A dye is an organic chemical or aromatic, polyaromatic, or heterocyclic compound used as a colorant to change the colour of a product [1]. Wastewater which containing these dyes are often effluents generated and discharged by the paper, printing, textile, leather, plastics, rubber, cosmetics and food industries. Effluents from the dye industry are considered highly toxic to aquatic organisms and they have been proven to disrupt the natural balance and affect symbiotic processes, by disrupting the photosynthesis activity and oxygen production due to water coloring [2]. Economically removing these dyes from wastewater remains a relevant problem for industries.
This study focuses on the CR azo-dye, named [1- naphthalenesulfonic acid, 3,30-(4,40-biphenylenebis(azo)) bis (4-amino)disodium salt). CR is a commonly used azo-dye and according to studies, between 5,000–10,000 tons of dyes are released into the waterways annually [3]. The degradability of CR in water-based environment is low. Additionally, it metabolizes to in water, to the carcinogen benzidine. Hence, the need of the hour is efficient systems of removal of dye from water before it is released into the environment.
The conventional methods of removing dyes from waste waters, which can are classified into- physical, chemical, and biological methods, are plenty [4]. Examples of them are biological degradation, membrane filtration, and ion exchange [5], electrochemical oxidation [6], reverse osmosis [7], photocatalytic degradation [8], and adsorption [9] and many more.
The dye removal effectiveness, the capital costs, and the operating costs for these methods vary.
Since adsorption process tends to be an inexpensive, effective and simple operation, this approach appears to an engaging, viable, and effective alternative to these strategies [10]. The adsorption process occurs when a substance in the gas or liquid phase comes into contact with a material, causing the substance to adhere to the phase of the material. A mass transfer process occurs when gases or solvents are absorbed from solids or liquids [11]. It very well may be utilized in a large number of enterprises. Importantly, it can be used to adsorb biomass as an adsorbent to remove certain colors from wastewater [12]. The primary objective of this study is to investigate the use of inexpensive and abundant agricultural waste biomass as an adsorbent, specifically water hyacinth and rice husk, and to determine the ideal conditions for adsorption-based removal of Congo red (CR) dye from wastewater [13].
The outer covering or shell of the rice grain is referred to as the "rice husk," which is a by-product of the rice milling process. It is one among the most prevalent agricultural waste products produced globally, especially in nations where rice is a main food. It ranks among the most significant agricultural residues in terms of volume. On a weight basis for the total rice, it makes up around 20% of the production [14]. Due to its granular nature, chemical stability, ease of availability, and affordable manufacture, RH is frequently used as a bio-adsorbent [15].
Water Hyacinth, or Eichhornia crassipes, is a free-floating aquatic plant native to South America, which has spread quickly to various parts of the world. Due to its rapid growth rate and ability to colonize freshwater bodies, it is considered an invasive species and poses significant environmental challenges in many areas.
Using it as an adsorbent could be a potential solution to these challenges. Water hyacinth possesses many plus points as an adsorbent for dye removal. The plant’s abundance makes it a low-cost and easily accessible option. Moreover, water hyacinth is a renewable and eco-friendly resource.
WH biomass possesses a high adsorption capacity due to its large area of surface, as well as its complex structure. Its biological composition, including cellulose, hemicellulose, and lignin, interacts effectively with dye molecules. These benefits collectively position water hyacinth as a promising adsorbent for efficient and environmentally friendly dye adsorption [16].
Many studies have reported the improvements in adsorption capacity of adsorbent materials after undergoing surface modification [17–20]. Hence, the biosorbents, i.e RH and WH are modified with citric acid with the aim of improving their dye adsorption ability. Citric acid activates the -OH groups from cellulose and carboxylic groups through esterification reactions. Additionally, citric acid-modification does not produce hazardous waste. Hence, this process was adopted [21].
Also, it is attempted to optimize the factors utilized in the CR adsorption process from recreated waste water on the surface of modified biomass and explore the impacts of changing adsorption factors like adsorption time, pH, and dose of biosorbent on CR color adsorption [22]. Since they don't consider the connection between the boundaries, conventional techniques that change just a single boundary at an at once for enhancement. In this manner, for multi-part improvement, the statistical techniques DoE and RSM were used [23].
RSM was used to plan, dissect, and upgrade the variables influencing the adsorption cycle [24]. The Box-Behnken Design (BBD) inside RSM was explicitly utilized for its adequacy in accomplishing better streamlining yield. The essential goal was to inspect the effect of functional circumstances on the adsorption productivity of the changed RH and WH utilizing BBD of RSM. The relapse model got from the BBD worked with the investigation of element connections through a three-layered reaction surface [25].
To find effectiveness of the adsorption, equilibrium data obtained from batch studies under optimized conditions were analysed, and their dye removal % and adsorption capacity. Additionally, the statistical models best fitted to the system were obtained using the DoE methodology. Overall, this study focused on surface modifying the biosorbents, optimizing operational conditions, exploring factor interactions, and understanding the adsorption mechanism of CR dye using modified RH and WH adsorbents.