Synthesis Of Activated Clay/starch/fe3o4 Nanocomposite For The Elimination Of Anionic Dye (reactive Red 198) From Aqueous Media

1.1 Background of the study

Dye-laden wastewater discharge into the environment has created severe environmental problems due to the persistent nature of dyes that signi?cantly affects water bodies. Dyes of synthetic origin have complex structures and most of them are non-biodegradable (Carmen & Daniel, 2012). Environmental pollution from wastewater containing synthetic-origin dyes, as well as its effect on the ecosystem, has been increasing and has become a serious concern for the general public at all levels (McKay, Porter & Prasad, 2019); Quan, Luo, Wu, Li, Cheng & Ge, 2017). In the industrial sectors such as food, textiles, leather, pharmaceutical, pulp, and paper, thousands of synthetic dyes are being used and discharged into the environment as wastewater. Among the different dye-consuming industries, the textile industry is the major industry that consumes a huge quantity of dyes and different hazardous chemicals that are discharged into the environment and water bodies as wastewater (Dizge, Aydiner, Demirbas, Kobya & Kara, 2018).

Of the different types of dyes used in the textile industry, reactive dyes such as RR198 are widely used due to their advantageous properties such as simple application techniques, bright colour, and low energy consumption (Dizge, Aydiner, Demirbas, Kobya & Kara, 2018). In their research ?ndings, (Carmen & Daniel, 2012) have reported that the ?xation rates of reactive dyes on the surface of materials range from 50 to 90%, while the remaining reactive dyes are discharged into the environment as effluent waste- water (Shoukat, Khan & Jamal, 2019) have investigated that a dye-laden water body has devastating effects on the photosynthesis of aquatic plants as well as aesthetic value due to the blocking of sunlight. The investigation conducted by (Dizge, Aydiner, Demirbas, Kobya & Kara, 2018) has also demonstrated that dye-containing wastewater has complex molecular structures and high solubility. Due to the synthetic origin of reactive dyes, conventional and biological treatment methods are not effective at meeting the required treatment levels. The mitigation of such hazardous dyes from wastewater has become a matter of urgent importance because of the increasingly alarming environmental threat posed by them, thus attracting the attention of researchers.

On the other hand, Mother Nature expresses itself in different spectrums of colours all around us, such that the world today would be unimaginable without colours. Industries such as textiles, leather, paper-making, plastics, food, rubber, and cosmetics use different types of dyes, which also appear in the effluents discharged from some of these industries (Ngulube et al., 2018). Generally, dyes are stable to light, heat and oxidizing agents and are usually non-biodegradable (Kono, 2015). The presence of dyes in water bodies cannot be easily ignored because they impart colour to water bodies (Kant, 2012). The presence of colour in water bodies affects aquatic diversity by blocking the passage of sunlight which is needed for photosynthesis. This colour in water bodies also has an adverse aesthetic effect. Furthermore, dyes are known to have toxic effects to some organisms due to the presence of aromatics and metals in their structures (Yagub et al., 2014). A major concern in wastewater treatment is the release of dyes and their metabolites into the environment as some may be mutagens and carcinogens. Some of these dyes are considered to be xenobiotic in nature and aerobically recalcitrant to biodegradation (De Jagger, 2013), and thus pose a threat to the environment when wastewater is disposed off, untreated. In that regard, there is a need to find treatment technologies that can decolourise the water and at the same time reduce the toxic effects of the dyes to within permissible limits as stipulated by water quality guidelines.

Being among the most demanding environmental tasks of the modern day, the increasing amount of toxic industrial waste has led to the development of various decontamination methods and techniques (Chollom, 2014). Usually, industrial wastewater is treated by various methods like adsorption, precipitation, chemical degradation, advanced oxidation processes, biodegradation and chemical coagulation (Kobya et al., 2007; Mohan et al., 2007; Du et al., 2010). Although these methods have been widely applied, they have some shortcomings (Pajootan et al., 2012). For example, biological methods are time consuming and are often ineffective in removing dyes which are highly structured polymers with low biodegradability and cannot be applied to most textile wastewaters due to the toxicity of most commercial dyes to the organisms used (Buthelezi, 2012). Chemical coagulation causes extra pollution due to the undesired reactions in treated water and produces large amounts of sludge (Kobya et al., 2007). Chemical degradation by oxidative agents such as chlorine is the most effective method, but it produces some very toxic products such as organochlorine compounds (Merzouk et al., 2011). Advanced oxidation processes such as ozonation, ultra violet and ozone– UV combined oxidation, photocatalysis, fenton reactive and ultrasonic oxidation are not economically feasible (Daneshvar et al., 2006). Furthermore, these methods are also usually expensive and treatment efficiency is inadequate because of the large variability of the composition of textile wastewaters (Drouiche et al., 2011).

Adsorption however seems to be one of the most preferable techniques used for water treatment because of low initial costs, simple design and ease of use and implementation even in small plants. Activated carbon adsorption has been widely used for industrial wastewater treatment but it has the disadvantage of high costs and difficultyof the regeneration process and a high waste disposal cost (Yagub et al., 2014). Because of very stringent laws regarding elimination of dyes from wastewaters before their discharge into water bodies and expensive conventional sorbents, there is an increasing tendency for the development of new, adequate, low-cost adsorbents with satisfactory adsorption properties and inexpensive to regenerate.

Various naturally occurring materials that are available and abundant have been explored as adsorbents for the removal of dyes from wastewater (Nandi et al., 2009; Rafatullah et al., 2010; Tehrani-Bagha et al., 2011; Ali et al, 2012; Ghosh and Reddy, 2013). A great interest has grown towards naturally occurring adsorbents, preferably the development of an adsorbent demonstrating both a high adsorption capacity and low cost for removing contaminants from polluted waters. Although a wide variety of adsorbents have been used for wastewater treatment, naturally available clays have been the adsorbents of choice in most developing countries (Weng and Pan, 2007; Nandi et al., 2009; Ali et al., 2012) It is extremely important to choose a technique that is cheap, requires only minor workloads and at the same time it should not be complicated. Obviously, the maintenance and repair should be easy and affordable. Using clays and earth minerals for industrial wastewater treatment fits well with all the above-mentioned parameters. 

Many modification and synthetic methods exist for preparing nanocomposites but only a few are ‘green’ and less costly. Methods like pillaring and intercalation (Gupta and Bhattacharyya, 2006; 2008), acid activation (Gupta and Bhattacharyya, 2011) and mechanochemical activation (Vdovi? et al., 2010) can be applied to enhance the adsorption capacity of clays. Amongst these, mechanochemical activation has proved to be preferable due its low cost and environmentally friendly properties. Several researchers studied the effect of mechanochemical activation on structural and morphological variations of clays (Lee et al., 2007; Ramadan et al., 2010; Vdovi? et al., 2010), but only a few studies explored the use of milling on their adsorption properties (Nenadovi? et al., 2009; Kumri? et al., 2013). Destruction, alteration, fragmentation of the crystalline network and reduction of particle size together with increased surface area and amorphization, can lead to enhanced removal of contaminants by modified clays (Makó et al., 2001; Hrachová et al., 2007). To overcome the restrictions and weaknesses of using raw and chemically modified clays as adsorbents, composite adsorbents can be prepared by way of grinding and/or milling. This is a novel study to evaluate the feasibility of applying mechanochemically synthesised clay-based nanocomposite for the decolouration of aqueous dye solutions.

Numerous studies have looked at the successful synthesis of adsorbent materials and their application in water treatment but there exist few studies that proceed to investigate what happens after the adsorbent material has been exhausted and can no longer be used for adsorption process. The available options currently used for managing post-sorbents are restricted to recovering the adsorbate, disposing of the adsorbent in a landfill and safe storage (Reddy et al., 2016). It should be highlighted that adsorption only transfers contaminants from one phase to the next and consequently generating sludge which must be regenerated, disposed off or managed by some other processes (Reddy et al., 2016a) which is a limitation as a method of removing dyes from aqueous solution. During the adsorption process, the physical structure of the adsorbent does not get altered much. When the adsorbent material is to be regenerated, the fate of the resultant concentrated sludge of dyes presents a problem of correct disposal (Guilane et al., 2016). Many adsorbent materials can be disposed off in landfills, however, this must be done in compliance with standard disposal regulations (McKay, 1995). Disposal of the adsorbent material is subject to certain requirements including using a landfill that demonstrates safe immobilisation of the absorbed liquid so as not to allow free liquid to leak into waste repositories (Feng et al., 2013). Apart from regenerating and disposing of spent adsorbents there are value added applications of spent adsorbents. It is essential to find sustainable ways of effectively using contaminant loaded adsorbents as value-added products to solve economic, management and toxicity issues associated with the spent adsorbent materials (Dodson et al., 2015). The combination of activated clay, starch, and Fe3O4 in a composite material provides a synergistic approach with enhanced functionalities. These composite materials can be utilized in various fields, including water treatment, drug delivery, environmental remediation, and more, offering tailored solutions for specific applications (Fayazi, Afzali, Taher, Mostafavi and Gupta, 2015).

1.2 Problem statement

Dye wastewater is complex and consists of concentrated waste process water which contains a wide and varied range of dyes and their metabolites (Shoukat et al., 2017). This wastewater contains non-biodegradable constituents and the impartation of colour to water bodies affects aquatic diversity by blocking the passage of sunlight. Due to the complexity of dye wastewater, some industrial companies do not meet environmental discharge standards, causing detrimental problems if the wastewater is discharged into the sewage system and eventually into the aquatic environment (Ngulube et al., 2017). Unless the wastewater is properly treated before it is discharged, it will contaminate the environment.

Three processes are used for the treatment of textile wastewater: biological, physical and chemical methods (Akram et al., 2017; Nouren et al., 2017). However, this manner of treating textile waste water is not sufficient. Physical treatment methods and physico-chemical methods such as flocculation are limited due to the high electrolytic strength of the dye liquid (Kausar et al., 2018). Chemical methods such as ozonation are also used, however, degradation products which result from ozonation and chlorination increase in the recycled liquid and act as colourless dyes which obstruct the dyeing process (Iqbal and Bhatti, 2015). Some biological treatment methods are toxic to the microorganisms used and adsorption using activated carbon seems a very viable method; colour removal is limited because only a single dye can be treated in a run (Chollom, 2014).

Moreover, the cost of treating these effluents is on the increase. Consequently, most textile industries have their own effluent treatment plants where most of the waste coming from various processes is combined and treated prior to its discharge to the environment. Biological and chemical methods are usually applied for end-of-pipe treatment (Pirkarami et al., 2013). The effluents from different processes are collected and treated together. The mixture of these streams makes the characteristics of the effluent complex. Therefore, the water quality obtained from these treatments is not satisfactorily high because most of the compounds cannot be easily degraded (Mart?nez-Huitle and Brillas 2009). The reclaimed water still contains colour. In addition, these processes are unable to decrease salinity and most of them are expensive and require expertise in the operation of the equipment employed (Daneshvar et al., 2006; Drouiche et al., 2011). The conventional methods are therefore limited because the quality of water produced does not meet the water quality guidelines.

It is, therefore, imperative to use cheap treatment methods that can treat the effluent to water for reuse in order to save on the costs incurred by treating the effluents and disposing into rivers and reduce fresh water consumption. This can be done using the adsorption treatment option in the dyeing process such that the most polluted streams are treated at the point of collection. Adsorption using clay-based materials is relatively cheap because it uses materials that are naturally available in abundance, the materials do not need pre-chemical treatment before use, and they are not toxic to the environment hence are very important products in the green nanoscale technology of wastewater treatment.

1.3  Aim and objectives

The general objective of this study is to investigate the synthesis of activated clay/starch/fe3o4 nanocomposite for the elimination of anionic dye (reactive red 198) from aqueous media. Specific objectives are:

I. To synthesize a nanocomposite adsorbent using activated clay, fe3O4 and starch nanoclay using a vibratory ball miller.

II. To identify the physicochemical characteristics of activated clay, fe3O4 and starch nanoclay and their corresponding nanocomposite using different characterization techniques.

III. To optimize conditions that are suitable for the elimination of selected anionic dyes from aqueous media via a batch study using activated clay, fe3O4 and starch nanoclay composite and establish the mechanisms governing the elimination of those dyes using different mathematical adsorption models.

IV. To optimize conditions that are suitable for the elimination of a selected dye from aqueous media via a column study using activated clay, fe3O4 and starch nanoclay and their nanocomposite thereof.

V. To determine the regeneration potential of the activated clay, fe3O4 and starch nanoclay and their nanocomposite thereof.

1.4Hypothesis

A nanocomposite prepared from activated clay, fe3O4 and starch nanoclay using a vibratory ball milling technique can successfully eliminate selected anionic dyes from aqueous media.

1.5 Significance of study

Many investigations have been done on the remediation of dye wastewaters (Zhou et al., 2014; Han et al., 2016; Yang et al., 2018). However, environmentally friendly and low-cost techniques that can be used by small scale industries are hardly available. Conventional advanced wastewater treatment technologies such as ozonation, membrane separation and electrolysis are costly and hence the driving force is to search for creative, low cost and environmentally sound ways of treating wastewater as environmental laws are becoming stringent (Yagub et al., 2014; Ngulube et al., 2017). Recently, the development of green nanotechnology in wastewater treatment is growing very fast (Karishma and Mehali, 2015). Green nanotechnology aims at developing environment safe and less harmful nano products from nanocomposites and nano powders.

Most conventional methods require high capital investment and are high in operational costs and create sludge disposal problems (Rafatullah et al., 2010). Although the adsorption technique is relatively cheap, there is a pressing need to replace commercial adsorbents like activated carbon, zeolite, activated alumina and silica gel with the low-cost adsorbents. The method of adsorption using commercial activated carbon is costly, especially for developing countries such as South Africa. Thus, ready to use adsorbent materials are desired considering the difficulties faced during commercial activated carbon regeneration and the disposal problems posed by regeneration solutions.

This calls for research efforts to develop an industrially feasible, cost effective and environmentally compatible adsorbent for wastewater treatment. Therefore, the viable option is to look for locally available and cost-effective adsorbents. Therefore, it is of utmost importance that naturally occurring materials be assessed for their potential in dye wastewater treatment hence, this study seeks to investigate the dye removal efficiency of a modified clay-based nanocomposite.

A greater motivation is the ability of the adsorbent to be used again after adsorption processes. The major drawback of adsorption is that it is a nondestructive technology where the materials do not degrade or disappear after use hence, they need safe disposal. However, post sorbent materials from naturally occurring earth materials like clays can be utilized in various value-added materials like sintered grains in ramming mixes, gunning mixes and bricks, insulator for high temperatures and as desiccants.

This work will provide textile and clothing industries which not only utilise large volumes of water, but also produce large quantities of wastewater containing dyes with the following options: 1) meeting the water quality discharge standards and reducing their discharge costs; 2) re-using the treated wastewater to reduce their carbon footprint by utilising less water from the municipality, thus reducing their impact on the environment; and 3) decreasing their annual expenditure on water, since the treated water would be available for re-use.

1.6 Contribution to the body of knowledge

Much work on the use of various adsorbents for wastewater treatment has been reported by different researchers. This research is novel in the following aspects: At the inception of this research project, to date, to the best of the author’s knowledge, there were no reported investigations of the ability of activated clay, fe3O4 and starch nanoclay nanocomposite to decolourize wastewater containing synthetic dyes. Magnetite based materials and nanoclay have been used separately to eliminate dyes from aqueous media but the synergistic effect of combining the two materials and assessing the performance on the removal of dyes has not yet been explored. This study will produce an optimised procedure for synthesising the nanocomposite as well as a procedure for decolourising water resources using a mechanochemically engineered nanocomposite. Although numerous dye elimination studies exist, many researchers concentrate on the batch adsorption mode using different adsorbents. While adsorption capacity parameters obtained from batch experiments are useful in providing information about the effectiveness of the adsorbate - adsorbent system, the data is not applicable to most industrial treatment systems. Adsorption on continuous flow fixed bed columns is preferred since it can be up scaled from a laboratory set up to a field set up. Hence, there is a definite need to perform both batch and continuous flow experiments in a fixed bed. Many studies have been done on the removal of dyes using clay-based adsorbents under different experimental conditions. However, most of the studies focus on an individual dye and only focus on synthetic dye solutions. In this study, the adsorbents were tested for the removal of four different dyes (two cationic and two anionic). This provided an opportunity to deliver a comparative analysis on how the materials work with differently charged dyes under the same experimental conditions. Moreover, attempts have not been made to use clay-based adsorbents on real effluents, and to fill that gap, the performance of our adsorbent materials on real industrial effluent was also tested. Most of the research into these adsorbent materials has focused on improved performance and increased removal rates, whereas less emphasis has been placed on the storage, disposal and reuse of these adsorbents once they are spent. To fill that gap, this work discussed the use of post-adsorbents, which are adsorbents with bound materials applied to meet environmental, energy, agricultural and dietary needs. The key contributions to knowledge of this work is the synergistic interaction between activated clay, fe304 and starch nanoclay which led to the formation of a novel adsorbent with distinct structures possessing different adsorbent capabilities that gave rise to excellent elimination of RR198 dye.