Removal of Sulfate from Water and Wastewater by Surfactant- Modified Coir Pith, an Agricultural Solid ‘waste’ by Adsorption Methodology

The surface of coconut coir pith, a lignocellulosic polymer was modified with a cationic surfactant, hexadecyltrimethylammonium bromide and used as an adsorbent for the removal of sulfate from aqueous solutions. Optimum pH for maximum removal of sulfate was found to be 2.0. Adsorption has been found to be concentration dependent and follows pseudo second order kinetics. Langmuir, Freundlich and Dubinin-Radushkevich isotherms were applied to describe the equilibrium data and the system followed all the three isotherms and the Langmuir adsorption capacity of the adsorbent was found to be 8.8 mg g. Desorption studies showed that recovery of sulfate from the spent adsorbent was feasible. Effect of foreign anions on the adsorption of sulfate was also examined. Removal of sulfate by surfactant modified coir pith from industrial wastewater was also tested. *Corresponding author Email: [email protected] INTRODUCTION Numerous applications of coconut coir pith, an agricultural solid waste obtained during the separation of fiber from coconut husk, arise from their properties of high porosity and adsorption capacities. It is estimated that the production of coir pith in India is about 7.5 million tons per year [1]. Raw coir pith consists of 35.0% cellulose, 25.2% lignin, 7.5% pentosans, 1.8% fats and resins, 8.7% ash content, 11.9% moisture content and 10.6% other substances [2]. α cellulose and lignin are important sources of relatively easily accessible hydroxyl units that can be used for the attachment of several functional groups [3]. Coir pith shows good adsorption towards dyes [4]. The availability of coir pith in large quantities as solid waste has stimulated further research for new applications. One such new field is anion adsorption. Raw coir pith does not show any affinity for anions without prior modification. In order to enhance its sorption capabilities towards anions, the surface of coir pith was modified using a cationic surfactant, hexadecyltrimethylammonium (HDTMA) bromide. After modification, coir pith exhibits high sorption capacity for anionic contaminants [5,6]. Sulfate is a common constituent of many natural waters and wastewaters, and is sometimes present in high concentrations. Industrial wastewaters are responsible for most anthropogenic emissions. Certain industrial effluents may contain several thousands of mg L domestic sewage contains typically less than 500 mg L [7]. The damage caused by sulfate emissions is not direct, since sulfate is a non-toxic compound. However, high sulfate concentrations can unbalance the natural sulfur cycle [8]. Several processes can be applied to promote the removal of dissolved sulfate. Chemical precipitation through the addition of barium or calcium salts is an alternative, mainly applied to the treatment of wastewaters that contain high sulfate concentrations. Using membranes is another alternative. However, the relatively high cost and energy consumption, proportional to the sulfate concentration, should be taken into account. Adsorption is an effective and economical method to treat sulfate-containing wastewaters. So this paper examines the sorption of sulfate present in aqueous solution using surfactant-modified coir pith as adsorbent and its applicability is also extended to fertilizer industry wastewater. Effects of contact time, adsorbent dose, pH and temperature on the removal process have been investigated. 130 J. Environ. Eng. Manage., 17(2), 129-135 (2007) MATERIALS AND METHODS The experimental solutions of sulfate were prepared using analytical grade sodium sulfate, obtained from s.d. Fine Chemicals, Mumbai, India. All reagents used are of analytical grade chemicals and were obtained from Merck and Loba Chemie. 1. Preparation and Characterization of Surfactantmodified Coir Pith (CPHDTMA) Waste coir pith was collected from local coconut coir industries and dried in sunlight. Then it was ground and sieved to obtain the particle size of 150250 μm. The surfactant modified coir pith was prepared as follows: coir pith and 2% HDTMA solution was taken in a 500 mL conical flask and agitated at 200 rpm on a shaker machine for 5 h. Then the suspension was left undisturbed to separate the liquid and the coir pith. The liquid was discarded and the modified coir pith was washed with distilled water several times to remove superficially held surfactant. The modified coir pith was dried in a hot air oven at 60 °C for 8 h. Then it was characterized by Fourier transform infrared (FTIR) spectra, CHN analysis and surface area analysis. Leaching of the surfactant was studied by agitating 0.5 g of the adsorbent with 50 mL of distilled water, at different pH values for 2 h. After agitation, the supernatant was discarded and the adsorbent was filtered using Whatman filter paper and washed gently with water to remove any superficially held surfactant. Then it was dried in hot air oven and analyzed for C, H and N using CHN analyzer (Vario EL III, Germany). pH measurements were made using a pH meter (Elico, model LI-107, Hyderabad, India). The surface area was measured by a Quantasorb model surface area analyzer. The IR spectra of the samples were recorded on a Shimadzu FTIR spectrophotometer. Orbital shaker machine (Scientific Systems, Chennai, India) was used for batch adsorption studies 2. Batch Mode Adsorption Studies Batch mode adsorption studies were carried out by shaking 100 mL conical flasks containing 0.20 g of CPHDTMA and 50 mL of sulfate solution of desired concentration on an orbital shaker machine at 160 rpm, 32 °C and at an initial pH 2.0. The solution pH was adjusted with 0.1 M HCl and 0.1 M NaOH solutions. At the end of the adsorption period the supernatant was separated by centrifugation at 3000 rpm for 30 min. Then the concentration of the residual sulfate ion was determined by turbidity-based method [9]. The amount of sulfate adsorbed was calculated from the concentration in the solution before and after adsorption. Effect of pH on the adsorption of sulfate onto CPHDTMA was studied for the sulfate concentrations of 20 and 40 mg L. Effect of contact time was studied by withdrawing the samples from the shaker at predetermined time intervals and residual sulfate concentration was analyzed as above and bromide ion release from the adsorbent surface was also analyzed simultaneously by phenol red method [9]. Effect of adsorbent dose was studied with different adsorbent doses (100-1000 mg 50 mL) for the sulfate concentrations 10-50 mg L. Langmuir, Freundlich and Dubinin-Radushkevich (DR) isotherms were used to analyze the equilibrium adsorption data. 2.1 Effect of foreign ions Effect of Mo(VI), Cr(VI), PO4 , SCN, Cl, V(V) and NO3 at various concentrations on adsorption of sulfate by CPHDTMA was studied. 2.2 Temperature studies Effect of temperature on adsorption of sulfate was studied using 10-50 mg L sulfate and 0.20 g of the adsorbent at 32, 40, 50 and 60 °C in the thermostated rotary shaker machine. 2.3 Batch mode desorption studies The adsorbent used for the adsorption of 20 and 40 mg L of sulfate was separated from the solution by suction-filtration using Whatman filter paper and washed gently with water to remove unadsorbed sulfate. Several such samples were prepared. Batch desorption experiments were conducted as a function of desorbing solution pH. To each 100 mL conical flask, 0.2 g of sulfate loaded adsorbent and 50 mL of desorbing solution at pH 2.0-11.0 (adjusted using 1 M HCl/NaOH) were added and agitated for equilibrium time. Then the desorbed sulfate was estimated as before. RESULTS AND DISCUSSION 1. Characterization of the Adsorbent The percentages of C, H and N present in the modified and unmodified coir pith are presented in Table 1. The increase in the C, H and N content is due to the adsorption of HDTMA onto coir pith surface. It was found that 198 mg g of HDTMA was adsorbed on to coir pith surface. The Brunanuer Emmett Teller (BET) surface area of CPHDTMA (1.9 m g) was lower than that of unmodified coir pith (2.3 m g). Sorption of HDTMA on coir pith caused a decrease in the surface area relative to the unmodified coir pith. Apparently, the adsorption process led to a constriction of the pore channels, as a result of attachment of the surfactant moieties to the internal framework surfaces. Similar reduction in the BET surface area was Namasivayam and Sureshkumar: Sulfate Removal by Modified Coir Pith 131 4 0 0 0 3 5 0 0 3 0 0 0 2 5 0 0 2 0 0 0 1 5 0 0 1 0 0 0 5 0 0 c b a % T r a n s m it ta n c e ( A r b it r a r y u n it s ) W a v e n u m b e r ( c m 1 ) Fig. 1. FTIR spectrum of (a) raw coir pith, (b) CPHDTMA and (c) sulfate loaded CPHDTMA. 2 4 6 8 10 12 0 10 20 30 40 50 60 70 80 90 100 0 10 20 30 40 50 60 70 80 90 100