A comparative removal of two dyes from aqueous solution using modified oak waste residues: Process optimization using response surface methodology

Highlights

• Solution pH is the most effective experimental variable on dyes removal.

• The performance of each applied stabilizer to enhance adsorptive removal of dyes is as follows: NaOH > HCl > ethanol > chloroform.

• The adsorbent dosage has a direct relationship with MB and AR 73 removal.

• Adsorptive removal rate of both dyes are decreased significantly by increasing initial dye concentration.

• MOW can be employed as an efficient and cost-effective adsorbent for removal of dyes from contaminated aqueous solution.

Abstract

In this study, optimization of adsorptive removal of two cationic (methylene blue (MB)) and anionic (acid red 73 (AR 73)) dyes with modified oak waste (MOW) adsorbent was carried out using response surface methodology (RSM). The input variables included initial solution pH (2–10), contact time (20–200 min), adsorbent dosage (0.5–2.5 g/L) and initial dyes concentrations (10–90 mg/L). Furthermore, the surfaces of the adsorbent were modified with different modifiers to find the most efficient adsorbent. The obtained results of comparative studies on dyes adsorption confirm that sodium hydroxide, compared to other modifiers, has the highest ability to activate the surfaces of the oak waste (OW) to remove dyes from the solution. Using NaOH, the removal efficiency of MB and that of AR 73 were obtained to be 96% and 29%, respectively. In addition, results of the optimization study showed that RSM could significantly predict the experimental data of dyes removal with a high correlation coefficient (0.991 for MB and 0.993 for AR 73). The optimum experimental conditions for dyes removal were pH of 6.2, 160 min contact time, 2.0 g/L adsorbent dosage and 70 mg/L initial dye concentrations. Under this condition, the removal efficiency for MB and AR 73 reached 85.36% and 41.27%, respectively. Ultimately, from the findings of this study, it is observed that MOW can be used as an efficient and cost-effective adsorbent for the removal of dyes from the contaminated aqueous solution.

Introduction

Agricultural activities and Industries, including dyestuff, textile, paper, and plastics, are responsible for large quantities of wastewater generation. Textile and other industries release the harmful effluents directly or indirectly into water sources. Dyes have been identified as the main contaminant in effluents. Approximately 2% of dyes have been estimated to be directly discharged from manufacturing plants into effluents, and roughly 10% of dyes are lost throughout the textile dyeing process [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14]. The noticeable fact is that dyestuff wastewaters are toxic, thereby they potentially impose hazardous impacts upon a wide range of water bodies as well as the plants and animals depending on such ecosystem. Also, even in small volumes, effluents are extremely visible and adversely affect the aesthetic nature of water. Other problems resulted from water contamination include, inter alia, interference with the photosynthesis process and food chain destruction in water ecosystem [15]. Due to the fact that contaminated water contains dyes and metabolites which are extremely toxic [16], [17] and inherently carcinogenic [18], [19], it would be detrimental to human beings and animals as well. Regarding its prominent qualities, i.e., applicability, simplicity, and efficiency, adsorption [20], [21], [22] has been proved to be an effective method to remove dyes from effluents in addition to other techniques, including electro kinetic coagulation [23], chemical coagulation [24], oxidation and ozonation [25], membrane separation [26] and ion exchange [27]. Accordingly, a plethora of studies has dealt with the adsorption of dyes in a wide range of materials. To this end, various cost-effective adsorbents, e.g., activated sludge [28], rhizopus oryzae [29], coconut bunch waste [30], waste oak [31], kaolinite [32], castor seed shell [33], bottom ash and deoiled soya [34], were investigated to evaluate their potential for wastewater treatment. Over recent years, versatile methods have been introduced for wastewater treatment using affordable adsorbents [8], [11], [15], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44], [45], [46], [47], [48], [49], [50]. For example, Hameed reported that pineapple stem (PS) waste had a significant ability to remove MB from the aqueous solution [51]. Furthermore, findings of Arami revealed that soy meal hull is an excellent adsorbent to remove a wide range of dyes like C.I. direct red 80 (DR80), C.I. direct red 81 (DR81), C.I. acid blue 92 (AB92) and C.I. acid red 14 (AR14) [52]. However, their application has been restricted because of the contaminants leaching into the solution. Hence, the chemical and physical modification of adsorbents has been recommended as a promising approach to not only enhance the functional groups onto their surfaces that lead to increased adsorption capacity but also to transform their physical structure that stimulates the adsorption of higher pollutants [53]. In the present study, oak waste (OW) pieces obtained from oak trees that are common in western and southwestern Iran were used as adsorbents. In addition, some acids, solvents and bases were used as surface modifiers to compare their effects on improving oak waste capacity for dyes removal. Conventional optimization approach, one-variable-at-a-time, which is applied for observing the effect of experimental variables is time-consuming and expensive. Hence, the multivariate statistical methods have to be employed for optimizing the influential factors. Response surface methodology (RSM) is a combination of statistical and mathematical methods that uses the polynomial equation to fit the experimental data [54]. This model is especially practical when some responses are affected by different input variables. It will help the researchers to use a novel method with the minimum number of experiments [55]. Up to now, a wide range of remediation techniques, such as reduction [54], [56], adsorption [57], photocatalysis [58] and biosorption [59], have been optimized by RSM. However, to the best of our knowledge, no study has so far been reported on the optimization of dye removal using surface-modified oak waste. Therefore, the principal objective of the present study is surface modification of oak waste with some substances to remove MB and AR 73 from the aqueous solution. In addition, the optimization of dyes removal was carried out using RSM at different input variables.