbents. Humic acids and coexisting anions commonly found in wastewater have minimal effect on the adsorption performance of SL/PEI/SA. In the column system, 1 g SL/PEI/SA can treat 8.1 L secondary electroplating wastewater at a flow rate of 0.5 mLmin^-1, thereby enabling the concentration of Cr(VI) in secondary electroplating wastewater to meet the discharge standard (< 0.2 mg·L^-1). It is worth noting that the concentration of competitive ions in secondary electroplating wastewater is more than 500 times higher than that of Cr(VI). These results demonstrate that the novel SL/PEI/SA beads can be effectively applied in the removal of Cr(VI) in wastewater.

Rationally designed carboxymethylcellulose-based sorbents crosslinked by targeted ions for static and dynamic capture of heavy metals: Easy recovery and affinity mechanism

A separable spherical bio-adsorbent (CMC-Cr) was prepared for capturing heavy metal ions by simple coordination and cross-linking between targeted ions of Cr³⁺ and carboxymethyl cellulose (CMC). A simple alternation of the CMC incorporation allowed the interconnected networks within the microspheres of preformed solid CMC to be adjusted. The excellent network structure could achieve the maximum collision between the adsorbent and the heavy metal cations in the wastewater. Through investigations, CMC-Cr-2 beads were determined as the optimal adsorbent. The adsorption performance of novel materials was evaluated by examining their adsorption behavior on Pb(II) and Co(II) under both static and dynamic conditions. The results showed that the adsorption behavior of CMC-Cr-2 beads on both two heavy metal cations could be fully reflected by the Freundlich model. Under the theoretical conditions, the maximum adsorption capacities were 97.26 and 144.74 mg/g. The kinetic results for the adsorption of two heavy metal cations on CMC-Cr-2 beads were consistent with the Pseudo-second-order kinetic model. Moreover, the correlation coefficient of the Thomas model was significant in the dynamic adsorption performance tests. Five regeneration cycle studies were successfully carried out on CMC-Cr-2 beads to evaluate reusability and stability. The applicability of CMC-Cr-2 beads in authentic aqueous solutions (both the single and binary pollutant systems) was also studied, and the results indicated that CMC-Cr-2 beads had a high potential for practical implementation. Furthermore, by analyzing the surface interactions of two heavy metal cations with the CMC-Cr-2 beads based on FTIR and XPS characterization, a basic understanding of the interaction between bio-sorbents and pollutants in wastewater can be obtained.