A new sand adsorbent for the removal and reuse of nickel ions from aqueous solutions

Nickel ions(Ni(II)) are among the toxic heavy metal ions in wastewater that cause serious environmental problems and do harm to the health of people (Fargasova 2012). Many methods have been developed for the purification of the Ni-polluted wastewater. Compared with other methods, the adsorption of Ni(II) from wastewater using adsorbents is simple, efficient and economic. Different types of adsorbents have been synthesized and used for the removal of Ni(II) from synthetic or industrial wastewater. Amino modified SiO₂-based adsorbents were shown to be effective for removing Ni(II) from aqueous solutions (Heidari et al. 2009; Hao & Hou 2013). Natural minerals as adsorbents are advantageous over synthetic ones due to their low cost and local availability. However, to improve their performance, modifications might be required. Iakovleva et al. (2015) proved that limestone modified with NaCl or mining processing wastewater could purify wastewater containing Ni(II). The efficiency of modified adsorbents was four times higher than an unmodified one. Yadav et al. (2013) simply soaked riverbed sand in 40% H₂SO₄ for 4 h and used the modified sand for the treatment of Ni(II) from aqueous solutions with varied initial ion concentrations under the conditions of pH = 4.0–8.0 and an adsorbent dosage of 1.0–2.0 g. They showed that the removal efficiency decreased from 68.76 to 54.09% when the concentration of Ni(II) was increased from 5 to 15 mg/L. Removal was found to be highly pH dependent, and the maximum removal was achieved at a pH of 8.0. The removal process was exothermic, and followed the first-order kinetics. Boujelben et al. (2009) first proved that natural iron oxide-coated sand extracted from an iron ore located in the north-west of Tunisia had a maximum nickel ion retention of 1 mg/g, and followed the Langmuir adsorption model. The same group also reported that modification of the sand by employing iron oxide and manganese oxide coated sand (ICS and MCS) improved the nickel adsorption. The maximum sorption capacities were 2.73 and 3.33 mg Ni/g ICS and MCS, respectively. The nickel uptake process followed the pseudo-second-order rate expression (Boujelben et al. 2010).

The regeneration of spent adsorbents has been considered by many researchers (Lata et al. 2015). For example, EDTA and 1 M HCl solutions were used to strip off the adsorbed Ni(II) in the afore-mentioned phosphorylated cellulose triacetate/silica and phosphine-functionalized PVA/SiO₂ composite adsorbents (Islam et al. 2015; Srivastava et al. 2016). However, reuse cases for the recovered Ni have been seldom reported. We found one example in the literature showing that the recovered nickel in resin was first desorbed with HCl solution, followed by precipitation to form nickel hydroxide, which was further successfully used to synthesize electrode material in a nickel/metal hydride cell (Priya et al. 2009).

In this work, targeted at the reutilization of the spent adsorbent with the adsorbed nickel, a new sand adsorbent was synthesized. Briefly, sand used for glass production was first coated with a layer of porous silica, which was further functionalized with aminopropyltriethoxysilane (APTES). Due to the surface coating and functionalization, the sand gained significantly enhanced capability toward adsorption of Ni(II) from aqueous solutions. In the meantime, the nature of the sand as a glass ingredient (Burciiard 1905) was not greatly influenced, facilitating the reutilization of the spent sand in colored glass preparation.

The original sand was taken from a glass factory. Other reagents were of analytic grade. The preparation of the sand adsorbent mainly comprised two steps: a surface coating followed by an amino-functionalization. The details were as follows. Briefly, 6.0 g of silica sand, 1.08 g of hexadecyltrimethyl ammonium bromide (CTAB) and 100 ml of distilled water were mixed under shaking in a water bath at 60 °C for 10 min. In this mixture, 30 ml of NaOH solution (0.3 mol/L) and tetraethyl orthosilicate (TEOS) were sequentially added. After reacting for 3 h, the resultant product was carefully washed and filtrated. The collected sample was dried at 90 °C. Afterwards, the surface coating procedures were repeated five times. The final product was calcined at 550°C for 6 h to remove the organic template, forming a nanoporous silica coated silica sand. For the amino-functionalization of the nanopores (Araghi et al. 2015), 5.0 g of porous silica coated sand was mixed with 100 ml of anhydrous toluene under stirring in a three-necked flask. The mixture was heated in an oil bath to 115°C. Then, 3 ml of APTES was added. After refluxing for 9 h, the solid was collected by filtration, followed by washing with toluene and methanol. The dried sample was collected for the following adsorption tests.

NiSO₄·6H₂O was used to prepare Ni(II) solutions with different concentrations. In 50 ml of the solution, the prepared sand was added as the adsorbent. The influence of pH, initial Ni(II) concentration (C₀), temperature (T), contact time (t), and adsorbent dosage (m) on the removal efficiency of the synthesized adsorbent toward Ni(II) in the aqueous solutions were studied. The Ni(II) adsorption experiments were performed by adding the sand adsorbent into the Ni(II) solutions in glass beakers, which were under shaking for a period of time in a water bath kept at a particular temperature.

To check the feasibility of using the spent sand adsorbent for the colored glass preparation, the adsorption of Ni(II) was performed in 50 ml solution under the conditions of pH = 6, C₀ = 50 mg/L, m = 0.5 g, T = 25°C and t = 6 h. The spent adsorbent was collected to prepare a glass with a composition of Na₂O 22, CaO 12, SiO₂ 60, MgO 4, and Al₂O₃ 2 by weight percentages. The SiO₂ component was supplied by the spent adsorbent. To get enough SiO₂, parallel adsorption tests were conducted. Apart from SiO₂, the other components in the glass were introduced by chemical reagents. Following the general procedures for glass preparation, the batch containing the necessary fluxing and refining agents was melted in a resistance furnace at 1,350 °C for 2 h. The melt was formed and annealed. After the samples were cooled down to room temperature, they were cut and polished for the visible light transmission spectrum measurement.

The light transmission spectrum of the prepared glass was recorded on a 722 spectrometer (Jinghua, China) over the visible light range.

A modified sand adsorbent with a porous silica coating and surface area of 150 m²/g was developed for the adsorption of nickel ions from aqueous solutions. The adsorption is pH dependent, with a sharp increase in the pH range of 2 to 4. The adsorption reaches equilibrium within 30 min. The maximum removal efficiency and ion retention in per unit mass of the adsorbent could be 100% and 5.78 mg/g under the specified experimental conditions. The kinetics and isotherm of the adsorption follow the pseudo-second-order and the Freundlich multilayer models, respectively. The spent adsorbent was successfully applied in preparing a brown colored glass. The present study provides a new adsorbent facilitating the reuse of the recovered nickel from wastewater.

This work was financially supported by the National Natural Science Foundation of China under 51372102.