Sorption of organic compounds by hydroxy-interlayered clays through chelation and humification processes

This experiment was carried out to study the interaction of citric acid, hydroquinone and catechol on Montmorillonite and hydroxy-interlayered Montmorillonite clays. Montmorillonite (Mt) and its interlayered, viz., hydroxy-Fe (FeMt), mixed hydroxy-Fe,Al (Fe,AlMt), and hydroxy-Al (AlMt) counterparts were used for the study. Clay–organic complexes were prepared by aging clay suspensions at adjusted pH of 6, for 30 days after adding organics at the rate of 300 cmol/kg of clay. Controls without organics, namely, Mt, FeMt, Fe,AlMt, and AlMt, were aged and separated similarly. The CEC, specific surface area and total C were determined, and X-ray diffraction patterns were obtained for the separated solid phase. Final pH of each suspension was recorded and the absorbance spectra between 400 and 600 nm were obtained in the supernatants of clay–catechol and clay–hydroquinone suspensions. Final average pH of the clay suspensions in citrate (Ct), hydroquinone (HQ) and catechol (CC) systems were 6.8, 5.9 and 4.7, respectively. Specific surface area and CEC in FeMt, Fe,AlMt and AlMt clays increased significantly after aging with citric acid. Amounts of Fe and Al determined in the supernatant solutions of clay–catechol and clay–hydroquinone systems were in the range of 0 to 18 cmol(+)/kg of clay while in clay–citrate systems were 170 to 347 cmol(+)/kg of clay. The d-spacings of AlMtCt and Fe,AlMtCt were 1.26 nm and 1.24 nm, respectively, which were lower than the original values of 1.40 nm and 1.43 nm for AlMt and Fe,AlMt. Similarly, FeMtCt reported lower d-spacing of 1.26 nm in comparison to its original d-spacing of 1.33 nm in FeMt, after aging with citrate. Hydroxy-interlayered clays aged with HQ and CC did not show large changes in d-spacings. The absorbance values measured at 400 nm in catechol and hydroquinone systems were in the range of 0.16–0.95 and 0.05–0.21, respectively. The amounts of organic C retained by the solid phases in catechol systems were significantly greater than that for hydroquinone or citrate systems. Results of the study showed that the citric acid had caused the dissolution of the hydroxy-interlayer polymers in hydroxy-interlayered clay minerals, while hydroxy-interlayered clays catalyzed the formation of catechol- and hydroquinone-derived humic substances.