Iranian Journal of Environmental Health, Science and Engineering Iranian Association of Environmental Health (IAEH) ISSN: 1735-1979 Vol. 5, Num. 2, 2008, pp. 125-130

Iranian Journal of Environmental Health Science & Engineering, 2008, Vol. 5, No. 2, pp. 125-130

Application of Impregnated Almond Shell Activated Carbon by Zinc and Zinc Sulfate for Nitrate Removal from Water

*A. Rezaee, H. Godini, S. Dehestani, A. Khavanin

Department of Environmental Health, Medical Sciences Faculty, Tarbiat Modares University, Tehran, Iran
*Corresponding author: abbasrezaee@yahoo.com Telefax: +98 21 8288 3575

Received 4 December 2007; revised 8 February 2008; accepted 3 March 2008

Code Number: se08023

ABSTRACT

In this study impregnated almond shell activated carbon by Zn° and ZnSO₄ were used as adsorbent with a particle size of 10-20 mesh. The objective of this research was to determine the ability of impregnated activated carbon in nitrate removal. The modified activated carbon had 1mm effective size, with a uniformity coefficient of 1.18. Potassium nitrate solution was used in batch adsorption experiments for nitrate removal from water. The effects of nitrate concentration, activated carbon dosage and time of contact were studied. Experimental data showed that modified activated carbon by Zn° and ZnSO₄ was more effective than virgin almond activated carbon for nitrate removal. The maximum nitrate removal was 64%-80% and 5%-42% for modified activated carbon and virgin activated carbon, respectively. While virgin activated carbon used, nitrate-N decreased from 20 to 15mg/L in 30min reaction. The final nitrate concentration was not in the standard range of WHO recommendations for water quality; while impregnated activated carbons were used, nitrate drcreased to <10mg/L. Maximum removal was over 16-17mg nitrate-N per 1g activated carbon for impregnated activated carbon. The experiments were conducted at pH=6.2, 20ºC and initial concentrations of 20mg/L nitrate-N. Increase in modified activated carbon dosage increased the nitrate removal efficiency. The equilibrium time was found to be 45min for modified activated carbon.

Key words: Activated carbon, nitrate, adsorption, zinc, zinc sulfate

INTRODUCTION

Nitrate is a wide spread contaminant in both ground and surface water due to excessive use of nitrogenous fertilizers in agricultural activities and disposal of untreated sanitary and industrial wastes (Hudak, 2000; Almasri and Kaluarachchi, 2004). Elevated nitrate concentrations in drinking water are linked to health problems such as methemoglobinemia in infants and stomach cancer in adults. Effluents with high concentrations of nitrate may also promote algal bloom in water reservoir (Zhi-Wei et al., 2005). The maximum contaminant level (MCL) for nitrate set by the World Health Organization (WHO) and the US Environmental Protection Agency (EPA) for drinking water are 50 and 45mg/L of NO₃ respectively (USEPA, 2000; WHO, 2004). Activated carbon (AC) adsorption is one of the recommended technologies for nitrate removal from water (Mizuta et al., 2004). Mizuta conducted nitrate removal with bamboo powder charcoal as adsorbent.

Activated Carbon mainly contains micropores and is efficient to remove some pollutants. In some cases, it needs mesopores to improve the adsorption velocity and mass transferring. Thus, it is necessary to increase the mesopore content in AC. One of the most effective approaches to increase mesopore and macropore volumes of AC is to catalyze the steam activation reaction of carbon by using the transition metals or rare earth metal compounds, which can promisingly promote the mesopore formation (Shen et al., 2006). The generation mechanism of mesopore and macropore is the activation reaction taking place in the immediate vicinity of metal particles, leading to the formation of mesopore and macropores by pitting holes into the carbon matrix.The impregnation optimizes the existing properties of the AC giving a synergism between the chemicals and the carbon (Shen et al., 2007; Namasivayam et al., 2006). This facilitates the cost-effective removal of certain pollutent which would be impossible otherwise. The unique structure of AC produces a very large surface area. AC has an extraordinarily large surface area and pore volume that gives it a unique adsorption capacity (Baker et al., 1992). It has both chemical and physical effects on substances where it is used as a treatment agent. Activity can be separated into (I) adsorption, (II) mechanical filtration; (III) ion exchange, and (IV) surface oxidation. Adsorption capacity depends on physical and chemical characteristics of the adsorbent, physical and chemical characteristics of the adsorbate, concentration of the adsorbate in liquid solution, characteristics of the liquid phase (e.g. pH, temperature) and the time the adsorbate is in contact with the adsorbent (Cheremishinoff et al., 1978).

Electrical forces between the AC surface and some contaminants may result in adsorption or ion exchange. Adsorption is affected by the chemical nature of the adsorbing surface (Saswati and Kumar, 2005). The chemical properties of the adsorbing surface are determined to a large extent by the activation process. AC materials formed from different activation processes will have chemical properties that make them more or less attractive to various contaminants. For example, chloroform is adsorbed more efficiently by AC that has the least amount of oxygen associated with the pore surfaces (Bakasi et al., 2003). A general rule of thumb is that similar materials tend to associate. Organic molecules and AC are similar materials; therefore there is a stronger tendency for most organic chemicals to associate with the AC rather than dissolved solute in water. Generally, the least soluble organic molecules are most strongly adsorbed. Often the smaller organic molecules are held the tightest, because they fit into the smaller pores (Bakasi et al., 2003).

Adsorption usually increases as pH and temperature decrease. Chemical reactions and forms of chemicals are closely related to pH and temperature. When pH and temperature are decreased, many organic chemicals become more adsorbable (Bakasi et al., 2003). The AC is not widely used in practice for nitrate control due to low capacities and slow adsorption kinetics of nitrate by commercially available activated carbons. In the present work, nitrate removal was carried out for treatment of water by a series of modified and well characterized almond shell-based activated carbon with Zn° and ZnSO₄.

MATERIALS AND METHODS

Impregnated activated carbon modification

Almond shell-based activated carbon was supplied from Kimia Carbon Co., Arak, Iran. The AC was then modified using the surface modification pathway as follows:

Almond activated carbon was stirred in a boiling solution containing, Zn° and ZnSO₄ (5wt.% based on AC) at room temperature, filtered and dried at 120°C (Cheremishinoff et al., 1978). The modified activated carbon (MAC) was then extensively washed with normal perchloride acid and deionized water. After washing, the AC was dried at 90°C in a drying oven and stored in a dessecator until use. The AC particles between 10 and 20 mesh sizes with 1mm effective size and 1.18 uniformity coefficient were used in all experiments.

Experimental design

Batch adsorption experiments were carried out by taking various dosages (1.5, 3, 4.5 and 6g AC/50mL water) of adsorbent, nitrate concentration (10, 20, 30, 40 and 50mg/L NO₃-N) and contact time variations (10, 20, 30, 40, 50, 60, 90 and 120min), with 50mL of nitrate solution of known initial concentration in different conical glass flasks in a shaking thermostat with a constant speed of 120rpm. All expriments were conducted in triplicate and average values are presented in subsequent sections.

Potassium nitrate was used as the source of nitrate in all the experiments (Merck). Percentage of nitrate removed was measured calorimetrically using a UV/visible spectrophotometer (Philips PU 8700 Series model). The experiments were conducted at 20°C for the impregnated AC and AC. The effect of pH was evaluated by adjusting pH using HCl and NaOH. Distilled water prepared in the laboratory was used for preparation of all reagents and all experimental works.

Analytical procedures

pH was monitored by a digital pH meter (Sens Ion 387, HACH model). Nitrate was analyzed using UV-visible spectrophotometer. Ammonia was estimated by phenate method at wavelength of 640nm after removing divalent magnesium ion at higher pH, and nitrite was analyzed by sulphanilamide method at 543nm. Zn and sulfate measurements were regularly checked for the untreated and treated effluents according to the Standard Methods (APHA, 1995). AC properties such as apparent and bulk densities, iodine adsorption test, moisture content (%) and particle size were analyzed according to ASTM D2854, ASTM D4607, ASTM D2867 and ASTM D5158, respectively. Surface morphology of AC was estimated by scanning microscopy (XL30 Philips model). The metals on AC were demonstrated by SEM equipped by EDX (Energy Dispersive X-ray microanalysis) and analysis system of ZAF software. For preparation of SEM, AC was dried in a CO₂ atmosphere under critical conditions.

RESULTS

After modification of activated carbon, physicochemical characteristics were determined. Physicochemical characteristics of almond shell AC and impregnated AC are shown in Table 1. The results were obtained from the mean of triplicate samples for every variable. The SEM micrographs of AC and impregnated AC were displayed in Fig. 1. The x-ray microanalysis of AC is demonstrated by Fig. 2. The allmond shell activated carbon has not Zn of total elements (carbon exception) on the surface while in the impregnated ACs, the Zn increased at surface by 80.67 of total elements composition (carbon exception). The form of Zn° on the AC was zinc oxide. The effects of contact time on the removal of nitrate by AC, AC-Zn° and AC-ZnSO₄ with the initial concentration of 20mg/L NO₃-N, 20^oC temprature and pH=6.2 are shown in Fig. 3 and 4. Adsorption of nitrate ions onto the adsorbent was studied by batch experiments. The effect of carbon dosages for the adsorption process is shown in Fig. 5. Nitrate isotherms of diferent type of activated carbons (AC-0, AC-ZnSO₄ and AC-Zn°) in mass basis are shown in Fig. 6. Water quality after ACs treatment is shown in Table 2.

DISCUSSION

The nitrate could be effectively removed by AC that was impregnated in the presence of Zn⁰ and ZnSO₄. Bulk density and apparent density increased by modification of almond shell AC (Table 1). The SEM micrographs of AC_s (Fig. 1) shows that original AC (a) was dark after modification (b and c); surface of modified AC lost the metallic glaze and the color of the surface also turned black. Comparing Fig. 1 a, b and c show that by modification of AC, macropores were filled by Zn° and ZnSO₄ and micropores were formed. Also, zinc presented at carbon surface as ZnO and increased positive charge of AC. This phenomenon increased ions adsorption such as nitrate. The SEM micrographs and energy despersive x-ray microanalysis of ACs (Fig. 2) supports this subject. It was show that the AC-0 had 0% Zn of total elements (carbon exception) on the surface (Fig. 2), while with modification by Zn° and ZnSO₄, Zn increased at surface by 80.67% and 34.89% of total elements composition (carbon exception). The studies conducted by Shen et al., (2004) and Namasivayan et al., (2006) supports this subject.

Fig. 3 and 4 show the effect of different ACs on nitrate removal from solution with 20mg/L nitrate- N concentration. While AC was used, the amount of nitrate removal reached 15mg/L as nitrate in 30min reaction. The final nitrate concentration was not in the standard range of WHO recommendation for water quality; while AC-Zn° and AC-ZnSO₄ were used, nitrate removal was lower than 10mg/L as nitrate-N. It was obvious that the modification of AC with Zn° and ZnSO₄ greatly enhanced the nitrate removal. This phenomenen may be caused by adsoption and chemical reactions. As for the AC-0, AC-Zn° and AC-ZnSO₄ at AC series, the AC-ZnSO₄ and AC-Zn° showed the highest removal capacity for nitrate, which was near to 60-80% in 30min reaction. The equilibrium time was found to be 45 min for impregnated ACs. (Namasivayam et al., 2006) experimented removal of anions, heavy metals, organics and dyes from water by adsorption on to ZnCl₂ activated coir pith carbon. Results showed that nitrate was removed as 95% by ZnCl₂ coir pith carbon at initial concentration of 20mg/L, temprature of 35°C, 200rpm and agitation time of 3h, while this reduction for coir pith carbon without ZnCl₂ modification was found to be negligible.

The mechanisms of nitrate removal included adsorption and chemical reactions with lower performance of chemical reactions. In the modification process, after impregnation of AC with Zn° and ZnSO₄, micropores were formed. The SEM micrographs showed that MAC had a superior micropore content than its parent sample (Fig. 1).

The other advantage of impregnation by Zn° and ZnSO₄ is increasing of AC positive charges. This phenomenon increased anions adsorption such as nitrate. The studies conducted by the same experiments confirmed the results (Mizuta et al., 2004; Cheng et al., 2005; Shen et al., 2006) studied removal of nitrate from drinking water using bamboo powder charcoal. Adsorption isotherm of nitratenitrogen onto commercial AC at 10^oC showed that maximum amount of nitrate adsorption was 1.17 and 0.93 for bamboo powder charcoal and AC, respectively. Shen et al., (2006) studied the effect of AC fiber structure and loaded metals on the adsorption of dichloroethylene. In their work, the AC was modified and impregnated with copper, which was tranceformed to metal oxides and reduced to elemental substance; the adsorption properties of dichloroethylene on the AC fiber was investigated. The results showed that both the pore structure of AC fiber and metal/oxide loading affected the adsorption capacity of dichloroethylene. The amount of AC adsorption increased with the increase in carbon dosage and reached a maximum value after a particular dosage. Effect of quantity of adsorbent AC and variation of percentage of nitrate removal with weight of adsorbent on nitrate removal is shown in Fig. 5. Study conducted in the same experiments showed that an increase in adsorbent dosage increased the percent removal of nitrate and reached a maximum value after a particular dosage and our results were the same (Ozturk and Bektas, 2004).

Nitrate isotherms of all AC in mass basis are shown in Fig. 6. Ozturk (2004) and Bektas (2003) studied nitrate removal from aqueous solution by adsorption into various materials. The equilibrium time was found to be 45min for AC. Maximum amount removal by time was 4mg NO₃ per gram of AC while in this paper; maximum removal was more than 16 and 17mg nitrate-N per 1g AC for AC-Zn° and AC-ZnSO₄, respectively.

ACKNOWLEDGMENTS

This research was supported by the Department of Environmental Health, Tarbiat Modares University of Iran. Authors are also grateful to Mr. A. Rezaee, Tarbiat Modarres University, For providing SEM photographs.

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