Adsorption of anionic species on hydrotalcite-like compounds: effect of interlayer anion and crystallinity

Hydrotalcite-like compounds (HT) and their calcined product (HT500) remove diverse anionic pollutants from waters by two mechanisms: anionic exchange for HT and reconstruction for HT500. and both can be affected by structural characteristics of these sorbents such as degree of layer substitution, interlayer anion and crystallinity. This work deals with assessing the effect of crystallinity and interlayer anion of the sorbent on the adsorption of two anionic organic contaminants in water: 2.4.6-trinitrophenol (TNP) and dodecylbenzylsulfonate (DBS). [Mg3Al(OH)(8)](2)CO3. nH(2)O (HTCO3) and Mg3Al(OH)(8)Cl . nH(2)O (HTCl) were prepared by the coprecipitation method and subjected to hydrothermal treatment in air at 80 degreesC and 130 degreesC for 24 h. The adsorption of TNP and DBS was followed by adsorption isotherms, qunntitatively described by application of the Langmuir equation. The adsorption of TNP and DBS on HT by anionic exchange is dramatically affected by the interlayer anion. On the calculated anion exchange capacity (AEC) basis, HTCO3 adsorbed 23% of TNP and 0% of DBS, whereas HTCl adsorbed up to 100% of both anionic contaminants. The specific affinity of carbonate anion for HT layer strongly complicates its displacement. These exchange reactions were scarcely modified by the crystallinity, more important in the case of carbonate-samples. inversely, the adsorption of TNP and Dss by reconstruction on HT500 was affected by the crystallinity in all cases: however. HT500 derived from HTCO3 samples adsorbed better than those derived from HTCl samples. The adsorption of TNP and DBS on HT500 (from the untreated samples) was about 50% in both sorbents, but TNP adsorption increased up to 77% on HTCO(3)500 and only up to 52% on HTC1500 (from 130 degreesC treated samples), and DBS adsorption increased up to 100% on HTCO(3)500 and 70% on HTC1500. A crystallinity increase of HT seems to render probably a better ordered HT500, favouring the memory effect. (C) 2001 Elsevier Science B.V. All rights reserved.