La(OH)3-modified magnetic sodium carboxymethyl cellulose for sequential removal of pollutants: adsorption of phosphate and subsequent photocatalytical reduction of Cr(VI)

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.

Polysaccharide-based (nano)materials for Cr(VI) removal

Chromium is a potentially poisonous and carcinogenic species, which originates from human activities and various industries such as leather, steel, iron, and electroplating industries. Chromium is present in various oxidation states, among which hexavalent chromium (Cr(VI)) is highly toxic as a natural contaminant. Therefore, chromium, particularly Cr(VI), must be eliminated from the environment, soil, and water to overcome significant problems due to its accumulation in the environment. There are different approaches such as adsorption, ion exchange, photocatalytic reduction, etc. for removing Cr(VI) from the environment. By converting Cr(VI) to Cr(III), its toxicity is reduced. Cr(III) is essential for the human diet, even in small amounts. Today, biopolymers such as alginate, cellulose, gum, pectin, starch, chitin, and chitosan have received much attention for the removal of environmental pollutants. Biopolymers, particularly polysaccharides, are very useful compounds due to their OH and NH₂ functional groups and some advantages such as biodegradability, biocompatibility, and accessibility. Therefore, they can be widely applied in catalytic applications and as efficient adsorbents for the removal of toxic compounds from the environment. This review briefly investigates the application of polysaccharide-based (nano)materials for efficient Cr(VI) removal from the environment using adsorption/reduction, photocatalytic, and chemical reduction mechanisms.

Facile synthesis of Zr(IV)-crosslinked carboxymethyl cellulose/carboxymethyl chitosan hydrogel using PEG as pore-forming agent for enhanced phosphate removal

Zr(IV)-crosslinked carboxymethyl cellulose/carboxymethyl chitosan hydrogels were prepared using polyethylene glycol (PEG) as pore-forming agent. The hydrogels exhibited porous structure and the pore size increased with the PEG content. The obtained hydrogels were used as adsorbents for removal of phosphate from aqueous solutions. The hydrogel prepared with most amount of PEG displayed largest adsorption amount of phosphate. This result might be attributed to improvement of surface area and pore volume caused by introduction of PEG, which could boost exposure of adsorption sites in the hydrogel and enhance the diffusion of phosphate into interior of the hydrogel. The phosphate adsorption achieved equilibrium within 400 min and the removal process followed pseudo-second-order kinetic model. The maximum adsorption capacity was reached at pH = 2.0, and the adsorption capacity gradually increased with temperature. In the presence of coexisting anions, SO₄^2- ions exerted much more negative effect on phosphate adsorption than Cl^- and NO₃^- ions. The phosphate adsorption mechanism was related to electrostatic interaction, ligand exchange and ion exchange. Moreover, the hydrogel exhibited relatively stable adsorption capacity after six adsorption/desorption cycles. The present study presented a facile method for preparing an excellent hydrogel adsorbent for phosphate removal from aqueous solutions.

Evaluation of phosphate adsorption by porous strong base anion exchangers having hydroxyethyl substituents: kinetics, equilibrium, and thermodynamics

Phosphate anions are recognized as the main responsible for the eutrophication of surface waters. In this work, two strong base anion exchangers having either N,N-dimethyl 2-hydroxyethylammonium (SBAEx.2M) or N,N-diethyl 2-hydroxyethylammonium (SBAEx.2E) functional groups, as highly efficient sorbents in the removal of phosphate anions, are presented. The influence of the main parameters (pH, contact time, initial concentration of phosphate, temperature) on the adsorption performances was investigated in batch mode. Modeling the kinetics data by Lagergren, Ho and McKay, and Elovich kinetic models indicated chemisorption as the main mechanism of sorption. The sorption at equilibrium was modeled with Langmuir, Freundlich, Sips, Dubinin-Radushkevich, and Temkin isotherm models. The experimental isotherms were the best fitted by Langmuir and Sips isotherms, the maximum sorption capacity for phosphate anions being 233.88 mg g^-1 SBAEx.2M and 223.5 mg g^-1 SBAEx.2E, at pH 3, and 23 °C. Adsorption of phosphate anions in competitive conditions showed that the interference with co-existing anions was low in the case of Cl^- ions and much higher with SO₄^2- ions, the ion exchange having an important contribution in the adsorption process. The adsorption was spontaneous and endothermic, the degree of spontaneity increasing with the increase of temperature. The high level of reusability, the adsorption capacity decreasing with only ~ 7% in the case of SBAEx.2E and with ~ 9% in the case of SBAEx.2M, after five sorption/desorption cycles, recommends these SBAEx as promising adsorbents for phosphate removal.