Removal of Cr(VI) from wastewater by modified montmorillonite in combination with zero-valent iron

For this study, the authors synthesised a composite sorbent based on montmorillonite: (Na,Ca)_0.33(Al,Mg)₂(Si₄O₁₀)(OH)₂·nH₂O denoted by MM, that contains Fe₃O₄ particles and dodecyldimethylbenzylammonium chloride: C₂₁H₃₈NCl as a cationic surfactant (CS) denoted by MM:Fe₃O₄:CS. X-ray phase analysis showed that the interlayer distance of montmorillonite expanded by 0.3 Å in the process of modification by Fe₃O₄, and by another 2.5 Å with further addition of the surfactant. The expansion of the interlayer space of MM acts as evidence of the penetration of the modifying particles into the MM structure and the formation of a composite with properties that differ from the initial MM. This modification promotes an increase in the adsorption efficiency of Cr(VI) ions from their 50 mg/L acidic solutions by more than 5 times compared to the initial MM, and up to 2.8 times when compared to MM:Fe₃O₄ composite. The adsorption of Cr(VI) on MM:Fe₃O₄:CS is accurately described by the Langmuir model (r² > 0.99); the maximum Langmuir capacity at pH = 5 and 290 K is 38.8 mg/g. The authors also investigated the thermodynamics of the sorption process; it has been shown that the process is endothermic, the determining factor in it is the structural changes of the sorbent. It has been shown that complete desorption of chromium is possible using a 2 M NaOH solution. The composite sorbent was tested on removing Cr(VI) from a real wastewater sample. It has been shown that purification to the maximum permissible concentration level in drinking water is possible when using a combination of adsorption with post-purification with zero-valent iron sol. On the basis of this combination, a scheme is proposed in this paper for the utilisation of chromium-containing solutions to obtain water of an acceptable quality that can be discharged into surface water, and a chromium-containing solution for the extraction of pure chromium compounds.