Potential of Soyabean Husk as Metal Sorbent and its Application in Water Pollution Remediation Studies

Dr. Naisergik Deepika Khanna¹, Dr. Anil Kumar²

¹ Assistant Professor, Department of Chemistry, Govt. College Hamirpur

² Assistant Professor, Department of Chemistry, Govt. College Nadaun

ABSTRACT:

On the global scale, loss of biodiversity is recognized as one of the most critical environmental problems facing mankind. Heavy metal pollution occurs in many industrial waste waters such as those produced by metal plating facilities, mining operations, battery processes, production of paints and pigments, and the glass production industry. This waste water commonly includes Cu, Ni, Cr, Cd and Pb. These heavy metals are not biodegradable and their presence in streams and lakes leads to bioaccumulation in living organisms, causing health problems in animals, plants and human beings. Therefore their presence in wastewater is of great environmental concern. Adsorption has advantages over the other methods because of simple design within a sludge free environment and also involved low investment in term of both initial cost and land required. The agricultural wastes such as sawdust, rice husk, peanut, coconut shell and etc. are easily and widely available. These materials cause a significant disposal problem. Efforts have been made to use these materials as adsorbents to adsorb heavy metals from aqueous solutions. The aim of the present study is to utilize the effectiveness of locally available soyabean husk for Cr⁶⁺ removal from synthetic waste water. In the present sorption studies we analyze the Langmuir and Freundlich adsorption isotherm and characterize soybean hulls and modified soybean hulls by FTIR spectroscopy.

1. INTRODUCTION:

Heavy metal pollution is one of the main problems. The presence of heavy metals over permissible levels causes hazardous effect on soil and aquatic system of any kind, consequently having concern on human health. Toxic metal compounds coming to earth’s surface not only reach earth’s water (seas, lakes, ponds), but also contaminate underground water in trace amount by leaking from the soil after rain and snow. Therefore earth’s water may contain various toxic metals. One of the most important problems is the accumulation of toxic metals in food structures. The contaminated food can cause poisoning in humans and animals. Thus removal of heavy metals from wastewater is important for the protection of environment and human health. Several industrial and agricultural processes and mining activities have increased the concentration of toxic contamination in water and wastewater around the world. The main sources of heavy metals includes waste from the electroplating and metal finishing industries, metallurgical industries, tannery operation, chemical manufacturing and ground water contamination from hazardous sites^[1,2]. Heavy metals that get into the environment could cause permanent negative effects.

Chromium (Cr) is one of the undesirable heavy metals which affect human physiology. Accumulates in the food chain and causes several ailments^[3]. It is an element which can exist in several oxidation states, the most common are the trivalent (Cr³⁺) and hexavalent (Cr⁶⁺) forms, the latter is the more toxic. Cr⁶⁺ in natural waters has attracted much attention since it is known to be carcinogenic (even at low concentrations, e.g. [less than or equal to 50 ppb), mutagenic and teratogenic^[4]. Cr⁶⁺ compounds are introduced to the environment through the wastes of a variety of industries like chrome plating, electronic, metallurgical, timber and leather-tanning^[5-6]. Due to the toxicity and commercial value of heavy metals, it has become increasingly urgent to develop new technologies for its reuse.

A number of technologies for the removal of metal ions from aqueous solutions have been developed over the years. The most important of these techniques include chemical precipitation, filtration, ion-exchange, reverse osmosis, membrane systems, etc. However, all these techniques have their inherent advantages and limitations in application. In the last few years, adsorption has been shown to be an alternative method for removing dissolved metal ions from liquid wastes^[7]. Adsorption is a very effective process for a variety of applications and now it is considered an economical and efficient method for metal ions removal from wastewaters. The most generally used solid adsorbent is activated carbon which is used as a very efficient solid adsorbent in many different applications^[8]. However, activated carbon is expensive and for effluents containing metal ions activated carbon requires chelating agents to enhance its performance, thus increasing treatment cost.

In order to minimize processing costs, several recent investigations have focused on the use of low cost adsorbents, e.g. agricultural by-products^[9], waste materials^[10], biosorbents^[11], slag^[12] and clay materials^[13]. Adsorbents, mainly clay minerals, are readily available, inexpensive materials and offer a cost-effective alternative to conventional treatment of such mentioned waste streams^[14]. Adsorption to remove various heavy metals from waste streams, tailings and such solutions is remains an important unit operation and is often the process of choice.

The application of low-cost adsorbents obtained from plant wastes as a replacement for costly conventional methods of removing heavy metal ions from wastewater has been reviewed^[15]. It is well known that cellulosic waste materials can be obtained and employed as cheap adsorbents and their performance to remove heavy metal ions can be affected upon chemical treatment. In general, chemically modified plant wastes exhibit higher adsorption capacities than unmodified forms. Numerous chemicals have been used for modifications which include mineral and organic acids, bases, oxidizing agent, organic compounds, etc. Plant wastes as adsorbents including rice husks, soybean hulls, sawdust, sugarcane bagasse, fruit wastes, weeds and others show good adsorption capacities for Cd, Cr, Cu, Pb, Zn and Ni.

The soybean (Glycine max) is often called the miracle crop and is the world's foremost provider of protein and oil. The mature soybean is about 38% protein, 30 % carbohydrate, 18 % oil, and 14 % moisture, ash, and hull. Soybeans contain all three of the macro-nutrients required for good nutrition: complete protein, carbohydrate and fat, as well as vitamins and minerals, including calcium, folic acid and iron. The soybean straw was water or base washed and modified with citric acid (CA) to enhance its nature adsorption capacity^[16]. The porous structure, as well as high amounts of introduced free carboxyl groups of CA modified soybean straw makes the adsorbent be good to retain Cu²⁺. The adsorption capacities increased when the solution pH increased from 2-6 and reached the maximum value at pH 6 (0.64 mmol g⁻¹ for the base washed, CA modified soybean straw (CA-BWSS)). Both the Langmuir and Freundlich adsorption isotherms were tested and the Freundlich model fitted much better than the Langmuir model.

Soybean hulls, extracted with 0.1 N NaOH (BE) and modified in the presence of 0.6 M CA, were compared to similarity treated peanut shells and the hulls of almonds, cottonseed and macadamia nut for their ability to absorb copper ion Cu²⁺ as a typical metal ion^[17]. BE, CA-modified soybean hulls had the highest metal ion adsorption but similarly treated almond hulls had the highest total negative charge. BE, CA-modified soybean hulls also were compared to BE hulls modified in the presence of 0.6 M concentrations of four different dicarboxylic acids (maleic, malic, succinic, tartaric) for their Cu²⁺ adsorption potential. Hulls modified with CA had the highest adsorption of Cu²⁺ by virtue of their largest total negative charge. Adsorption capacities and affinity constants for the metal ions Cd²⁺, Cu²⁺, Ni²⁺, Pb²⁺ and Zn²⁺ were determined for BE, CA-modified hulls at pH 4.8. Adsorption capacities for all ions were greater than 1.0 mmol g^-1 hull.

Wartelle et al.^[18] developed a method to enhance metal ion adsorption of soybean hulls for wastewater treatment using Cu²⁺ as a typical metal ion. Hulls, extracted with 0.1 N NaOH, were modified with different CA concentrations (0.1-1.2 M) at 120⁰C for 90 min. CA-modified hulls had adsorption capacities for Cu²⁺ from 0.68 to 2.44 mmol/g, which was much higher than for unmodified hulls (0.39 mmol/g). CA-modified, non-extracted (NE) and CA-modified, BE hulls were compared for adsorption kinetics and adsorption capacity. For BE, CA-modified hulls, increasing the temperature from 25^oC to 60^oC appeared to have no effect on the rate of Cu²⁺ removal from solution. CA modification of soybean hulls greatly enhanced metal ion removal and resulted in a product with possible commercial potential for metal ions remediation.

Marshal and Wartelle^[19] used agricultural by-products, base-extracted and reacted with CA, were compared to demonstrate their ability to adsorb Cu²⁺ from solution. Soybean hulls exhibited the highest Cu²⁺ uptake (1.44 mol/g) of the 12 biomaterials tested. The by-products with the highest bulk densities (>0.6 g/cm³), namely pecan, black walnut and English walnut shells, showed the lowest Cu²⁺ uptake after CA modification. Those materials with a bulk density less than 0.6 g/cm³ and low lignin content had the best potential of becoming ion exchange resins using CA modification. Tang et al.^[20] prepared new biosorbent using chemically modified orange peel and its biosorption for Pb²⁺ ions was studied. The experimental results show that biosorption equilibriums were rapidly established in about 1h and the reaction could be explained as pseudo-first-order kinetic processes. The Pb²⁺ adsorption was strictly pH dependent, and maximum uptakes of Pb²⁺ on different biosorbents were observed at pH range of 4.5-6.0. The maximum adsorption capacity of Pb²⁺ was obtained as 1.22 mol kg⁻¹.

The advantages and drawbacks of conventional and non-conventional heavy metal removal methods are critically discussed by Ledezma et al^[21] given particular attention to those related to adsorption, nanostructured materials and plant-mediated remediation. Heavy metals removal by adsorption on biochar's surface studied by Pathy et al^[22] has shown promising results in the remediation of contaminated soil and water. The adsorption capacity of biochar can be altered by pre- or post-pyrolysis activation. The main objective of this work is to use the effectiveness of soybean hulls for the removal of Cr⁶⁺ ions from synthetic waste water.

2. EXPERIMENTAL:

2.1. Materials:

Soybean Hulls (SH) were collected from District Mandi Himachal Pradesh, washed with water to remove dust, sun dried for 3-4 days and stored for further use. Potassium dichromate (K₂Cr₂O₇) (Qualigens Fine Chemicals, Mumbai), Diphenylcarbazide (Loba Cemi Pvt. Ltd.), Sodium acetate (Qualigens Fine Chemicals, Mumbai), Glacial acetic acid (Qualigens Fine Chemicals, Mumbai) and Sodium hydroxide (Qualigens Fine Chemicals, Mumbai) were used as received. All the weights were taken on Shimadzu Balance having minimum readability of 0.001 g.

2.2. Reagents prepared:

The stock solutions of K₂Cr₂O₇ (100 ppm, 300 ppm, 500 ppm and 750 ppm)  were prepared in sodium acetate buffer (pH 3.6, 5.0) and were used in sorption experiment. A solution of diphenyl carabzide is prepared by dissolving 0. 250 g of diphenyl carabzide in 50 mL of acetone.

2.3. Modification of soybean hulls by pre-treatment:

Acidic and basic hydrolysis of SH was carried out by immersion of 0.500 g weight in 250 mL of 0.1M  NaOH and 0.1 M  HCl for 2h. After that, the solution was filtered and residue was dried and used for sorption study. The SH treated with NaOH and HCl were abbreviated as NaOH-SH and HCl-SH respectively.

2.4. Method for Cr⁶⁺ ions uptake:

Dried and weighed SH (0.100-1.000 g) was suspended in a known volume (50 ml) and concentration (100, 300, 500 and 750 ppm) of metal ions solution in a beaker maintained at constant temperature (30⁰-70⁰C) and pH (3.6, 5.0). After the stipulated time interval (15-360 min) 1 ml of metal ions solution was pipette out. Same set-up as mentioned above was carried out to evaluate the efficiency of HCl-SH and NaOH-SH in sorption studies.

To each test tube containing 1.0 mL of solution from experimental set-up added 9.0 mL buffer and 0.1 mL of diphenyl carbazide. Optical density was measured at λ_max 545 nm at 25⁰C using Ultraviolet spectrophotometer (Elico, Dolphin PG College of Life Sciences, Chunni Kalan, Punjab). The unknown concentration of solution from experimental set-up was detected by using standard curve. The standard curve was generated by taking ten standard concentrations.

2.5. Relationships used to express adsorption results:

Where, Ci is the initial concentration of Cr⁶⁺ ions in the solution (ppm)

Cf is the final concentration of Cr⁶⁺ ions in the solution (ppm)

V is the volume of the solution (mL)

W is the weight of biosorbent (g)

2.6. Characterization:

Physical characterization of SH, HCl-SH and metal ions loaded HCl-SH has been carried out by FTIR-spectroscopy (Perkin Elmer, P.U. Chandigarh) using KBr pallets.

3. RESULT AND DISCUSSION:

3.1. Effect of pretreatment and contact time on Cr⁶⁺ removal:

The SH is pretreated with 0.1 N HCl (HCl-SH) and 0.1N NaOH (NaOH-SH). The modified soybean hulls were used to adsorb Cr⁶⁺ ions from potassium dichromate solution for 15, 30, 60, 120, 240 and 360 min. The comparison of using modified and unmodified SH for the adsorption is shown in Fig. 3.1. It is found that the HCl-SH is shown better adsorption of Cr⁶⁺ ions than the NaOH-SH followed by the unmodified SH. The maximum metal ions uptake, 95.9 % in 360 min and 90.89 % in 240 min is shown by HCl-SH and NaOH-SH respectively. The unmodified soybean hulls gave maximum ion uptake 86.0 % in 240 min which is less than modified soybean hulls. The absorption capacity for HCl-SH, NaOH-SH and SH is 2.39 mg/g in 360 min, 2.25 mg/g in 240 min and 2.15 mg/g in 240 min respectively.

The influence of contact time on Cr⁶⁺ removal by HCl-SH is illustrated in Fig. 3.1. The rate of Cr⁶⁺ removal is rapid at the initial stage of sorption of Cr⁶⁺ onto the external surface of HCl-SH. The Cr⁶⁺ sorption on HCl-SH is 71.1% in 30 min, after that, the rate of Cr⁶⁺ removal increased gradually and reached the equilibrium value within 120 min. This process is controlled by the pore diffusion velocity of Cr⁶⁺ into the intraparticle matrix of SH.  

The maximum metal ion removal was obtained with HCl-SH therefore it is used for further studies and the equilibrium is reached with in 120 min.

Table 3.6a. Parameters of Langmuir and Freundlich models

Table 3.6b. The values of R_L for adsorption of Cr⁶⁺ on HCl-SH

Table 3.6c. Characteristics of adsorption-Langmuir isotherm

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