Removal of selected anions by raw halloysite and smectite clay

Abstract

The structure of clay minerals is made up from tetrahedral and octahedral sheets, which can be stacked in different ways, forming 1:1, 2:1 and 2:1:1 layers. Halloysite, which belongs to the kaolin group, is composed of 1:1 layers while smectite group minerals exhibit 2:1 dioctahedral layered structure. The most important feature of these minerals is the presence of numerous active centers on their surface and/or in the interlayer space, which allows them to attract and exchange ions from aqueous solutions. These makes them perfect candidates which could be used for the purification of wastewaters from harmful ions (Bhattacharyya & Gupta 2008; Lee & Tiwari 2012). The aim of this work was to examine the sorption capacity of natural halloysite and smectite clay towards P(V), As(V) and Cr(VI).

Two samples used in the research came from Polish deposits. Natural halloysite (H) was obtained from Dunino deposit, while smectite clay (SC) was obtained from Bełchatów Lignine Mine where it forms an overburden cover. For the both raw samples the XRD patterns and FTIR spectra were collected. The sorption of P(V), As(V) and Cr(VI) was conducted as a function of anions concentration in the range from 0.05 to 50 mmol/L for initial pH 5 in a single-element system. The suspension of H or SC and an adequate solution (solid/solution ratio: 20 g/L) was shaken for 24 h at 25°C. Afterwards the anions concentration in the supernatant solution was measured using colorimetric methods. The P(V) and As(V) concentration was determined with molybdenum blue method, while Cr(VI) concentration was measured with diphenylcarbazide method.

The XRD pattern of the H sample showed a basal peak at 7.20 Å, which confirms the presence of dehydrated halloysite-(7 Å). In turn, the SC exhibited a peak centered at ~12.5 Å with an asymmetric profile starting from ~15.0 Å. Such reflection suggests the presence of smectite which has both Na⁺ and Ca²⁺ cations in its interlayer space. The peaks at 4.26 Å and 3.34 Å are associated with quartz. The IR spectra of the H showed bands characteristic for kaolin group minerals related to the OH-stretching region (3700-3620 cm^-1), vibrations of water molecules (~1630 cm^-1) and bands assigned to stretching and bending vibrations of aluminosilicate framework (1200-400 cm^-1). The IR spectrum of SC shows bands characteristic for smectite minerals i.e. 3623 cm^-1 band attributed to OH hydroxyl located inside the 2:1 layer and a broad band centered at ~3400 cm^-1 due to interlayer water surrounding cations. Also the structural vibrations of the 2:1 layer may be observed in the 1200-400 cm^-1 region.

The results of the experiment indicated that the sorption capacity of the H sample and the SC sample were relatively high. The amount of removed P(V) was the highest for both materials, where the sorption was respectively equal to: 201 and 256 mmol/kg. The sorption capacity of As(V) for the H was equal to 168 mmol/kg, while it was significantly lower on the SC (96 mmol/kg). In the case of H the Cr(VI) sorption reached only 36 mmol/kg and for the SC it was equal to 104 mmol/kg. In most cases the sorption isotherms were fitted to the Freundlich model. The only exception was for P(V) sorption on the H sample, which was better described by Langmuir model. The specific surface areas (S_BET) of the studied materials do not differ significantly: SC = 69.10 m²/g and H = 49.52 m²/g. The sorption centers that may attract anions in both, H and SC samples, were limited, because isomorphic substitutions in tetrahedral and/or octahedral sheets generate positively charged sites, which attract cations. It is believed that the mechanism responsible for the adsorption of anions on both materials is mainly surface complexation, which occurs at the crystals edges (Bradl 2004). The sorption capacity of the H and the SC was significantly lower than that reported for hydrotalcite-based anion-exchange materials (HTLc). For comparison, the sorption capacity towards P(V), As(V) and Cr(VI) on uncalcined HTLc was as follows: 498 mmol/kg (Kuzawa et al. 2006), 596 mmol/kg (Wu et al. 2013) and 314 mmol/kg (Alvarez-Ayuso & Nugteren 2005). Nevertheless, the examined mineral samples might be useful as sorbents for industrial wastewater treatment involving the removal of P(V) and As(V).