Elsevier

Applied Clay Science

Volume 38, Issues 3–4, February 2008, Pages 237-249
Applied Clay Science

Adsorption mechanism of humic and fulvic acid onto Mg/Al layered double hydroxides

https://doi.org/10.1016/j.clay.2007.02.010Get rights and content

Abstract

The adsorption of humic and fulvic acids onto nitrate, chloride and carbonate intercalated layered double hydroxides with Mg/Al ratios ranging from 85/15 to 60/40 was studied by adsorption isotherms at different ionic strengths and the characterization of the preferentially adsorbed humic substance size fractions. The adsorption of humic and fulvic acids onto LDHs occurred by ion exchange with both the intercalated and surface anions of the LDH and ligand exchange reactions with surface groups. The contribution of both mechanisms to the total HA and FA adsorption was estimated. Lower molecular weight humic and fulvic acids were preferentially adsorbed because these fractions can more easily enter the mesoporous LDHs and contain more carboxylic groups, which are known to be involved in ligand exchange reactions with e.g. surface Al-OH groups. Intercalation of the entire HA or FA molecules in-between the LDH sheets is unlikely to occur.

Introduction

For a number of reasons humic substances may pose a problem in drinking water production from surface waters and in water treatment in general because: a) the colour, taste and odour that accompany it, b) their forming potential of carcinogenic disinfection by-products (DPBs) (e.g. trihalomethanes, haloacetic acids) during the chlorination of drinking water, and c) their fouling potential of membranes and ion-exchange resins (Abdel-Jawad et al., 1997, Hong and Elimelech, 1997, Cho et al., 1998, Huber, 1998, Wilbulswas et al., 1998, Bolto and Dixon, 2002). Processes such as chemical coagulation and membrane separation have been developed for removing humic substances from water. Although coagulation using alum has widely been used, it incurs a high operational cost and generates high volumes of extra sludge. Humic substances also tend to foul membranes seriously and thus limit membrane applications in this field Bai and Zhang, 2001). Therefore, there is a great interest in new adsorbents for effective removal of humic substances from water. Many insoluble materials other than activated carbon such as natural and synthetic zeolites, clays and modified clay minerals, aluminas and resins have the potential for removing humic substances from water (Bolto and Dixon, 2002, Jiang and Cooper, 2003, Vreysen and Maes, 2006a, Vreysen and Maes, 2006b). In this paper the use of layered double hydroxides for the removal of humic and fulvic acids (HA and FA) from water is investigated.

LDHs are effective adsorbents for the removal of humic substances from water (Amin and Jayson, 1996, Önkal-Engin et al., 2000, Seida and Nakano, 2000, Bin Hussein et al., 2001). Önkal-Engin et al. (2000) found that, in tap water (unknown pH and ionic strength), carbonate intercalated LDHs had a lower Aldrich HA adsorption capacity (18.6 mg/g) than chloride intercalated LDHs (93.8 mg/l). These authors concluded that the HA adsorption mechanism was ion exchange rather than surface adsorption, as the interlayer anion CO32− or Cl exchanges with the present HA anion in water. On the other hand, Bin Hussein et al. (2001) concluded from XRD spectra and the characterization of the surface properties of the LDHs (surface area, micropore surface area and pore size distribution) that HA sorbs onto LDHs by surface adsorption processes. These authors suggested that intercalation of humic substances could still be possible, but they did not find any evidence of the occurrence of this process in their experimental observations. Seida and Nakano (2000) stated that the removal of unpurified Aldrich HA from water by LDHs occurs in two phases: in the first phase, the HA adsorption occurred rapidly within a few hours by both HA intercalation into the positive charged interlayer (anion exchange) and HA adsorption onto surface-hydroxyl groups. In a second phase, the small amounts of hydroxides, which are produced from the slight dissolution of the layered double hydroxides, coagulate the humic substances. The contribution of these different adsorption mechanisms to the total HA removal by the LDHs and the influence of the nature of the intercalated anion and the M2+/M3+ ratio on the aforementioned processes, however, is not yet understood. The purpose of this paper is to present a more detailed study of the parameters influencing the adsorption of humic and fulvic acids onto chloride, nitrate and carbonate intercalated LDHs with Mg/Al ratios ranging from 85/15 to 60/40.

Section snippets

LDH synthesis

Mg–Al–NO3 and Mg–Al–Cl LDHs were prepared by co-precipitation. Solutions of Mg(NO3)2·6H2O and Al(NO3)3·9H2O or MgCl2 and AlCl3 (with Mg/Al ratios 85/15 to 60/40) were added to bidistilled water to which NaOH (0.1 M) was continuously added to in order to maintain pH 10. Previous studies showed that carbonate contamination of the LDHs was unavoidable, even when the LDHs are prepared under nitrogen atmosphere and washed with decarbonated water (Inacio et al., 2001, Moujahid et al., 2003, Zhao and

Adsorption kinetics

Fig. 1 shows the adsorbed HA concentration (mg/g dry LDH) onto NO3- and Cl-LDHs versus the equilibration contact time. Initially HA adsorption is fast: after respectively 15 min (first data point of each curve), 4 h and 20 h respectively 85%, 95% and 98% of the maximum HA adsorption occurred. In further experiments HA and FA adsorption isotherms onto the LDHs are made after a contact time of at least 40 h to ensure adsorption equilibrium conditions.

Generally, three steps are involved during the

Discussion

The layered double hydroxides have a permanent positive charge originating from the isomorphic substitution of bivalent Mg2+ by trivalent Al3+ ions, and amphoteric charges resulting from surface-hydroxyl groups (broken edges). Delgado et al. (2004) observed that these surface groups develop a variable charge through protonation–deprotonation reactions. The excess of permanent positive charges, however, results in a net positive charge in an ample pH range. Indeed, Châtelet et al. (1996) and

Conclusion

The HA and FA adsorption isotherms onto NO3, Cl- and CO3-LDHs with Mg/Al ratios ranging from 85/15 to 60/40 showed that the adsorption of humic substances onto LDHs is influenced by the size of the humic substance, the ionic strength, the intercalated LDH anion and the Mg/Al ratio of the LDH. HA and FA adsorption onto NO3- and Cl-LDHs at low ionic strength occurred by ligand exchange reactions with LDH surface groups and anion exchange with both the intercalated and surface anions of the LDH,

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