# Nascent Rice Husk as an Adsorbent for Removing Cationic Dyes from Textile Wastewater
## 3 Results and Discussion
### Adsorption Isotherm  
The equilibrium adsorption isotherms are indispensable in understanding the interactive nature between adsorbents and adsorbates, as they are useful in designing adsorption systems. The influence of different MB and CV concentrations (0-120 mg/L) was tested and presented in Figure 5, where equilibrium of the different concentrations in an aqueous phase, C\({}_{\text{e}}\) (mg/L), and the amount of MB and CV adsorbed on the RH, q\({}_{\text{e}}\) (mg/g), were plotted as a function of the equilibrium concentration of MB and CV in aqueous solution. With increasing concentrations of MB and CV in the aqueous phase (Figure 3a,b), the adsorbed amounts of MB and CV, respectively, increased linearly; each isotherm represented the observed data fairly well. The selection of best fit model was based on the \(R^{2}\) value provided in Table 2. Thus, the adsorption data fit well with the Langmuir isotherm, with R\({}^{2}\) values of 0.972 and 0.979, rather than the Freundlich isotherm, for both MB and CV adsorption by the RH. The Langmuir isotherm is an equation assuming monolayer adsorption onto a uniform surface with no interaction between the adsorbed molecules, suggesting the adsorption of MB and CV on the RH surface as a homogenous monolayer [68]. The isotherm implies a monolayer of MB and CV over a homogenous RH surface, reaching saturation after attaining equilibrium, and making it impossible for further adsorption [69]. The CV adsorption onto almond skin waste followed a similar Langmuir isotherm [70]. Furthermore, the values of 1/\(n\) for MB and CV were determined to be 0.32 and 0.28, respectively. The 1/n values between 0 and 1, indicate a favorable adsorption process [71]. Figure 5: Freundlich and Langmuir adsorption isotherms using rice husk for (**a**) methylene blue (MB) and (**b**) crystal violet (CV). The adsorption capacities of RH for MB and CV were compared with other adsorbents. The maximum amounts of MB and CV adsorbed were found to be 25.46 mg/g and 24.48 mg/g, respectively. The RH showed a favorable adsorption capacity, even though it was low in comparison to that of various adsorbents, as shown in Table 3 for MB, and in studies by [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON] [44] and [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON] [72] for CV. The maximum reported capacities of the adsorbent materials ranges from 20.29 mg/g to 555.56 mg/g; that of RH (25.46 mg/g) lies at the tail end of the table. It is worth mentioning that most adsorbents ranking higher also had a smaller size (nano or micron). Nano/micro-sized particles cannot be used in filtration systems without post separation process, to separate micro particles from water after treatment. The low adsorption capacity of RH could be attributed to its chemical composition, which includes lignin and silica, that cause a reduction in the bonding strength between functional groups available on its surface and an adsorbate; thus, resulting in low adsorption of MB and CV. Additionally, the surface of RH is smooth, containing wax and fats that shelter the grain, chemically and physically affecting its adsorption properties [73]. However, RH is readily available in Myanmar and Vietnam at an affordable price of \(\sim\)USD 0.025 per kg [38]. Thus, the RH treated at 75 \({}^{\circ}\)C (RH-75) for 24 h, with particle size of 0.075-1.16 mm, is environmentally friendly and more cost effective as compared to other adsorbents. Firstly, the granular nature of the adsorbent is advantageous as it is easily separated after filtration and sedimentation without forming a sludge. Secondly, with regard to recycling, by utilizing an agricultural by-product, it helps to valorize waste as a resource to minimize environmental hazards.