Removal of hexavalent chromium from aqueous solution by calcined Zn/Al-LDHs

Chromium is a highly toxic pollutant generated from many industrial processes such as electroplating, leather tanning, textile industries, metal finishing and chromate preparation (Gupta et al. 2011). It is considered by the International Agency for Research on Cancer to be a powerful carcinogenic agent that modifies the DNA transcription process, causing important chromosome aberrations (Benoit 1976; Rengaraj et al. 2001). Moreover, chromium ions hardly degrade into harmless end products (Khezami & Capart 2005). Therefore, the chromium ions in industrial wastewaters would endanger public health and the environment if they were discharged without adequate treatment. Generally, the effluent from the above-mentioned industries may contain chromium at concentration ranging from dozens to hundreds of milligrams per litre, whereas the National Institute for Occupational Safety and Health recommends that the level of chromium in industrial wastewater should be reduced to less than 0.1 mg/L (Pérez-Candela et al. 1995; Khezami & Capart 2005).

Chromium exists in trivalent and hexavalent forms in aqueous systems. The trivalent chromium (Cr(III)) is a trace metal nutrient for humans, but hexavalent chromium (Cr(VI)) is toxic, carcinogenic and mutagenic in nature (Rojas et al. 2005; Garg et al. 2007). Several approaches such as reduction and precipitation (Chang 2003), adsorption (Mathew et al. 2015), ion exchange (Dąbrowski et al. 2004), biological degradation (Cheung & Gu 2007), extraction (Chakraborty et al. 2005), ultrafiltration (Aroua et al. 2007), reverse osmosis (Padilla & Tavani 1999) and electrochemical methods (Rana et al. 2004) are available for removing chromium from effluents. Of these methods, adsorption is regarded as one of the most promising technologies because of its cost-effectiveness, versatility and operability (Álvarez-Ayuso & Nugteren 2005). The key is to develop innovative adsorbents with high chromium adsorption capacity and fast process kinetics.

Layered double hydroxides (LDHs), also known as lamellar compounds, feature with a general formula [M_1−x²⁺M_x³⁺(OH)₂]^x+(Aⁿ⁻)_x/n•mH₂O consisting of octahedral brucite-like host layers (M²⁺/M³⁺: divalent and trivalent metal cations; x = 0.2–0.33), charge-balancing anions (Aⁿ⁻), and interlayer water molecules (Ma et al. 2007; Zhang et al. 2014). Because LDHs can offer a large interlayer surface to host diverse anionic species and have the additional advantage of being potentially easily recyclable, they are attracting current research interest for the removal of heavy metals ions in effluents (Wang et al. 2006; Dadwhal et al. 2009; Türk & Alp 2014).

Although there are several reports concerning the preparation and properties of Zn/Al-LDHs, the applications of calcined Zn/Al-LDHs (C-Zn/Al-LDHs) for Cr(VI) removal are limited. The objective of this study is to investigate the performance of C-Zn/Al-LDHs in removing Cr(VI) from water. The parameters that influence adsorption, such as initial Cr(VI) concentration, absorbent dose, pH value, adsorption time and temperature, were investigated.

Zn(NO₃)₂·6H₂O, Al(NO₃)₃·9H₂O, NaOH, HNO₃ and Na₂CO₃ were purchased from Sinopharm Chemical Reagent Co., Ltd, China. All reagents were analytically pure and used as raw materials without further purification. Deionized water was used throughout the experimental processes.

Zn(NO₃)₂ solution (1.5 M, 100 mL) and Al(NO₃)₃ solution (1 M, 50 mL) were mixed together to form solution A. NaOH (1 M, 150 mL) and Na₂CO₃ (0.5 M, 150 mL) were mixed together to form solution B. Solution A and B were added into a 1,000 mL flask drop by drop at room temperature (25 °C) under mechanical stirring (300 rpm), and the pH value of the reaction mixture was controlled to a constant value (7, 8, 9, 10, 11 or 12) by the dropping speed of solution B. Then the reaction mixture was crystallized at 70 °C for 48 h to obtain a precipitate. Afterwards, the precipitate was filtrated, washed with deionized water and dried at 80 °C for 48 h to obtain Zn/Al-LDHs. Then Zn/Al-LDHs were calcined at 400 °C for 4 h to obtain C-Zn/Al-LDHs.

Powder X-ray diffraction (XRD) patterns of the Zn/Al-LDHs were collected using a Bruker D8 Advanced XRD diffractometer at Cu Kα radiation and a fixed power source (40 kV and 40 mA, λ = 0.15406 Å). Fourier transform infrared (FTIR) spectra were recorded in the range of 4,000–400 cm⁻¹ at a 2 cm⁻¹ resolution on a Bruker EQUI-NOX55 spectrometer using KBr pellet technique. A scanning electron microscope (SEM) (FEI Quanta250) was used to study the surface morphology of Zn/Al-LDHs.

The effect of temperature on the Cr(VI) adsorption by the C-Zn/Al-LDHs was examined at the temperature range of 10 to 90 °C keeping other parameters constant (chromium concentration = 100 mg/L, adsorption time = 80 min, adsorbent dose = 1 mg/mL, pH = 3). It can be seen in Figure 4(b) that the maximum of chromium adsorption occurs at 25 °C. It was reported that the temperature has two main effects on the adsorption process (Khezami & Capart 2005). On the one hand, the increase of temperature decreases the solution viscosity and increases the diffusion rate of the adsorbate molecules across the external boundary layer and internal pores. On the other hand, the increase of temperature decreases the equilibrium adsorption capacity of the adsorbent for a particular adsorbate.

Adsorption experiments were conducted by varying the adsorption time from 20 to 140 min keeping other parameters constant (chromium concentration = 100 mg/L, temperature = 25 °C, absorbent dose = 1 mg/mL, pH = 3). As exhibited in Figure 4(c), the adsorption amount of chromium on the C-Zn/Al-LDHs increases with increasing adsorption time, and reaches the adsorption equilibrium at ca. 80 min, indicating that highly effective adsorption of chromium could be obtained over a relatively short period. Therefore, an 80 min adsorption time was used for further experiments.

The adsorption experiments of Cr(VI) on the C-Zn/Al-LDHs were studied at different adsorbent dose range of 0.2 to 1.6 mg/mL keeping other parameters constant (chromium concentration = 100 mg/L, temperature = 25 °C, pH = 3, adsorption time = 80 min). The results show that the adsorption amount of chromium on C-Zn/Al-LDHs increases from 79.25 to 109.65 mg/g with the adsorbent dose increasing from 0.2 to 0.6 mg/mL (Figure 4(d)), which may be due to the increase of adsorbent surface area and adsorption sites (Garg et al. 2004). However, the adsorption capacity of chromium significantly decreases with further increase in the adsorbent dosage, probably due to the overlap of adsorption sites resulting from overcrowding of adsorbent particles (Namasivayam et al. 1998).

The adsorption experiments were performed by varying chromium concentration from 20 to 140 mg/L keeping other parameters constant (adsorbent dose = 0.6 mg/mL, pH = 3, temperature = 25 °C, adsorption time = 80 min). As shown in Figure 4(e), the adsorption capacity of chromium on the C-Zn/Al-LDHs increases from 29.63 and 120.83 mg/g with the increase of Cr(VI) concentration from 20 to 100 mg/L, then has a little decrease with the further increase of Cr(VI) concentration.

Adsorption performance of Cr(VI) on the C-Zn/Al-LDHs and other adsorbents reported in the literature are given in Table 1. The results show that the C-Zn/Al-LDHs have an acceptably high adsorption capacity for the removal of Cr(VI), indicating the C-Zn/Al-LDHs have a good application potential for the removal of Cr(IV) from wastewater.

where k₁ and k₂ are the rate constants of the pseudo-first-order and the pseudo-second-order kinetics, respectively.

where Q (mg/g) is saturated monolayer adsorption capacity, and b is Langmuir constant (L/mg). The slope and intercept of the linear plot of 1/q_e versus 1/c_e are the values of b and Q, respectively. The constants k_f and n of the Freundlich model are related to the strength of the adsorptive bond and the bond distribution. The linear plot of lnq_e versus lnc_e gives a slope of 1/n and intercept of ⁠.

Zn/Al-LDHs with high degree of crystallinity and layered structure were synthesized by a co-precipitation method, and then C-Zn/Al-LDHs were used to remove Cr(VI) from aqueous solution. The adsorption of Cr(VI) on the C-Zn/Al-LDHs is rapid during the initial periods and reaches the adsorption equilibrium at ca. 80 min. The maximum adsorption capacity of Cr(VI) on the C-Zn/Al-LDHs is over 120 mg/g. The kinetic and isotherm of the adsorption of Cr(VI) on the C-Zn/Al-LDHs can be suitably described by the pseudo-second-order kinetics and Langmuir isotherm models, respectively. The findings demonstrate that C-Zn/Al-LDHs have a good application potential for the removal of Cr(VI) from aqueous solutions.

This work was subsidized by the Fundamental Research Funds for Central Universities (China University of Mining & Technology; Grant 2015QNA25) and a project funded the Priority Academic Program Development of Jiangsu Higher Education Institutions.