Adsorption of methylene blue from aqueous solutions using water treatment sludge modified with sodium alginate as a low cost adsorbent

The presence of industrial effluents in the environment is associated with many harmful effects and currently considered to be one of the world's major environmental problems (Méndez-Paz et al. 2005). Dyes are an important class of pollutants which may exist in the industry effluent and can even be identified by the naked eye. Industries such as textile, painting, rubber, leather, cosmetics, pharmaceutical, food, paper, and plastics are some of the sources for dye effluents (Gupta 2009; Moghaddam et al. 2011). Various dyes such as radioactive, azo, cationic, and basic dyes are used in these industries (dos Santos et al. 2007). The discharges of organic dyes in aquatic environments can reduce light penetration and photosynthesis (Khataee et al. 2013). Furthermore, disposal of highly colored effluents from these industries poses significant environmental risks and can be harmful to aquatic life and human beings due to their toxicity and non-biodegradability properties (Modirshahla et al. 2012). In addition, some dyes and their metabolites have also mutagenic and carcinogenic effects, hence posing a potential health hazard to all forms of life (Modirshahla et al. 2011). Methylene blue (MB) is one of the most commonly used dyes in industrial applications. Nevertheless, it has various dangerous effects on humans and the environment. For example, it can cause high pulse rate, nausea and vomiting (Pavan et al. 2008; El-Mekkawi et al. 2016; Li et al. 2016). Therefore, the discharge of dyes in precious water resources must be avoided. A variety of treatment technologies, including biological (Hayat et al. 2015), adsorption process using several adsorbents (Madrakian et al. 2012; Hassani et al. 2014), coagulation and flocculation (Mahmoud et al. 2013), chemical oxidation (Modirshahla et al. 2011), and membrane processes (Wu et al. 1998), have been used or are in use for the removal of various dyes from wastewater or aquatic environments (Gupta 2009). Notwithstanding, there are a few drawbacks encountered in some of these methods, which could limit their applications. For example, although biological methods are environmentally friendly techniques (Moussavi et al. 2009), they have some inhibitory effects on microbial metabolism particularly at high concentrations because of low biodegradability of some dyes (Ghoreishi & Haghighi 2003). Chemical techniques impose some operational costs such as electric power and materials shipment. Furthermore, some chemicals (e.g. ozone) which are used in advanced oxidation processes are highly corrosive and toxic (US EPA 1999). Membrane processes have also undesirable effectiveness in the removal of dyes because of membrane fouling issues and vulnerability of the membranes to organic solvents of effluents (Zazouli et al. 2010).

Adsorption, the most popular and effective economical alternative, has attracted considerable attention compared to other techniques because of the advantages in terms of cost, the recovery of pollutant from waste streams, ease of operation, flexibility and simplicity of design, production of a high quality effluent, etc. (Liu et al. 2011). Accordingly, a variety of natural and synthetic materials, as adsorbents, have been used to treat colored wastewaters.

Activated carbon is an adsorbent which is widely used for removal of different contaminants from aqueous solutions and wastewater (Ramakrishna & Viraraghavan 1997; Babel & Kurniawan 2003). Nonetheless, it has a few downsides and shortcomings such as high cost, difficulties in regeneration, non-selectivity properties, and unsuitable for dispersed dyes. Thus, nowadays a huge range of different substances have been used by a number of researchers to develop highly efficient and low cost alternative adsorbent to treat contaminated environments. Large quantities of drinking water treatment sludge (DWTS), as an inevitable water industrial waste, are produced every day all over the world. DWTS mainly consists of removed solids (both dissolved and suspended), coagulants and other chemicals that have been used to separate water impurities. This waste has been disposed mainly by landfilling, incineration and even discharge to ocean. Because of the increases of sludge disposal cost, environmental effects consideration and reduction in landfill capacity, the reuse of DWTS has received more interest in recent decades (Yang et al. 2015). The use of DWTS has been studied extensively as a low cost adsorbent for removing some pollutants from the aqueous solutions or wastewater. For instance, Changhui Wang used water treatment residuals to remedy soil contaminated with multiple metals in China (Wang et al. 2012) and found that the treatment of soil containing heavy metals with water treatment residuals resulted in its transformation into more stable forms. Chiang et al. (2012) showed that at the highest DWTS dosage of 250 mg/L, concentrations of the cationic contaminants (heavy metals) decreased by at least 80%. The use of adsorbent derived from DWTS was also investigated by other researchers to remove cadmium (Siswoyo et al. 2014), ammonium (Yang et al. 2015) and chromium(VI) (Shams Khorramabadi et al. 2012) from the polluted environments. The corresponding removal efficiencies were 70–90%, 80–90% and about 55%, respectively. Adsorption of MB using chemically and physically treated adsorbents from DWTS (alum sludge and mud sludge) has also been reported in the recently published work (Rashed et al. 2016). Based on the study, the optimum adsorption conditions for the adsorption are initial MB concentration of 100 ppm, contact time of 1 hour, solution temperature of 250 °C, solution pH of 7 and adsorbent dosage of 0.25 g/L. Given the better fitness of experimental data with the Langmuir model, the maximum adsorption capacity of alum and mud sludge was 70.4 and 65.79 mg/g, respectively.

Therefore, the aim of the present research was to study the possibility of employing modified sludge from a water treatment plant as a low cost adsorbent for removal of MB. The modification was conducted using sodium alginate (SA) and adsorption experiments were carried out under different concentrations of adsorbent, adsorbate, pH and contact time. Alginate (as calcium and sodium salts) has been applied as one of the most effective biological agents for the immobilization of various fine adsorbents due to its special properties such as biodegradability, water solubility, low cost and non-toxicity towards aqueous environment (Martinsen et al. 1989; Hassan et al. 2014). The obtained data were also applied to find out the kinetic and isotherm of MB adsorption from solutions. Furthermore, the role of functional groups was also discussed. To the best of our knowledge, the adsorption of MB on DWTS modified by SA has not been previously reported.

The characteristics and molecular structure of the MB as a typical organic dye are shown in Table 1. SA (BDH, UK) and MB (Merck, Germany) were of analytical grade. All solutions were made with double distilled water. The sludge existing in the drying bed of a water treatment plant was used as starting materials to prepare adsorbent. The pH of the solutions was adjusted with hydrochloric acid (HCl) and sodium hydroxide (NaOH) (0.1 N). The stock MB solution was prepared by dissolving the appropriate amount of MB of purity 97% in a solution containing distilled water and then it was stored in a glass container at room temperature. Required working concentrations of MB standards were instantaneously prepared through appropriate dilution of this stock solution for each experiment series.

Table 1

Characteristics of MB dye (Reddy et al. 2016)

Trade name .	Chemical structure .	Molecular formula .	Molecular weight (g/mol) .	λmax .
MB		C16H18ClN3S	319.85	664

Trade name .	Chemical structure .	Molecular formula .	Molecular weight (g/mol) .	λmax .
MB		C16H18ClN3S	319.85	664

View Large

A scanning electron microscope (SEM) equipped with an energy dispersive X-ray microanalysis (LEO-1430 VP) was used to determine the surface morphology and the porous structure of the adsorbent. The surface functional groups of prepared adsorbent were determined using Fourier transform infrared spectroscopy (FTIR) at wavenumbers ranging from 400 to 4,000 cm⁻¹.

The pH at point of zero charge (pzc) (pH_pzc) can be used to determine the quality of the relationship between pH and cephalexin adsorption. The pzc was determined using 0.01 molar NaCl as an electrolyte by adding 0.1 N NaOH or 0.1 N HCl solutions. For this matter, 50 mL of the electrolyte was introduced into eight beakers and then the pH was adjusted to the required value in the range 2–12. After that, 0.3 g of adsorbent was added into each beaker and the beakers were shaken for 2 h. Then, the adsorbent was filtered and the final pH of the filtrate was measured. By plotting the initial pH vs. the pH after 2 h, the pzc of developed adsorbent was determined from the intersection of these curves (Kalhori et al. 2013).

Concentration of MB in supernatant solution was measured by a spectrophotometer at a wavelength of 662 nm (Mahmoud et al. 2013). Other parameters were measured according to the methods well described in the Standard Methods (APHA 2005). The effect of pH, adsorbent dosage, reaction time, and MB concentration was examined in the range of 2–12, 0.1–2 g/L, 0–120 min, and 50–500 mg/L, respectively.

This study was investigated in batch mode and adsorption experiments were performed in 250 mL Erlenmeyer flasks. The contents of all Erlenmeyer flasks were mixed thoroughly using magnetic stirrers with a fixed setting to achieve a constant speed. For every experiment, 250 mL of samples with a given concentration was added into the Erlenmeyer flask. The desired level of pH for each experiment was adjusted using 0.1 N HCl or NaOH. Then, a determinate dose of adsorbent was added into the Erlenmeyer flask and was shaken immediately at a regulated speed of 150 rpm by shaker. After the desired contact time, to separate adsorbents from aqueous solution, the samples were filtered through Whatman filter paper (0.45 μm) and then centrifuged at 5,000 rpm (5810 R, Eppendorf Biotech Company) for 15 minutes to determine residual MB concentration. All the experiments were carried out at room temperature in triplicate and mean values of data were reported to ensure the reproducibility of the results. Since standard deviations never exceeded ±1.5%, the error bars are not shown in the figures.

In all kinetic experiments, pH of the solution was kept at optimum condition (≈8), which was predetermined by studying the pH effects on the removal efficiency. For equilibrium studies, various concentrations of MB ranging from 50 to 200 mg/L were contacted and shaken in several 250 mL flasks with a constant 0.3 g/L dose of adsorbent at room temperature. The flasks were sealed and the contents were stirred for 2 h.

The kinetics of MB adsorption were analyzed by fitting data from the experiment of this step with pseudo-first-order and pseudo-second-order models as shown in Table 3. In this table, k₁ and k₂ are constants of adsorption rate, q_t is adsorption capacity at time t, and q_e is adsorption capacity at equilibrium condition. Regarding the adsorption isotherm analysis, it is crucial to understand the interaction between an adsorbate and the adsorbent or to describe the nature of the adsorption (Samarghandi et al. 2015). Thus, to provide quantitative information, these data were fitted by the Langmuir, Redlich–Peterson and Freundlich isotherm models by using the equations presented in Table 3. The q_m parameter is the maximum adsorption capacity (mg/g), b is a constant related to the energy of adsorption (L/mg), k_f is a constant related to the adsorption capacity (mg/g) and can be defined as the distribution coefficient, where, with the increases of k_f values, the adsorption capacity of the adsorbent was also increased, A and B are the Redlich–Peterson isotherm constants, and g is an exponent which lies between 0 and 1. The parameters of the Redlich–Peterson isotherm can be evaluated from the plot between log(AC_e/q_e−1) and log C_e. As this model contains three parameters, the iteration method was used to estimate the parameter A (L/g) with the maximum R² value. To perform the iteration, Solver add-in of Microsoft Excel was used.

Table 3

The name and equations of the studied isotherm and kinetic models

Model types .	Name .	Equation .	Ref. .
Isotherm models	Langmuir		Tarlani Azar et al. (2016)
	Freundlich		Tarlani Azar et al. (2016)
	Redlich–Peterson		Kumar (2007)
Kinetic models	Pseudo-first-order		Lagergren (1898)
	Pseudo-second-order		Ho & McKay (1999)

Model types .	Name .	Equation .	Ref. .
Isotherm models	Langmuir		Tarlani Azar et al. (2016)
	Freundlich		Tarlani Azar et al. (2016)
	Redlich–Peterson		Kumar (2007)
Kinetic models	Pseudo-first-order		Lagergren (1898)
	Pseudo-second-order		Ho & McKay (1999)

View Large

Adsorption kinetics can explain the solution uptake rate and show the adsorption efficiency of the adsorbents (Zhua et al. 2010). Kinetics experiments were conducted in Erlenmeyer flasks containing 250 mL of MB solution with initial MB concentrations of 50, 100, and 200 mg/L, pH = 8 and 0.3 g of adsorbent, which were agitated at room temperature. After regular time intervals, 5 mL of solution was sampled from the mixtures and MB concentration was analyzed until the reaction reached equilibrium. Two of the most widely used kinetic models, namely, Lagergren first-order kinetic model and pseudo-second-order model (Table 3), were applied to understand the adsorption kinetic behavior of MB onto SA modified sludge adsorbent.

The value of R_L indicates the adsorption nature to be either unfavorable (R_L > 1), linear (R_L = 1), favorable (0 < R_L < 1) or irreversible (R_L = 0). The Langmuir constant K_L was found to be 0.018. This value also means that a higher affinity of MB for sorbent is expected. Moreover, the value of R_L obtained for MB was 0.18 to 0.53, indicating favorable adsorption of MB onto sorbent as it lay between 0 and 1 for the prepared adsorbent.

In this study, the sludge stabilized by SA was used for removal of MB from aqueous solutions. The highest removal efficiency was observed at initial concentration of 50 mg/L, adsorbent dose of 0.3 g/L, reaction time of 60 minutes, pH 8, and room temperature. The results showed that removal efficiency has a direct relationship with adsorbent dose, reaction time, and pH, and an inverse relationship with initial concentration of dye. The study of the kinetics and isotherms of the adsorption showed that the MB removal capacity of sorbent is substantially and significantly high. Based on the isotherm analyses, the monolayer sorption capacity for MB by adsorbent was found to be 909.1 mg/g. The sorption kinetics of MB on adsorbent could also be explained by the pseudo-second-order model. From the experimental results we can conclude that the developed low-cost sorbent from DWTS has a considerable potential for removal of MB and could be considered as a promising material for the removal of dyes and probably other organic pollutants from aqueous solutions.

The authors would like to acknowledge Hamadan University of Medical Sciences for financial and instrumental supports.

The authors declare no conflict of interest.