Abstract
Removal of Cr(VI) from the environment represents a stringent issue because of its tremendous effects on living organisms. In this context, design of sorbents with high sorption capacity for Cr(VI) is getting a strong need. For this purpose, poly(vinylbenzyl chloride), impregnated into porous silica (PSi), was cross-linked with either N,N,N’,N’-tetramethyl-1,2-ethylenediamine (TEMED) or N,N,N’,N’-tetramethyl-1,3-propanediamine, followed by the reaction of the free -CH2Cl groups with N,N-diethyl-2-hydroxyethylamine to generate strong base anion exchangers (ANEX) inside the pores. The PSi/ANEX composite sorbents were deeply characterized by FTIR spectroscopy, SEM-energy dispersive X-ray spectroscopy (EDX), thermogravimetric analysis (TGA), and water uptake. The sorption performances of composites against Cr(VI) were investigated as a function of pH, contact time, initial concentration of Cr(VI), and temperature. It was found that the cross-linker structure and the silica morphology are the key factors controlling the sorption capacity. The adsorption process was spontaneous and endothermic and well described by pseudo-second-order kinetic and Sips isotherm models. The maximum sorption capacity of 311.2 mg Cr(VI)/g sorbent was found for the composite prepared with mesoporous silica using TEMED as cross-linker. The PSi/ANEX composite sorbents represent an excellent alternative for the removal of Cr(VI) oxyanions, being endowed with fast kinetics, equilibrium in about 60 min, and a high level of reusability in successive sorption/desorption cycles.
Highlights
Nowadays, finding highly efficient systems for the removal of heavy metal ions (HMIs) from the aquatic environment is a stringent issue in front of the scientists all over the world, because they are highly toxic, not biodegradable, and have an accumulation tendency in living organisms—most of them being carcinogenic
The influence of the cross-linker type used in the synthesis of strong base anion exchangers (ANEX)
Embedded into the pores of two porous silica, a mesoporous one having Ssp of 95.097 m2 /g (PSi1), and a macroporous having Ssp of 23.963 m2 /g (PSi2), and of the silica morphology on the sorption capacity of Cr(VI) oxyanions in aqueous solution was deeply investigated in this work
Summary
Nowadays, finding highly efficient systems for the removal of heavy metal ions (HMIs) from the aquatic environment is a stringent issue in front of the scientists all over the world, because they are highly toxic (even at low concentrations), not biodegradable, and have an accumulation tendency in living organisms—most of them being carcinogenic. Various techniques are available for the removal of Cr(VI) such as ion exchange, chemical precipitation, reduction [13,14], reverse osmosis, foam flotation, electrolysis, membrane filtration [15,16], ultrafiltration [17], sorption, and biosorption [18,19,20,21,22,23,24,25,26,27,28,29,30,31] Some of these techniques have disadvantages of either producing toxic sludge as is the case of chemical precipitation, or asking for high capital costs (reverse osmosis), recovery of value metal being difficult [27,28]. Macroporous strong base anion exchangers (SBAE) bearing quaternary ammonium salt groups have been synthesized and successfully used by our group in the removal of Cr(VI) either as single anion exchangers [25] or embedded in a composite consisting of chitosan and poly(vinyl amine) as cryobeads [24], their maximum sorption capacity being of 200–320 mg Cr(VI)/g sorbent
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