Phosphate uptake behavior of ZnAlZr ternary layered double hydroxides through surface precipitation P. Koilraj, S. Kannan * Discipline of Inorganic Materials and Catalysis, Central Salt and Marine Chemicals Research Institute, Council of Scientific and Industrial Research, GB Marg, Bhavnagar 364 002, India article info Article history: Received 28 May 2009 Accepted 28 September 2009 Available online 2 October 2009 Keywords: Layered double hydroxides Phosphate removal Surface precipitation Hopeite Zirconium abstract Tetravalent Zr 4+ ion incorporated ZnAl hydrotalcite-like material with varying Zn/(Al + Zr) atomic ratio (2–4) and Al/Zr atomic ratio 0.7:0.3 with CO 2 3 as interlayer anion were successfully synthesized and characterized by various physicochemical methods. The phosphate uptake studies were carried out over these materials by batch method with the objective of enhancing the uptake through creation of higher positive charge on brucite-like layers by the incorporation of Zr 4+ . The studies revealed that ternary ZnAlZr hydrotalcites show good uptake capacity for phosphate, among which ZnAlZr4-HT depicts a max- imum uptake of around 91 mgP/g. We observed that phosphate uptake results in the formation of layered hopeite mineral (Zn 3 (PO 4 ) 3 4H 2 O) on the surface of material by the surface precipitation of dissolved zinc ion through the outer-sphere complex formation of phosphate. The phosphate uptake increases with a decrease in Zn/Al and Zn/(Al + Zr) atomic ratios. Effects of initial phosphate concentration, contact time, adsorbent amount and temperature variation on phosphate uptake were studied. Co-presence of compet- itive anion like nitrate enhances the uptake of phosphate to 148 mgP/g. Recycle studies of the material shows a decrease in the uptake of phosphate with the number of cycles due to an increase in the crys- tallinity. Very high uptake of phosphate by these materials even in the presence of other anions/cations suggests their potential role in waste water remediation. Ó 2009 Elsevier Inc. All rights reserved. 1. Introduction Phosphate is an essential nutrient for human, plant and for the growth of microorganisms. Currently water pollution is a major problem in our society, among which phosphate is one of the most important pollutant which causes the algal bloom in the receiving water bodies that leads to the depletion of dissolved oxygen con- tent (eutrophication). Phosphate mainly comes from the fertilizing industry, agrochemical industry, agriculture and household appli- cations. The removal of phosphate from the aquatic system is important to control eutrophication. The need for selective uptake of phosphate in trace concentration levels has been well recog- nized. The sorption reaction by solid absorbents has been largely used for the removal of undesired anions from water and waste water. Many techniques like ion-exchange, adsorption [1,2], chemical precipitation, coagulation–precipitation [3,4] are widely accepted for phosphate removal at industrial level, but anion- exchangers requires high regeneration cost and precipitation or coagulation processes create the problem of sludge disposal. Previ- ous work over phosphate uptake includes: long term stability of phosphorus over phosphate sorbed Al/Fe hydroxides and influence of pH on phosphate-enriched sea water over Zr hydroxides [5,6]. The adsorption using inorganic sorbent like ZnCl 2 activated coir pith carbon [7], goethite (a-FeOOH), akaganeite (b-FeOOH) [8,9], dolomite, alums and zeolite materials are widely studied for phos- phate removal [10,11]. New type of polymeric ion-exchangers (chelating resins) [12], ion-exchange fibers [13,14], ammonium- functionalized MCM-48 [15] were also studied. The main disad- vantage of sorption process of phosphate is the non availability of suitable environmentally friendly and inexpensive sorbents. Layered double hydroxides (LDHs), also otherwise known as hydrotalcite-like materials (HT-like) or anionic clays, consist of the positively charged brucite or brucite-like layers through iso- morphous substitution of divalent cation by trivalent cation. The general formula of HT-like materials is [M(II) 1x M(III) x (OH) 2 ] x+ [A n x=n ]mH 2 O, where M(II) and M(III) are the divalent and trivalent metal ion, A n is the interlayer anion that may be organic, inorganic, carboxylate, oxoanion, coordination compounds and polyoxometalates and x generally can have the values between 0.1 and 0.33. During the last three decades, extensive research has been done on the synthesis and applications of hydrotalcite- like materials. Recently the LDHs are received incisive attention in catalysis [16,17], pharmaceuticals [18,19], polymer blends [20,21], adsorbents [22,23] and ion-exchangers [24,25]. Hydrotal- cite-like materials were found to be selective adsorbents for the removal of phosphate from sea water [26,27]. 0021-9797/$ - see front matter Ó 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2009.09.059 * Corresponding author. Fax: +91 278 2567562. E-mail addresses: skannan@csmcri.org, kanhem1@yahoo.com (S. Kannan). Journal of Colloid and Interface Science 341 (2010) 289–297 Contents lists available at ScienceDirect Journal of Colloid and Interface Science www.elsevier.com/locate/jcis