The role of sawdust in the removal of unwanted materials from water
Introduction
Environmental pollution and its abatement have drawn keen attention for a long time. The problem of removing pollutants from water and waste water has grown with rapid industrialization. Heavy metals, dyes, oil and other salts, which are toxic to many living life and organisms, are present in the waste water streams of many industrial processes, such as dyeing, printing, mining and metallurgical engineering, electroplating, nuclear power operations, semiconductor, aerospace, battery manufacturing processes, etc. [1], [2], [3], [4]. All of them have faced increasing pressure regarding environmental and waste-related concerns as a result of the quantity and toxicity of generated waste waters. The waste generated in large volumes is high in pollutant load and must be cleaned before it is released. Many methods have been used to remove the dye component, oil pollutants, and heavy metals as well from the textile effluent, namely, membrane filtration [5], coagulation [6], [7], adsorption [8], [9], [10], oxidation [4], ion exchange [10], [11], [12], [13], [14], [15], [16], precipitation [6], [15], [17], [18], etc. have been reported in the literature, but few of them were accepted due to cost, low efficiency, inapplicability to a wide variety of pollutants.
The most widely used method for removing pollutants is coagulation and precipitation [6], [7], [15], [17], [18]. Heavy metals, for example, can be precipitated as insoluble hydroxide at high pH [19], [20], [21] or sometimes as sulfides [22], [23]. A major problem with this type of treatment is the disposal of the precipitated waste. Another weak point is that in most time the precipitation itself cannot reduce the contaminant far enough to meet current water-quality standards. Ion exchange treatment is the second most widely used method for metal removal. This method does not present a sludge disposal problem and has the advantage of reclamation of metals. It can reduce the metal ion concentration to a very low level. However, ion exchange does not appear to be practicable to waste water treatment from a cost stand point [24]. Adsorption with activated carbon can also be highly efficient for the removal of numerous trace elements from water, but the high cost of activated carbon inhibits its large-scale use as adsorbent. Then, the need for effective and economical removal of unwanted materials resulted in a research for unconventional methods and materials that might be useful in this field.
The trace elements in water are so numerous that only a combination of various treatment processes can provide the effluent quality desired and only a non-specific process such as adsorption appears appropriate for their removal [1]. Scientists have, therefore, centered their interest on adsorbents research in recent years. The utilization of agricultural waste materials is increasingly becoming of vital concern because these wastes represent unused resources and, in many cases, present serious disposal problems. During the past decade, a great deal of attention has been given to methods of converting these materials into useful products. Among those, sawdust is one of the most appealing materials for removing pollutants, such as, dyes, salts and heavy metals, etc. from water and waste water. This study presents a brief review on the role of sawdust in the removal of unwanted materials from waters. Application and comparison of different methods are discussed and a number of references are available in this article. The principal, affecting factors and our recent study of sawdust adsorption are also discussed.
Section snippets
A brief review of the adsorption of unwanted materials by sawdust
Numerous studies on adsorption properties of naturally occurring and low cost adsorbents, such as agricultural by products or natural fibers, have been documented. Namely, barley straw, tree bark, peanut skins, human hair, waste tire rubber, and moss peat [25], etc. have been reported in recent years. Studies have shown that sawdust, among the low cost adsorbents mentioned, is the most promising adsorbent for removing heavy metals, acid and basic dyes, and some other unwanted materials from
Adsorption mechanism
Based on the behavior of heavy metal adsorption on sawdust, scientists have speculated that ion exchange and hydrogen bounding may be the principal mechanism for the removal of heavy metals [30]. There are a great deal of facts to support this speculation, including the components and complexing properties of the sawdust, the properties of heavy metals and the adsorption behavior, such as the effect of pH of the aqueous media.
It has long been recognized that heavy metal cations are readily form
Adsorption isotherms
The Langmuir and Freundlich isotherms were both used to describe observed sorption phenomena of various metal ions on sawdust materials [2], [29], [30], [31], [33], [37], [47]. The Langmuir isotherm applies to adsorption on completely homogenous surfaces with negligible interaction between adsorbed molecules. For a single solute, it is given byHowever, the linear form of the equation can be written asWhere Ce is the equilibrium concentration of the solution, x/m is
Effect of pH of the aqueous media
An important influencing factor for heavy metal adsorption on sawdust has been referred to as pH in most studies published in the literatures [31], [37], [38], [44], [46], [47]. In a certain pH range, most metal adsorption increases with increasing pH upto a certain value and then decreases with further pH increasing. This is readily explained by the adsorption mechanism. Therefore, there is a favorable pH range for the adsorption of every metal on a certain sawdust material. Favorable pH
Sawdust regeneration and packed column studies
To make the adsorption process more economical and practical it is necessary to regenerate the spent sorbent and pack the sorbent in a certain container, such as packed column. Studies on sawdust regeneration and dynamic adsorption of sawdust in packed column have been reported by a number scientists [29], [35], [37], [46].
Conclusions
The role of sawdust materials in the removal of pollutants from aqueous wastes has been investigated. The investigations are quite useful in developing an appropriate technology for waste water treatment. Some valuable information or guidelines can be drawn from the review and discussions.
Sawdust material has proven to be a promising material for the removal of contaminants from waste waters. Not only is sawdust abundant, but also it is really an efficient and economic adsorbent that is
Acknowledgements
S.S.S. and J.L.M. would like to acknowledge financial support from the Welch Foundation.
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