Modified grape stem as a renewable adsorbent for cadmium removal

Recovering the Soybean Hulls after Peroxidase Extraction and Their Application as Adsorbent for Metal Ions and Dyes

This study is aimed at extending the soybean hulls’ lifetime by their utilization as an adsorbent for metal ions (Cd2+ and Cu2+) and dyes (Reactive Yellow 39 (RY 39) and Acid Blue 225 (AB 225)). ATR-FTIR spectroscopy, FE-SEM microscopy, and zeta potential measurements were used for adsorbent characterization. The effect of the solution’s pH, peroxidase extraction, adsorbent particle size, contact time, the pollutant’s initial concentration, and temperature on the soybean hulls’ adsorption potential was studied. Before peroxidase extraction, soybean hulls were capable of removing 72% Cd2+, 71% Cu2+ (at a pH of 5.00) or 81% RY 39, and 73% AB 225 (at a pH of 3.00). For further experiments, soybean hulls without peroxidase were used for several reasons: (1) due to their observed higher metal ion removal, (2) in order to reduce the waste disposal cost after the peroxidase (usually used for wastewater decolorization) extraction, and (3) since the soybean hulls without peroxidase possessed significantly lower secondary pollution than those with peroxidase. Cd2+ and Cu2+ removal was slightly increased when the smaller adsorbent fraction (710-1000 μm) was used, while the adsorbent particle size did not have an impact on dye removal. After 30 min of contact time, 92% and 88% of RY 39 and AB 225 were removed, respectively, while after the same contact time, 80% and 69% of Cd2+ and Cu2+ were removed, respectively. Adsorption of all tested pollutants follows a pseudo-second-order reaction through the fast adsorption, intraparticle diffusion, and final equilibrium stage. The maximal adsorption capacities determined by the Langmuir model were 21.10, 20.54, 16.54, and 17.23 mg/g for Cd2+, Cu2+, RY 39, and AB 225, respectively. Calculated thermodynamic parameters suggested that the adsorption of all pollutants is spontaneous and of endothermic character. Moreover, different binary mixtures were prepared, and the competitive adsorptions revealed that the soybean hulls are the most efficient adsorbent for the mixture of AB 225 and Cu2+. The findings of this study contribute to the soybean hulls’ recovery after the peroxidase extraction and bring them into the circular economy concept.

Chemical Modification of Teff Straw Biomass for Adsorptive Removal of Cr (VI) from Aqueous Solution: Characterization, Optimization, Kinetics, and Thermodynamic Aspects

Teff straw, a by-product of Teff, mainly available in Ethiopia, has not been studied much for biosorbent production. The present study has investigated the effects of modification and optimization of process parameters (viz., concentration of modifying agent (H3PO4 and KOH), modifying temperature, and modifying time) on the Cr (VI) removal efficiency of using chemically activated Teff straw biosorbent by RSM followed by BBD. The maximum Cr (VI) removal was obtained using an H3PO4-modified Teff straw biosorbent of 92.5% with 2 M concentration of the modifying agent, 110°C, and 4 h. Similarly, maximum Cr (VI) removal using KOH-modified Teff straw biosorbent of 95.2% was obtained with 1.5 M activating agent concentration, 105°C activation temperature, and 3.5 h activation time. In addition, the effects of adsorption parameters (viz., biosorbent dosage, temperature, initial concentration of Cr (VI), and contact time) were investigated. The maximum removal efficiency was attained at 2 g of biosorbent dosage, 4 h contact, 75 mg/L of initial Cr (VI) concentration, and 25°C sorption temperature. In addition, isotherm, kinetic, and thermodynamic studies for Cr (VI) biosorption were studied. The experimental adsorption data were well fitted with the Langmuir isotherm and pseudo-second-order kinetic model with higher correlation coefficient in both untreated and chemically modified Teff straw biosorbent. The investigated thermodynamic parameters ( $\Delta H^o$ , $\Delta S^o$ , and $\Delta G^o$ ) confirmed that Cr (VI) metal ions’ adsorption process onto Teff straw biosorbent was spontaneous and endothermic.

Ecofriendly Biosorbents Produced from Cassava Solid Wastes: Sustainable Technology for the Removal of Cd2+, Pb2+, and Crtotal

This research is aimed at investigating the possible use of cassava agroindustry solid wastes in manufacturing adsorbents and their use in removing heavy metals Cd2+, Pb2+, and Crtotal from water. Thus, a pilot study was conducted in two main steps: (1) obtaining and characterizing the adsorbents and (2) laboratory studies focused on the evaluation of critical physicochemical parameters on adsorption, such as pH of the solution containing heavy metals, the effect of adsorbent dose, besides kinetics and equilibrium adsorption and desorption studies. Three adsorbents were studied, cassava barks, bagasse, and their mixture. SEM, FTIR, pHPZC, acid digestion, and chemical composition analysis were employed for adsorbent characterization. The pH of the contaminated solution was evaluated within 4.0 to 7.0, while the adsorbent doses varied from 5.0 to 24.0 g L-1. The adsorption kinetics was evaluated within 5 to 180 minutes and interpreted using pseudofirst- and second-order models. Finally, equilibrium and desorption studies were performed by evaluating adsorbent performance within 5 to 200 mg L-1 of heavy metals, using several nonlinear models for results interpretation. SEM analysis reveals a heterogeneous structure full of cavities. FTIR before and after adsorption reveals gaps related to missing functional groups, suggesting a significant role of alkenes, carboxylic acid, alcohol, anhydride, and ether. pHPZC is found at pH 6.02, 6.04, and 6.26 for adsorbents derived from barks, bagasse, and their mixture. In low concentrations of metals, the higher adsorption capacities were found at pH 7.0 (94.9%) using 16 g L-1 of adsorbent, with the most cost-benefit dose found using 8.0 g L-1. The removal of metals reaches equilibrium within 5-10 minutes of contact time with pseudosecond-order best adjustments to the observed phenomena. The adsorption of metals by a cassava adsorbent is better adjusted to the Freundlich model, with significant and critical information provided by Sips, Redlich-Peterson, Temkin, Liu, and Khan models. Adsorption/desorption studies indicate that cassava adsorbent performs, on average, -10% of the adsorption of metals compared to activated carbon. Nevertheless, factors such as low cost and availability favor the use of such natural materials.

Effective Cd2+ removal from water using novel micro-mesoporous activated carbons obtained from tobacco: CCD approach, optimization, kinetic, and isotherm studies

PURPOSE

This research aimed to develop activated carbons from tobacco by double (thermal-physical) and triple activations (thermal-chemical-physical) for high-efficiency removal of Cd²⁺.

METHODS

The adsorbents were characterized by their chemical composition, point of zero charge (pH_PZC), SEM, FT-IR, BET, and BJH. The subsequent adsorption studies were conducted: optimal conditions (CCD on adsorbent dose versus pH of Cd²⁺ solution), kinetics, equilibrium, thermodynamics, and desorption studies.

RESULTS

The activated carbons have irregular and heterogeneous morphology, surface functional groups COO-, C-O, C-O-C, C=O and O-H, pH_PZC of 11.11 and 10.86, and enhanced SSA (especially for CT NaOH + CO₂ = 103.40 g m^-2). The optimal conditions for Cd²⁺ adsorption occur using 4.0 g L^-1, pH from 3.0 to 7.0, with most of the Cd²⁺ adsorbed in the first 10-20 min. The goodness of the fit found for pseudo-first order, pseudo-second order, intraparticle diffusion, Langmuir, Freundlich, Dubinin-Radushkevich, Sips, and Temkin suggest the occurrence of Cd²⁺ chemisorption and physisorption in mono and multilayers. The values of ∆G° < 0 kJ mol^-1 indicate that the observed phenomena are energetically favorable and spontaneous; the values of ∆H° < 0 and the effective desorption rates (58.52% and 44.64%) suggest that the adsorption of Cd²⁺ is ruled mainly (but not only) by physical interactions.

CONCLUSION

Our excellent results on Cd²⁺ removal allow us to state that tobacco use as a raw material for adsorbent development is a renewable and eco-friendly technique, allowing the production of highly effective activated carbons and providing an adequate destination for this waste.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s40201-021-00740-8.