A new biosorbent with controlled grain (I). Efficient elimination of cationic dyes from textile dyeing wastewater
Abstract
Environmental care is an increasing concern in our society, and therefore integrated, circular economy allowing to close the industrial cycle is an urging demand. This project employs a residue of the food industry to recover the wastewaters from the textile industry, allowing closing the loop in two different industrial processes. Orange peel is avery abundant residue in the food industry. By chemical modification of the orange peel, we aim to produce a biosorbent to be employed in the treatment of the textile industry wastewater containing cationic dyes. In this project, we evaluate the capacity of the treated orange peels as cationic interchanger with different dissolutions of copper (II). Finally, their capacities as biosorbents were evaluated with four cationic dyes, examining the influence of different parameters like: biosorbent concentration, contact time, temperature and pH of the medium. An adsorption between 51 and 92 % was reached with the first treatment and also the possibility of the biosorbent recovery.
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Introduction
A trend in wastewater treatment of industrial processes is to perform primary treatments in the most troublesome waters. In this first part of the study the new way to obtain a biosorbent with controlled particle size is presented and also the results of batch tests for the elimination of dyes, as well as the reuse of biosorbent that have allowed the subsequent development of continuous process of decoloration of wastewater from dyeing of textile materials.
Dyes are widely used in dyeing textiles and food, so they constitute one of the greatest challenges in the treatment of industrial wastewater due to its visual impact and increase of the organic load and toxicity (Vieira 2000). The dye molecules distinguish three functional parts: the chromophore, which is responsible forgiving the colour property to the dye molecule; the auxochrome, which provides affinity to the textile fibre and intensifies the colour, and the solubilizing groups (Zollinger2001).
These substances are persistent in wastewater and constituting a pollution problem . The dyes are found in the waste of the textile industries from their own production. Their main effect on the aquatic life is the limitation of photosynthetic activity as a result of the decrease in the light penetration and the toxicity affecting aquatic life due to the presence of aromatic and halogenated compounds and/or heavy metals(Robinson 2001).
The dyes currently used are mainly synthetic. Due to its diverse and complex reactive nature, the chemical stability of the dyes converts them in compounds difficult to treat with a general method. According to the conditions in which the dyeing process occurs, the dyes are classified into: acidic, cationic, direct, disperse, reactive, sulphur, vat and others (Aksu 2005). The textile finishing industry is an industrial sector that consumes water, energy and auxiliary chemical products; therefore, the treatment of wastewater is important. These effluents have significant concentrations of dye, organic contaminants, heavy metals, surfactants and chlorinated compounds.
The treatment of textile effluents is carried out in two stages: homogenization, and physicochemical or biological treatment. Within this scheme, it is possible to selectively treat the dyeing wastewater, discolour, and incorporate them in the overall treatment system. Each method has its own technical and economic limitations and, usually, the use of a single process is not efficient enough to ensure the colour degradation and the mineralization of the compounds formed (Supaka 2004; Buitrón 2004).
Biological processes have been considered as effective alternatives to treat coloured effluents (Van der Zee 2005;Pandey 2007) but the elevated permanence times needed of some dyes and auxiliary products are now the major constraints for their application (Rai 2005). There are many techniques used in dye removal, which include both physical and chemical processes, for example: ozonation, advanced oxidative processes, photochemical processes, membrane filtration, etc. (Robinson 2001) The lines of research to obtain new low-cost adsorbents materials have focused, primarily, to produce activated carbon. Different activated carbons have been prepared from shell Walnut (Yalcin 2000; Bello 2002) rice husks, peach stones (Abdel-Nasser 2001), and from other waste materials. However, due to the high cost of the aforementioned substances, we have also considered low cost biosorbents as, for example, agro-industrial waste without any type of treatment. Namely, rice husks, cork and orange peels have been found to yield results such as sufficient retention of dyes. In fact, the valorisation of vegetable residues such as biosorbents, is gaining increasing significance in the environmental field (Brown 2000). Adsorption is a transfer of matter that is being reintroduced as an alternative to dye removal. There are three kinds of adsorption according to the type of interaction given between the solute and the adsorbent. If the adsorption is done by anion exchange mechanism, the ions of the substance of interest are concentrated in an area of the adsorbent material as a result of the electrostatic attraction between the two; this is called electrostatic adsorption. However, if the adsorbed molecule is not fixed in a specific place of the surface, but it is rather free to move into the interface, the adsorption is done due to the Van der Waals forces and it is called physisorption. Therefore, if the adsorbate has strong links in the active sites of the adsorbent, one can say that adsorption is of chemical nature. It may be highlighted that, in the physisorption, the adsorbed species preserve its chemical nature, while during the chemisorption the adsorbed species undergo a chemical transformation, giving place to different species (Appelo 2005). The systems based on physisorption can allow the reuse of the adsorbent, probably better than the systems based on chemisorption are able to.
The main parameters are: the specific surface of biosorbent, pH, temperature, the nature of the adsorbent, the nature and concentration of the adsorbate, the contact time and even the solute ionic force (Santos 2003). In the interaction between adsorbate and adsorbent, the factors that affect the process are: the adsorbate solubility (at lower solubility, best adsorption); molecular structure of the solute (as more branched best adsorption); molecular weight (large molecules show better adsorption); polarity (lower polarity has better adsorption and degree of saturation) (Fetter 2001). The biosorption is an adsorption process that consists of the catchment of different chemical species by a biomass (living or dead), such as: algae, fungi, bacteria, shells of fruits, agricultural products and some types of biopolymers through physicochemical mechanisms as the adsorption or anionic exchange (Chojnacka 2010).
The biosorption process involves a solid phase-biomass- (sorbent or adsorbent) and a liquid phase (solvent) that contains the dissolved species (adsorbate), which is to be retained by the solid. To carryout this process affinity should exist between the adsorbent and the adsorbate, so that those are transported toward the solid, where they are retained by different mechanisms. This operation continues until a balance between the dissolved adsorbate and the adsorbent is established and bound to the solid. This process continues until a steady state of concentration is reached. The use of dead biomass has advantages compared to the use of living biomass, since it is not necessary to add nutrients to dead biomass. Additionally, the adsorbent is immune to the toxicity or to the adverse conditions of the operation so the processes are not governed by biological limitations anymore (McKay 1986).
The cellular walls of biosorbent materials contain polysaccharides, proteins and lipids, and, therefore, functional groups with capacity to bind heavy metals and cationic molecules in their surface. The main functional groups present here are the amino, carboxylic, hidroxilic, phosphate and thiol groups that differ in their affinity and specificity of joining different metal ions (Ghimire 2003).
The orange peel (Citrus sinensis) is obtained as a byproduct of orange juice manufacturing, and is eliminated as scrap. However, the orange peel and other citrus fruits have been widely used in the elimination of heavy metals and textile dyes (Annadurai 2002; Arami 2005; Pavan 2006;Pérez 2007;Popuri 2007;Hameed 2008; Li2008; Gupta 2009; Lu 2009; Arjona 2016).
The bioadsorption in orange peels is because they contain pectin in their composition. Pectin is a natural high molecular compound widely-existing in cell wall and middle lamella structure of all higher plants (Qiu, Tian,Qiao, & Deng, 2009). Pectin is usually considered as a complex polysaccharide which consists of α-1,4-linkedD-galacuronic acid, which is partly methyl esterified, and the side chain contains various neutral sugars, such as L-rhamnose,L-arabinose, and D-galactose (Mohnen, 2008; Xie, Li, &Guo, 2008). Pectin properties include gelatification, thickening and stabilization, giving it widespread use in food, medical, chemical, textile and other industrial fields (Sato et al., 2011). FIGURE 1: PECTIN A POLYMER OF α-GALACTURONIC ACID WITH A VARIABLE NUMBER OF METHYL ESTER GROUPS When the proportion of methoxy groups is low and, therefore, the proportion of COO-groups available is high, the links that are established between the molecules can be made through divalent cations (Ca 2+, Cu2 +, etc. . .). The main objective of this study is to develop and optimize the treatment of orange peel to obtain a reusable biosorbent, which will allow the removal of heavy metals and cationic dyes from wastewater.
Conclusion
The removal of heavy metals by treated orange peel is basically done through anionic exchange phenomenon between Ca (II) and the metal ions in solution until the equilibrium is reached. That way, Ca (II), linked to the polygalacturonic chains, is displaced by the metal ion.
The saponification process (desmetoxilation) and the pectins reticulation sensible to the Ca(II) ion are usually ,done in two steps. At the first step, the desmetoxilation with NaOH is done, while in the second step the reticulation by CaCl takes place. 2 In this project, both steps were simultaneously done using Ca(OH) . 2 The testing of this method has been rendered effective through a removal of Cu (II), using 0.5 grams of the biosorbent, through anion exchange mechanism, almost 100 ppm of dissolution of Cu (II). It is accepted that this removal is done mainly because of anionic exchanger process, in a way that Ca(II) is initially linked to the pectin chains and displaced by Cu (II) until reaching the equilibrium conditions. Furthermore, the biosorbent presents a high selectivity to the Cu (II) even in presence of other cations (Hang 1970).
In the light of these results, it is possible to assert that the removal of the cationic dyes used is not explained by a simple mechanism of ionic exchange, leaving the establishment of a better-adjusted mechanism according to experimental data for a later study.
The physical-chemical treatment developed in this work in the orange peel shell has achieved a final stable product (biosorbent), which can be reused several times.
Its application in the removal of cationic textile dyes has been tested yielding positive results at laboratory level. Nevertheless, the experimental results discard that, with this type of organic molecules, this biosorbent works as a cationic exchanger.
It has been demonstrated that 40 g/l of the biosorbent is effective in the removal of 30 ppm of cationic dyes, for 45 minutes of stirring, obtaining yields ranging from 52 to 92%.
It is possible to reach the total removal of the biosorbed dyes if the process is successively repeated, which suggests the developing of a new removal procedure as a continuous process.
The removal of cationic dyes has been studied in a solution with higher dyestuff concentrations (between 60 and 240 ppm) at the same conditions with higher or, at least, similar efficiency. This means that the dilution of the wastewater is not necessary to treat with this biosorbent. It has been demonstrated that it is possible to recover the biosorbent and to reuse it with a constant efficiency for at least four cycles.
