Color and cod removal from treated textile industry effluents with various adsorbents

2.1. Preparation of Adsorbents

In this study four adsorbents were used: the waste metal hydroxide (WMH), colemanite waste (CW) and ulexite waste (UW) and magnetic nanoparticles (Fe3O4) (MNP). WMH obtained from aluminum electroplating industry and was generated by alkaline precipitation of the metal ions present in wastewater. The sludge was dried at room temperature, ground, sieved and washed with distilled water several times. The powder of particle size smaller than 500 μm was used as the adsorbent [4]. Colemanite waste (CW) and ulexite waste (UW) collected from boron ore wastes and sieved with 1mm pore size sieve. MNP was preparated as follow: 6.1 grams of FeCl3⋅6H2O and 4.2 grams of FeSO4⋅7H2O were dissolved in 100 mL of distilled water and heated up to 90 oC. 10 mL 26% of ammonium hydroxide in 200 mL of distilled water were added to each other quickly. The solution was mixed for 30 minutes at 80 oC and cooled down to room temperature. The obtained MNP was precipitated for 1-2 minutes and then washed with distilled water for 3 times. After that, separated MNP is dried at 50 oC [5, 6]. MNP has 62.99% Fe content.

2.2. Wastewater Origin

In this study, biologically treated effluents of a local textile factory in Corlu which has COD: 200 mg/L and color: 39.4 m-1 was studied.

2.3. Batch Adsorption Studies

The adsorption studies were carried out at different adsorbent concentrations (2-30 g/L) to investigate the adsorptive capacity of the adsorbent. The adsorption of color and COD onto different adsorbents were studied at natural pH (pH 7.5), different adsorbent doses (2-30 g/L) and different contact times (0, 15, 30, 60, 90, 120, 150, 180 min). COD and color removal were studied by a series of equilibrium experiments. The contents of each flask were centrifuged at 3500 rpm for 5 min and COD and color were measured in the supernatant. The samples were taken at the end of the desired contact time to determine the effect of agitation time. The flasks were agitated at room temperature (25oC) at 200 rpm. The amount of color and COD adsorbed were calculated from the following equation (1): qe=V(Co-Ce)/W

where qe is the amount adsorbed (mg/g); Co and Ce are the initial and equilibrium color COD concentrations in the solution (mg/L), respectively; V is the solution volume (L); and W is the mass of adsorbent (g).

2.4. Analysis

The color (m-1) was measured according to EN ISO 7887 color measurement method and chemical oxygen demand (COD, mg/L) determined according to standard methods [7].

3.1. Removal Rates of COD and Color

The color removal efficiencies by WMH, MNP, UW, and CW at different contact times are shown in Figure 1 and Figure 2. As shown from the Figure 1 and Figure 2 that color removal efficiencies of WMH and UW were bigger than MNP and CW. So CW and MNP were not used in the further studies. As it can be seen from Figure 1 color removal efficiencies increased with increase of contact time. As seen from Figure 1 UW reached equilibrium at 120 min and WMH didn’t reach at 120 min and WMH achieved to equilibrium at 180 min (Figure 3).

Figure 1. Effect of contact time of various adsorbents on color removal efficiency

Figure 2. Comparing color removal efficiency of adsorbents at 120 min.

Figure 3. Effect of contact time on color removal efficiency for WMH Removal efficiency of COD and color is highly affected by adsorbent concentration. Raising 732

the WMH and UW doses from 2 g/L to 30 g/L the COD removal efficiency increased from 23% to 70% and 45% to 72% respectively (Figure 4). As it can be seen from Figure 4 that COD removal achieves to equilibrium at 30 g/L for WMH and at 20 g/L for UW. At equilibrium point Ce values were 60 mg/L for WMH and 54 mg/L for UW.

Figure 4. Equilibrium COD concentrations obtained for WMH and UW at different doses Raising the WMH and UW doses from 2 g/L to 30 g/L the color removal efficiency increased from 22% to 57% and 40% to 68%, respectively (Fig.5). Color removal achieves to equilibrium at 20 g/L for WMH and at 18 g/L for UW. At equilibrium point Ce values were 17.4 m-1 for WMH and 12.3 m-1 for UW.

Figure 5. Equilibrium color concentrations obtained for WMH and UW at different doses

3.2. Adsorption equilibrium study

Two isotherm models (Freundlich and Langmuir) were used in the present study to describe the adsorption data (Table 1). Table 1. Models used to describe the adsorption of COD and color Freundlich Langmuir

is a constant depicting adsorption capacity, is a constant indicating adsorption intensity. (L g-1) represent s the Langmuir constant and (mg g-1) is the maximum adsorption capacity for monolayer formation on adsorbent

It was found from the results that only the Freundlich isotherm model described and predicted COD and color adsorption on UW and WMH (Table 2). Values of the adsorption intensities (1/n) less than 1 show the favorable nature of adsorption on the adsorbents [8, 9]. As it is seen from Table 2 that the adsorption intensities are found higher than 1. If 1/n value is high it shows that the adsorbent used will lose its capacity more rapidly than the others [10]. So 1/n values found in this study show that these adsorbents will lose their capacity rapidly.

Table 2. Isotherm constants for the Freundlich isotherm Freundlich constants UW Parameters COD Color

Table 3 gives the COD removal rates for adsorption on various adsorbents obtained from this study and obtained in other studies. As it is seen from the Table 3 that WMH and UW could be used as low cost adsorbents to remove COD from biologically treated effluents and the new methods could be tested to increase COD and color removal efficiency by these adsorbents. Table 3. COD removal rates for adsorption on various adsorbents Adsorbent

Coconut shell carbon Rice husk carbon Fly ash Brick kiln ash Ulexite waste Waste metal hydroxide

Industrial Industrial Domestic Domestic Textile eff. Textile eff.