Effects of pH, particle size and porosity of raw rice husk and its silica on removing lead and hexavalent chromium from aqueous solution

Effects of pH, particle size and porosity of raw rice husk and its silica on removing lead and hexavalent chromium from aqueous solution

K. Moeinian¹, P. B. Abdul latif², T. Rastgoo³, S. M. Mehdinia⁴

1,3,4. Department of Environmental Health Engineering, Semnan University of Medical Sciences, Semnan, Iran

2. Department of Environmental Science, Faculty of Environmental Studies, University Putra Malaysia, Selangor, Malaysia

Corresponding author’s E-mail: smmehdinia@yahoo.ca

(Received: Jan. 20. 2017 Accepted: June 21. 2017)

INTRODUCTION

Over the course of the recent decades, human activities have led to a significant increase in environmental pollution. This is also why control and treatment of pollutants are the most important environmental challenges today. Lots of studies have reported a relationship between environmental pollution levels and human health problems (Koop et al. 2010; Mehdinia 2011). Heavy metals are some of the most important environmental pollutants which have some hazardous effects on human health at exceeded concentrations (Kobya et al. 2004). Currently, several methods have been used to remove heavy metals from the environment. Some of the most important methods utilized in removing them work by adsorption (Abdel Ghani et al. 2007; Mehdinia et al. 2014). Activated carbon has been generally used for adsorption of heavy metals from aqueous. Since the use of commercial activated carbon to remove pollutants is expensive, investigators are searching for other cost-effective adsorbents. Recently, wide attention has been devoted to the study of the heavy metals removal from water and wastewater with the application of agricultural wastes (Mahvi 2002; El-Ashtoukhy et al. 2004; Demirbas et al. 2004; Gueu et al. 2007; Shamohammadi et al. 2008; Montazeri et al. 2009; Khazaei et al. 2011; Salari et al. 2012; Kafia & surci 2012; Ghaneian et al. 2014; Mohseni et al. 2016). Adsorbent properties such as porosity, particle size, surface area and pore radius size as well as the properties of water and wastewaters such as pH and temperature are very important in determining the removal efficiency of pollutants. Therefore, this study investigates the effects of pH, particle size and porosity of raw rice husk and its silica on removing the lead and hexavalent chromium from water and wastewaters.

MATERIALS AND METHODS

Preparation of adsorbents

This research as an experimental study, was carried out in the School of Health, Semnan University of Medical Sciences in 2014. Raw rice husk was collected from the north of Iran. Rice husk silica was prepared according to Jamwal & Mantri method (2007). In this method, the rice husk was first washed with tap water to remove contaminants and was then oven-dried at 110°C for 24 h.  It was then subjected to acid leaching by reflux in 3% hydrochloric acid and 10% sulphuric acid for 2 h, at a ratio of 50 g husk per liter. The husk was completely washed with distilled water and oven-dried at 100°C for 4 h. Eventually, the clean and dried husk was burned inside a muffle furnace at 800°C in static air for 4 h.

Batch adsorbent experimental

According to   Souag et al. (2007), synthetic solution containing lead and chromium were prepared by dissolving a known amount of analytical-grade lead chloride and potassium dichromate in distilled water. The effects of the adsorbent particle sizes (0.07-0.1, 0.1-0.5, 0.5-1.0 and 1.0-1.5 mm), porosity and  pH from 2 through 8 were investigated by varying any of the process variables while keeping the other variables constant (adsorbent dosage = 1.5 g.l^-1, contact time = 60 min, chromium and lead initial concentration = 5 mg.l^-1). The pH  was adjusted from 2.0 through 8.0 by adding 0.1M HCl and 0.1M NaOH. All experiments were carried out in batch reactors  at room temperature of 25 ± 2°C. All experiments were carried out at three times and the averages were reported as results.

Data analysis

The concentration of lead and hexavalent chromium before and after the adsorption process were measured using atomic adsorption (Perkin Elmer-100 model). Wavelengths of 283.3 and 357.9 nm and detection limits of 0.05 and 0.02 mg.l^-1 were used for measurment of lead and Cr⁺⁶, respectively. Removal efficiency of the adsorbents was determined by the following formula:

RE  

where, C_i and C_o are the concentration of lead and hexavalent chromium before and after adsorption operating time, respectively (Mehdinia et al. 2011; Mehdinia et al. 2013).     

The surface morphology of the adsorbents were observed under a scaning electron microscope (SEM), using LEO operated at an accelerating voltage of 15 kV (Ros et al. 2007).  

RESULTS AND DISCUSSION

Effect of particle size

The results of this study showed that the removal efficiency of hexavalent chromium is dependent on the particle sizes of raw rice husk and its silica. Using 1.5 g.l^-1 of the adsorbents, 5.0 mg.l^-1 initial concentration of Cr⁺⁶ and 60 min contact time, the maximum removal efficiency of up to 98.8 and 88.4% was obtained at particle size of 0.07-0.1 mm by the application of raw rice husk and its silica, respectively. However, at the same condition but with changing the particle size to 1.0-1.5 mm, the maximum removal efficiency decreased up to 63.3 and 61.4%, respectively. The result of removal efficiency of Cr⁺⁶ using raw rice husk and its silica of different particle sizes and 5.0 mg.l^-1 initial concentration, 60 min contact time and 1.5 g.l^-1 adsorbents dosage are shown in Fig. 1.

This result is supported by Choudhury et al. (2012) who studied the adsorption of Cr (III) from aqueous solution using groundnut shell. They reported that: "The lower the particle size, the higher the chromium removal (%). By decreasing in particle size, the removal of chromium (III) increased from 47 to 97%".

Moreover, Wong et al. (2003) and Munaf & Zein (1997) reported that by elevating the adsorbent particle size, the removal efficiency of heavy metals will be decreased.

Fig. 1. Removal efficiency of Cr⁺⁶ using raw rice husk and its silica with different particle sizes,  5.0 mg.l^-1 concentration, 60 min contact time and 1.5 g.l^-1 adsorbent dosage.

In addition, in this research, we studied the correlation between decreasing adsorbents particle size and removal efficiency of lead using the Pearson’s correlation analysis.

So that, we assessed the correlation between decreasing the raw rice husk and its silica particle sizes (0.07-0.1, 0.1-0.5, 0.5-1.0 and 1.0-1.5 mm) as well as removal efficiency of lead at pH 6, adsorbents dosage = 1.5 g.l^-1, contact time = 60 min and initial concentration of lead = 5.0 mg.l^-1. A positive strong significant correlation was detected between decreasing the adsorbent particle sizes and lead removal efficiency in both adsorbents (P < 0.01).

The results showing the correlation between decreasing particle sizes and lead removal efficiency is illustrated in Fig. 2.

These effects could be due to the fact that the smaller adsorbent particles offer larger surface areas and greater numbers of adsorption sites (Choudhury et al. 2007).   

Effect of pH

The results of this study showed that the removal efficiency of hexavalent chromium is intensively pH-dependent. Using 1.5 g.l^-1 adsorbents, 5.0 mg.l^-1 initial concentration of Cr⁺⁶ and 60 min contact time, the maximum removal efficiency of 98.8 and 88.4% by the application of raw rice husk and its silica, were obtained at pH 2, respectively. However, at the same condition only by changing pH values, the removal efficiency decreased as the maximum RE at pH 7 reached up to 69.4 and 60.4, respectively. Unlike chromium, lead removal efficiency is not so pH-dependent. Using 1.5 g.l^-1 of the adsorbents, particle size = 0.5-1.0 mm, 5.0 mg.l^-1 initial concentration of lead and 60 min contact time, maximum removal efficiency of 69 and 97.4% were obtained by the application of raw rice husk and its silica, respectively at pH 6.

Therefore the removal efficiency did not

Change by changing pH values. Hasan et al.

(2004) studied the removal of hexavalent chromium from aqueous solutions using agricultural waste ‘maize bran’. They reported that the initial removal of hexavalent chromium increased by an increase in pH from 1.4 to 2.0, thereafter removal started to decrease by elevating pH from 2.0 to 8.5. This could be that since hexavalent chromium occurs in the form of oxyanions for example HCrO₄⁻, Cr₂O₇²⁻, CrO₄²⁻, etc. in acidic medium and the lowering of pH causes the surface of the adsorbent to be protonated to a higher amount, consequently a strong attraction exists between these oxyanions of hexavalent chromium and positively-charged surface of the adsorbent. Hence, the removal efficiency increases by a decrease in the pH of the aqueous (Hasan et al. 2008).

Demirbas et al. (2004) studied the removal of hexavalent chromium by three types of carbon at different pH levels at an initial concentration of 105 mg.l^-1 and particle sizes of 1.0 to 1.25 mm. They reported that optimum removal efficiency was observed at pH 1.0. They also reported that the dominant form of hexavalent chromium at pH 2 is HCrO₄^-. Increasing in the pH will shift the concentration of HCrO₄^- to other forms, CrO₄^2-and Cr₂O₇^2-.

The results of lead and hexavalent chromium removal efficiency using raw rice husk and its silica with different solution pH, 5.0 mg.l^-1 initial concentration, 60 min contact time and 1.5 g.l^-1 adsorbent dosages are shown in Fig. 3.

Fig. 2. The correlation between decreasing in particle sizes and lead removal efficiency at pH 6, adsorbents dosage of 1.5 g.l^-1, initial concentration of lead = 5 mg.l^-1 and contact time of 60 min by the two adsorbents.

Effect of adsorbent morphology

Scanning electron microscope (SEM) shows the morphologic images of the raw rice husk and the silica derived from modified rice husk. It showed that when the rice husk goes under on acid leaching and incineration at high temperature, it reduces the crystallization of cells and increases its porosity. This phenomenon can thus increase the potential of adsorbents to remove environmental pollutants. Mehrasbi & Farahmandkia (2008)

reported that modification of banana shell using acidic an alkaline agents results in increased porosity and surface area of the adsorbent (Mehrasbi & Farahmandkia, 2008). This result is also supported by Wang Ngah & Hanafiah (2008) who investigated the removal of the heavy metal ions from wastewater using chemically-modified plant wastes as adsorbents.

In order to compare the shape and pores of the raw rice husk with its silica, SEM images of rice husk silica with magnification of (a) 1000 and (b) 2000 are shown in Fig. 4 while the SEM images of raw rice husk with magnification of (c) 500 and (d) 1000 are shown in Fig. 5.            

Fig. 3. Removal efficiency of lead and Cr⁺⁶ using raw rice husk and rice husk silica with different solution pH and 5.0 mg.l^-1 initial concentration, 60 min contact time and 1.5 g.l^-1 adsorbents dosage.

Fig. 4. Scanning electron microscope (SEM) images of silica derived from raw rice husk with magnification of 1000 (a) and 2000 (b).

Fig. 5. Scanning electron microscope (SEM) images of raw rice husk with magnification of 500 (c) and 1000 (d).