International Journal of Environment Science and Technology Center for Environment and Energy Research and Studies (CEERS) ISSN: 1735-1472 EISSN: 1735-2630 Vol. 5, Num. 2, 2008, pp. 275-285

International Journal of Enviornmental Science and Technology, Vol. 5, No. 2, Spring 2008, pp. 275-285

Review Paper

Application of agricultural fibers in pollution removal from aqueous solution

*A. H. Mahvi

School of Public Health, Center for Environmental Research, Medical Sciences/ University of Tehran, Tehran, Iran
*Corresponding Author Email: ahmahvi@gmail.com Tel.:+98 21 8895 4914; Fax: +98 21 8895 0188

Received 19 January 2008; revised 10 February 2008; accepted 28 February 2008; available online 10 March 2008

Code Number: st08032

ABSTRACT

Discharging different kinds of wastewater and polluted waters such as domestic, industrial and agricultural wastewaters into environment, especially to surface water, can cause heavy pollution of this body sources. With regard to increasing effluent discharge standards to the environment, high considerations should be made when selecting proper treatment processes. Any of chemical, biological and physical treatment processes have its own advantages and disadvantages. It should be kept in mind that economical aspects are important, too. In addition, employing environmentfriendly methods for treatment is emphasized much more these days. Application of some waste products that could help in this regard, in addition to reuse of these waste materials, can be an advantage. Agricultural fibers are agricultural wastes and are generated in high amounts. The majority of such materials is generated in developing countries and, since they are very cheap, they can be employed as biosorbents in water and wastewater applications. Polluted surface waters, different wastewaters and partially treated wastewater may be contaminated by heavy metals or some organic matters and these waters should be treated to reduce pollution. The results of investigations show high efficiency of agricultural fibers in heavy metal and phenol removal. In this paper, some studies conducted by the author of this article and other investigators are reviewed.

Keywords: Biosorption, heavy metals, industrial effluents, aqueous solutions, phenol, adsorption

INTRODUCTION

In recent years, increasing awareness of water pollution and its far reaching effects has prompted concerted efforts towards pollution abatement (Dönmez et al., 1999). Contamination of aqueous environments by heavy metals is a worldwideenvironmental problem due to their toxic effects and accumulation through the food chain (Kapoor et al., 1999; Perez-Rama et al., 2002; Sternberg and Dorn, 2002). Heavy metals are major pollutants in marine, ground, industrial and even treated wastewaters (Valdman et al., 2001). The presence of heavy metals in drinking water can be hazardous to consumers; these metals can damage nerves, liver and bones and block functional groups of vital enzymes (Ewan and Pamphlet, 1996). Metal ions in water can occur naturally from leaching of ore deposits and from anthropogenic sources, which mainly include industrial effluents and solid waste disposal. Due to rapid development of industrial activities in recent years, the levels of heavy metals in water system have substantially increased over time (Apak, 1998; Nouri et al., 2006). Among these metal ions, the ions of Cd, Zn, Hg, Pb, Cr, Cu, etc. gain importance due to their high toxic nature even at very low concentrations. Various methods are available to isolate and remove these heavy metals from the environment. Adsorption is one of the easiest, safest and most cost-effective methods because it is widely used in effluent treatment processes (Balkose and Baltacioglu, 1992). Phenol is one of the most abundant organic pollutants in industrial wastewater (Alnaizy and Akgerman, 2000; Maleki et al., 2005b). It is released to the environment from industries such as petroleum refining, coal tar, steel, tanning, pesticides, pharmaceuticals, etc. (Lesko, 2004; Entezari et al, 2003; Lathasreea et al., 2004; Beltran et al., 2005). Phenol has attracted public attention due to its presence in groundwater, rivers and drinking waters (Entezari et al., 2003). Even in small quantities, phenol causes toxicity and foul odor to the water. Most countries specify maximum allowable concentration of phenol in effluent to be less than 1 ppm (Mahamuni and Pandit, 2005; Maleki et al., 2005). This makes it necessary to develop methods that allow one to detect, quantify and remove heavy metals and phenol from the effluents (Khalfaour et al., 1995). Techniques for removing heavy metals from industrial effluents include precipitation, ion exchange, adsorption, electrodialysis and filtration. However, these methods have limitations on selective separation and high cost of investment and operation of equipment (Ajmal et al., 2003; Cheung et al., 2001; Dae and Young, 2005; Dezuane, 1990; Peternele et al., 1999). By using natural agricultural waste fibers, the adsorption of pollutants from aqueous solutions can be much more economical with regard to other similar physico-chemical processes. In the last few years, adsorption has been shown to be an economically feasible alternative method for removing trace metals from wastewater and water supplies (Allen and Brown, 1995; Gabaldon, et al., 1996; Mahvi et al., 2004a). Adsorption is an effective purification and separation technique used in industry, especially in water and wastewater treatments (Al-Asheh et al., 2000). Several treatment methods such as chemical oxidation, biological treatment, wet oxidation, ozonolysis and activated carbon adsorption have been proposed for removing phenol from industrial effluents. In recent years, advanced oxidation processes (AOPs) have been developed (Akbal and Nuronar, 2003; Han et al., 2004). One of these technologies is photolysis. This method is based on supplying energy to chemical compounds as radiation which is absorbed by reactant molecules that can pass to excited states and have sufficient time to promote reactions (Esplugas et al., 2002). Direct photolysis has always been considered as one possible alternative because it is possible for the molecules of most organic compounds to transform, cleave bonds and even undergo complete destruction in the presence of UV eradiation (Bolton et al., 1994). Activated carbon is the mostly-used adsorbent; nevertheless, it is relatively expensive among other sorbents and its use depends on the degree of the required treatment process and the local availability of activated carbon (Bailey et al., 1999). Biosorption is the uptake of heavy metal ions and radionuclides from aqueous environments by biological materials, such as algae, bacteria, yeast, fungi, plant leaves and root tissues, which can be used as biosorbents for detoxification and recovery of toxic or valuable metals from industrial discharges (Davis et al., 2003; Figueira et al., 2000; Ma et al., 2003; Veglio and Beolchini, 1997). It has many advantages including low capital and operating costs, selective removal of metals, biosorbent regeneration and metal recovery potentiality, rapid kinetics of adsorption and desorption and no sludge generation. Biosorption technology has been shown to be a feasible alternative for removing heavy metals from wastewater (Benguella and Benaissa, 2002; Ma et al., 2003; Volesky, 2001). Certain waste materials from industrial or agricultural operations may be potential alternative biosorbents (Baylor et al., 1999; Mahvi et al., 2005). It has been reported that wood wastes such as sawdust, barks and tree leaves effectively adsorb cadmium species from aqueous systems (Kumar and Dara, 1982). The binding mechanisms of heavy metals by biosorption could be explained by the physical and chemical interactions between cell wall ligands and adsorbents by ion exchange, complexation, coordination, chelation, physical adsorption and micro-precipitation (Nouri et al., 2001). The diffusion of the metal from the bulk solution to active sites of biosorbents predominantly occurs by passive transport mechanisms and various functional groups such as carboxyl, hydroxyl, amino and phosphate existing on the cell wall of biosorbents which can bind the heavy metals. Tree leaves from agricultural operations have generally little or no economic value (Veglio and Beolchini, 1997; Volesky, 2001). Cost is an important parameter for comparing the sorbent materials (Bailey et al., 1999). By-products of soybean and cottonseed hulls, rice straw and sugarcane bagasse were evaluated as metal ion adsorbents in aqueous solutions (Marshall and Champagne, 1995; Marshall et al., 1993). Activated carbon prepared from rice husk, ground nut husk, fertilizer waste slurry, peanut hull, jute stick, moringa olefera seed husk, coconut husk and sawdust have been used for wastewater treatment and the potential of their ultimate usage may be determined by their adsorption capacity, regeneration characteristics and physical properties of subsequent products. In recent years, adsorption has emerged as a cost-effective and efficient alternative for removing heavy metals from lowstrength wastewaters(Manju and Anirudhan, 1997; Raji and Anirudhan, 1997; Warhust et al., 1997). Rice husk are an agricultural waste produced in excess of 100 million tons as a by-product of the rice milling industry of which 96% is generated in developing countries. The utilization of this source of biomass would solve some disposal problem as well as access to cheaper materials for adsorption in water pollutants control system (Williams and Nugranad, 2000). Since the main components of rice husk are carbon and silica (15-22% SiO₂ in hydrated amorphous form like silica gel), it has the potential to be used as an adsorbent (Khalid et al., 2000; Nakbanpote et al., 2000). When rice husk is burnt, about 20 wt% of the husk remains as ash. The rice husk ash has more than 95 wt% of silica with high porosity and large surface area, because it retains the skeleton of cellular structure. These properties of the rice husk ash could be used to synthesize siliceous rawmaterialssuch as claymaterials (Aksu and Yener, 2001).

Application of biosorbent in heavy metals removal

Cadmium is introduced into the bodies of water from smelting, metal plating, cadmium-nickel batteries, phosphate fertilizer, mining, pigments, stabilizers, alloy industries and sewage sludge. The harmful effects of cadmium include a number of acute and chronic disorders, such as “itai-itai” disease, renal damage, emphysema, hypertension, and testicular atrophy (Leyva-Ramos et al., 1997). There are two major sources of chromium contamination, wastewater metal finishing industries (hexavalent chromium) and tanneries (trivalent chromium). Chromium occurs most frequently as Cr (VI) or Cr (III) in aqueous solutions. Both valences of chromium are potentially harmful but hexavalent chromium has a grater risk due to its carcinogenic properties (Dakikiy et al., 2002). Hexavalent chromium, which is primarily present in form of chromate (CrO₄^2-) and dichromate (Cr₂O₇^2-), poses significantly higher levels of toxicity in comparision with other valence states (Sharma and Forester, 1995). Toxicity of hexavalent chromium, even in small concentrations, has been well documented (Masakazu, 2003). Since the addition of chromium ions through industrial waste effluents into natural bodies of water causes serious environmental disruption, strict wastewater standards have been setup in many countries. In Japan, the standard on wastewater quality states that the maximum level permitted in wastewater is 2 mg/dm³ for total Cr and 0.05mg/dm³ for Cr (VI) (Masakazu, 2003).Activated carbon from cheap and readily available sources such as coal, coke, peat wood, rice husk and tree leaves may be successfully employed for the removal of cadmium and other toxic heavy metals from aqueous solution (Elliott and Denneny, 1982; Dadhich et al., 2004; Khalid et al., 2000). Other adsorbents such as a wood charcoal, red mud, sun flower stalks, petioles felt-sheath and rice husk have also been used for the adsorption of cadmium (Lopez et al., 1995; Marzal et al., 1996; Namasivayam and Ranganthan, 1995). The feasibility of Platanus orientalis leaf and its ash, as adsorbent for removing cadmium from aqueous solution, was investigated by Mahvi et al. (2007b). The results show that adsorption of cadmium increased with increasing contact time and became almost constant after 60 min for POL and 60 min for its ash (Fig. 1). These results also indicate that the sorption process could be considered very fast because of the largest amount of Cd attached to sorbent within the first 60 min of adsorption. Experiments, concerning the effect of pH on the sorption, were carried out with the range of pH that was not influenced by the metal precipitation as metal hydroxide. The suitable pH ranges for cadmium was performed for the pH range variations of 3 to 9. Adsorption of cadmium on POL and its ash increased with increasing initial concentration of Cd. These results may be explained by an increase in the number of metal ions competing for the available binding sites in the adsorbent for complexation of Cd ion at higher concentration levels (Mahvi et al., 2007b). The percentage adsorption of cadmium on rice husk and its ash increased as the pH of the solution increased and reacheda maximum value at pH9 (Mahvi et al., 2005b). Adsorption of metal cation on adsorbent depends upon the nature of adsorbent surface and species distribution of the metal cation. Surface distribution mainly depends on pH of the system (Namasivayam and Ranganthan, 1995). Percent adsorption of metal ion decreased with the decrease in pH, because protons compete with metal ion for sorption sites on the adsorbent surface as well as the concomitant decrease of negative charge of the same surface (Sun and Shi, 1998). It has been reported that precipitation of cadmium starts at pH 83 (Ajmal et al., 2003; Sun and Shi,1998). The resultsshowthat adsorption of cadmium increased with increase in contact time and became almost constant after 45 min for rice husk and 30 min for its ash (Mahvi et al., 2005b). These results also indicate that the sorption process can be considered very fast because of the largest amount of cadmium attached to the sorbent within the first 30 min of adsorption.Similar results have been reported byAjmal et al. (2003) and Namasivayam and Panganathan (1995). The percentage adsorption increased from 95 to 97.8% for rice husk and from 96 to 99.4% for rice husk ash when adsorbents doses were increased from 0.5 to 10 g/L for both sorbents, but at the same time, adsorption density decreased from 38.02 to 1.95 mg/g for rice husk and from 38.4 to 1.99 mg/g for its ash (Mahvi et al., 2005b). Similar results were reported by other researchers (Cheung et al., 2001; Namasivayam and Ranganthan, 1995; Peternele et al., 1999). The efficiency of tea waste has been determined in processing heavy metal removal from both single metal solutions and various mixtures. Metals of interest were cadmium and Lead. They were chosen based on their industrial applications and potential pollution impact on the environment. Cadmium is a inessential and useless element to plants and animals. Lead is a hazardous waste and is highly toxic to humans, plants and animals (Low et al., 2000). In addition, Ni is also one of the important toxic metals to humans, plants and animals (Ajmal et al., 1998a). Biosorption of lead (II) and cadmium (II) from aqueous environments by brown algae Sargassum spp. biomass was studied by Nabizadeh et al. (2005). Fig. 2 shows the effect of pH on equilibrium uptake capacities of Sargassum spp. biomass for Pb²⁺ and Cd²⁺. The effect of Na⁺, K⁺, Mg²⁺ and Ca²⁺ on equilibrium capacities of Pb²⁺and Cd²⁺ biosorption by Sargassum spp. biomass is shown in Fig. 3 (Nabizadeh et al., 2005). Biosorption of lead (II)and cadmium (II) from aqueous solutions using various biomasses has been studied. Cadmium (II) biosorption on Asppergillus oryzae reached equilibrium in 1 h with 90% biosorption taking place in the initial 10 min (Kiff and Little, 1986). Kinetic data of cadmium (II) biosorption by chitin presented high correlation with the pseudo-second order rate equation (Benguella and Benaissa, 2002). Matheickal and Yu (1999) observed that the maximum uptake capacities of Durvillaea potatorum and Ecklonia radiata for Pb²⁺ were 1.6 and 1.3 mmol/g, respectively.

Scott and Karanjkar studied cadmium (up to 25 ppm) adsorption on to biofilm covered granular activated carbon (Scott and Karanjkar, 1995; Scott and Karanjkar, 1995). There has been no study on the removal of low concentration (less than 5 ppm) of cadmium by biofilm/ GAC. The underlying objective behind using GAC as a support for biofilm was to provide the foundation for remediation processes that could provide metal biosorption concurrently with removing non-metal contaminants such as organic compounds. The objective of this study was to investigate the adsorption characteristics of cadmium (less than 5 mg/ L) on to plain (non-biofilm) GAC, biofilm and biofilm/ GAC and to determine the effects of temperature and pH on the cadmium uptake isotherms by plain GAC and biofilm/GAC. The results of this study shows that adsorption coefficient (Kad) for BAC is 2 to 3 times greater than those with plain GAC and the bed volumes of water containing 0.5 mg/L Cd²⁺ treated at breakthrough for GAC, biofilm and BAC columns were 45, 85 and 180 BV, respectively. Also it was found that BAC is more efficient than GAC in the removing of Cd from water environment (Danati-Tilaki et al., 2004). Biosorption of Pb²⁺ and Cd²⁺ by protonated Sargassum glaucescens biomass in a continuous packed bed column was investigated by Naddafi et al. (2007). Mahvi et al. (2007c) performed an investigation on the efficiency of Platanus orientalis leaves (POL) and their ashes on removing chromium from diluted aqueous solutions. Adsorption of chromium increased with increase in contact time and became almost constant after 120 min for Platanus orientalis leaves and their ashes. These results also indicate that sorption process can be considered very fast because of the largest amount of chromium attached to sorbent within the first 60 min of adsorption. Adsorption of chromium on POL and its ash increased with increase in initial concentration of Cr(VI) to reach 20 mg/L. Light metals can disturb adsorption of heavy metals on any adsorbent. Ca⁺²and Mg⁺²had more effect than Na⁺ and K⁺ in decreasing qe for Cr(VI) adsorption (Mahvi et al., 2007c). Fig. 4 shows the effect of contact time on the removal of chromium(VI) byplant Ulmus (UL)and its ash. Experiments concerning the effect of pH on the sorption were carried out with the range of pH that was not influenced by the metal precipitation as metal hydroxide. The effect of initial metal ion concentration on the adsorption capacity of UL and its ash was studied under optimum conditions (pH = 6, Temp. = 2425 °C) (Gholami et al., 2006). It has been reported that precipitation of chromium starts at pH= 6.5 (Ajmal et al., 2003).

Feasibility of rice husk and its ash as an adsorbent for the removal of cadmium from aqueous solution was investigated by Mahvi et al. (2005a). The results of this study indicate that the adsorption efficiency is maximum for Pb and minimum for Cd. Fig. 5 represents the adsorption efficiency for various concentrations of lead. As it is obvious, tea waste is a wonderful adsorbent for removing lead from wastewater. Increase in adsorption capacity of the adsorbent seems to be a result of increase in total surface area of the adsorption sites. Thereby, it would increase adsorption by grinding the adsorbent. For example, 94% removal of lead from a 5 mg/L solution was possible by applying 0.5g tea waste whereas the similar amount of adsorbent was not enough to treat a 100 mg/L lead solution to more than about 76%. However, by increasing the amount of tea waste to 1.5 g, it was possible to increase the efficiency of adsorption to about 96.5% for the same n (100 mg/L Pb). Thus, there would be a better treatment through using excess tea waste (Mahvi et al., 2005a). As this adsorbent is cheap and available, there would be no problem to increase its consumption (Ajmal et al., 1998b).

By comparing the results of the study, it can be concluded that the adsorption efficiency is also dependent upon the type of metal, as for Ni, not more than 76% removal is achieved in the same conditions (0.5 g adsorbent in solutions of 5, 10, 15, 30 and 100 mg/L), but for Pb and Cd the efficiencies are reported to be 94 and 60%, respectively (Fig. 6 and 7). As mentioned before, the maximum and minimum removal efficiencies in the first stage of experiments with 0.5g adsorbent were 94 and 76.3% for Lead, 76 and 14.8% for Ni and 60 and 24.8% for Cd. But for the mixture of these metals, a 3.5% decrease has been observed for lead adsorption whereas Ni adsorption has decreased in most concentrations (12.2%) (Mahvi et al., 2005a). Some bacteria like the heavy metal resistant alcaligenes eutrophos CH34 strains are able to promote biomineralization, being the biologically induced crystallization of heavy metals. In the presence of heavy metals, this strain may create an alkaline environment in the periplasmic space and outer cell environment appropriate induction of heavy metals resistance mechanisms. In such an environment, metal hydroxides are formed together with metal bicarbonates resulting from the carbonates production by the cell. Also, metals bind to outside cell membrane proteins and the metal hydroxides and bicarbonates precipitate around these nucleation foci inducing further metal crystallization. Mahvi et al. (2004c) studied on biological removal of cadmium by alcaligenes eutrophus CH34. The result of this study is presented in Fig. 8. In these studies, it was shown that electrocoagulation process achieves a fast and effective reduction of cadmium (more than 99%) present in industrial effluents (Bazrafshan et al., 2006; Mahvi et al., 2007a).

Phenol removal

Phenols, as a class of organics, are similar in structure to the more common herbicides and insecticides in that they are resistant to biodegration. Phenol is very soluble in water. The odor threshold for phenol is 0.04 ppm (US EPA). Their presence in water supplies are noticed as bad taste and odor problems (Mostafa et al., 1989). In the presence of chlorine in drinking water, phenols form chlorophenol, which has a medicinal taste that is quite pronounced and objectionable (Rengaraj et al., 2002; Street et al., 1995). Phenols are considered as priority pollutants, since they are harmful to organisms at low concentrations and many of them have been classified as hazardous pollutants due to their potential harm to human health. Stringent US Environmental Protection Agency (EPA) regulation call for lowering phenol content in the wastewater to less than 1 mg/L (Banat et al., 2000). There are many methods such as oxidation, precipitation, ion change, solvent extraction and adsorption for removing phenols and its derivatives from aqueous solution (Aksu and Yener, 2001; Rengaraj et al., 2002). Because of the high cost and variable performance of carbon regeneration, single use materials are desirable (Kummar et al., 1987; Rengaraj et al., 2002). This has led many workers to look for more economical, practical and efficient techniques. Bottom ash, brick-kilm ash, fly ash, peat, soil, rice husk, wood, sawdust, bagasse and carbonized bark are some new adsorbent used for organic pollutants (Aksu and Yener, 2001; Edgehill et al., 1998; Kummar et al., 1987; Rengaraj et al., 2002; Srivastava et al., 1997; Street et al., 1995). The possibility of using rice husk and rice husk ash for removing phenol from aqueous solution was investigated by Mahvi et al. (2004a). The adsorption of phenol from aqueous solution is dependent on the pH of the solution, which affects the surface charge of the adsorbent, degree of ionization and speciation of the adsorbate species (Mahvi et al., 2004a). The adsorption of phenol by rice husk and its ash were studied at various pH values (Aksu and Yener, 2001; Caturla et al., 1998; Halouli and Drawish, 1995; Kummar et al., 1987; Street et al., 1995). The results are displayed in Fig. 9. As was expected, the adsorbed amount decreases with increasing the pH value. This can be attributed to the dependency of phenol ionization on the pH value (Banat et al., 2000). The results show that equilibrium time required for the adsorption of phenol on rice husk and its ash are almost 6 h and 3 h, respectively. These results also indicate that the sorption process can be considered very fast because of the largest amount of phenol attached to the sorbent within the first 120 min of adsorption. This indicates that rice husk ash would require less residence time for the complete removal of phenol compared to rice husk (Mahvi et al., 2004a). Samarghandi et al. (2006) investigated on phenol, lead and cadmium by means of UV/TIO₂/H₂O₂ processes. The results of this study are shown in Table 1. These results show that the removal of pollutants such as cadmium and phenol is dependent upon pH. As pH increases from 3.5 to 11, the efficiency removal increases from 26 to 94.5 and from 51 to 76%, respectively. This study indicates that the removal of lead is independent of pH.

CONCLUSION

The analyses of the results indicate that tea waste like most other natural absorbents can be used in the treatment process of heavy metals and the treatment efficiency may be as high as 100% by choosing the adsorbent amount precisely. The concentration of heavy metal has also an important effect on the treatment outcome. Tea waste is a cheep material and thus it would be convenient to use it in industrial wastewater treatment plants. Meanwhile, it is possible to increase the treatment efficiency by pretreatment with some chemicals such as acids, bases and detergents (Ajmal et al., 1998a). In case of platanus orientalis leaves, about 99% removal efficiency was observed within 3 hrs of contact time by adsorbent which was carbonized. Also, the removal efficiency of chromium (VI) by Ulmus leaves has been found to be greater than 85%. The kinetic of chromium (III) biosorption by Sargassum spp. biomass is fast, reaching 60% of the total uptake capacity within 10 min (Cossich et al., 2002). The biosorption of lead (II) by Durvillaea potatorum was rather rapid and 90% of the total uptake occurred within 30 min (Matheickal and Yu, 1999). Sargassum spp. can be classified as an efficient biosorbent because of rapid kinetic, remarkable biosorption capacity and selective removal of metals. Thus, the biosorbent has a high potential to be applied in full-scale for removing heavy metals from aqueous environments. Rice husk and rice husk ash adsorption capacity strongly depends on the pH of the solution. The sorption capacity is decreased with increase in pH and initial phenol concentration. Rice husk ash has higher adsorption capacity for phenol. However, the experimental results indicate that rice husk’s ability to adsorb phenol and consequently its possible utilization in the treatment of phenol-contaminated solution and its adsorptive capacity is limited.

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