Utilization of Calcined Gypsum in Water and Wastewater Treatment : Removal of Phenol

The release of phenol-containing effluents above the phenol permissible limit has triggered a lot of concern over the world due to their toxic nature. The adsorptive potential of gypsum on the removal of phenol was investigated. The effect of gypsum loading (0.5–3 g), contact time (2.5–20 min) and solution temperature (298 to 318 K) on the removal of phenol by gypsum was studied at neutral pH. The thermodynamics of the adsorption process was also studied. The kinetic data were fitted into the pseudo-second-order, Elovich, and intraparticle diffusion models. The removal efficiency of phenol increased along with the mass of gypsum, contact time and temperature. The results of the thermodynamics study indicate that the adsorption process is spontaneous and endothermic in nature. The change in free energy (ΔG0) was found to increase with temperature. The values of the estimated ΔG0 suggest that the phenol adsorption on gypsum is a physical adsorption process. Additionally, the kinetic data fitted best into the pseudo-second-order than the other kinetic models. This study proved that phenol can be used effectively for the reduction of phenol concentrations in water and wastewater.


INTRODUCTION
The excessive release of phenol-containing wastes into the aqueous ecosystem due to industrialization is one of the greatest threats to health (Bemosmane et al., 2018; Pourakbar et al., 2018;Kaczorek et al., 2016).The major sources of phenolic compounds include the wastes from paper mills, pesticides production, petrochemical, paint, textile plants, oil refineries, and pharmaceutical industries (Mahvi et al., 2007;Azevedo et al., 2009,Baird;1998).Phenols and phenolic compounds are considered as the priority and one of the most dangerous pollutants since they are harmful to organisms even at a low concentration (Rahmani et al. 2008;Pourakbar et al., 2018).Phenol and its degradation products are major aquatic pollutants since they are toxic and carcinogenic in nature (Ahmadi and Igwegbe, 2018; Kulkarni and Kaware, 2013).Phenol is lethal to people and can result in either acute (short-term) and chronic (long-term) effects (Villegas et al., 2016).It is highly corrosive and causes harmful side effects such as diarrhea, nervous breakdown, impaired vision, sour mouth, and excretion of dark urine (Kulkarni and Kaware, 2013).Besides, it is relatively stable and soluble in water (Azevedo et al., 2009.Therefore, considering the health implication, proper treatment techniques are necessary for the reduction of the phenol level.Owing to its high toxicity and difficulty to reduce biologically, the limits for their existence in drinking water and effluents were set up (Sharan et al., 2009;Roostaei and Tezel, 2004;Salari et al., 2018).The United States Environmental Protection Agency (USEPA) specified that the phenol concentration in drinking water and industrial effluents must be less than 1µg/L and 500 µg/L, respectively (Tao et al., 2013;Yan et al. 2006;Salari et al., 2018;Wu et al;2001).Additionally, the World Health Organization (WHO) considers the maximum permissible limit of phenol in drinking water to be 0.002 mg/L (Salari et al., 2018;Roostaei et al., 2004).Furthermore, the EPA has set the concentration of phenol in wastewater below 2 mg/L (EPA, 2015; Pourakbar et al., 2018).The presence of phenol in water contributes to taste deterioration and odor (Sharan et  However, amongst the above-mentioned methods, the adsorption process seems to be the most prospective for the removal of organic and mineral pollutants (Sarvani et al., 2018;Deng et al., 2010) and is effective either carried out on laboratory or industrial scale (Sarvani et al., 2018;Rodrigues, 2011).Adsorption has been extensively used to control the phenol pollution because of its cost-effectiveness, simplicity of operation, availability of adsorbent precursors and efficiency (Menkiti et al., 2018).Activated carbon is commonly used for adsorbing pollutants but it is very expensive and cannot be easily regenerated (Khoshnamvand et al., 2017).Ahmadi and Igwegbe (2018) studied the removal of phenol using acid-modified bentonite.The removal efficiency of 97.9% was obtained at contact time of 60 min, dosage of 0.6 g/L, pH of 6 and concentration of 50 mg/L; the experimental data was best described by the pseudo-secondorder kinetic model.Menkiti  Gypsum is more advantageous compared to other adsorbents because of its abundance in nature, low-cost and no pretreatment is required as an adsorbent (Rauf et al., 2009).Gypsum (CaSO 4 .2H 2 O) is a very soft sulfate mineral composed of calcium sulfate dihydrate (Bello et al., 2013;Cornelis and Cornelius, 1985).Gypsum was applied successfully and extensively for the removal of dye (Rauf et al, 2009), pharmaceutical drugs (Li et al., 2014) and heavy metals (Petruzzeli et al., 2015; Raii et al., 2014).Moreover, there is no report on the use of gypsum for the removal of phenol.
The present study aims at investigating the removal of phenol from aqueous solution using gypsum which is abundantly available.The effects of mass of adsorbent (gypsum), contact time and solution temperature on the removal of phenol were investigated using the batch method under neutral pH conditions.The thermodynamics and kinetics of the adsorption process were also studied.

MATERIALS AND METHODS
The gypsum was supplied by Rawabi for Mining (Tafila, South Jordan).It was crushed, milled and then calcined at 150-200 o C. Phenol (99% purity) was obtained from a local supplier.Phenol stocks were prepared by dissolving accurate weight of phenol in 1000 mL of DDW.This stock solution was then used in the preparation of synthetic wastewater samples with different concentrations.The batch sorption experiments were conducted in a series of 500-mL glass beakers containing 250 mL of the phenol solution.A determined amount of calcined gypsum (0.5-3.0 g) was added to these beakers.The experiments were conducted at an ambient temperature of 25 o C and the desired temperature with a stirring time of 5-25 min.After the completion of the experiments, the samples were filtered.The filtrate of phenol solutions was then subjected to UV-visible analysis in order to determine the percentage of phenol removal (SPECTRA-COMP 602; Advanced Products, Milan, Italy).A calibration curve of absorbance against different concentrations of phenol was constructed.The samples of phenol solutions had been analyzed by UV before and after treatment in order to determine the amount of phenol removed.The amount of phenol adsorbed on gypsum, q e (mmol/g) and the percentage of phenol removed (%R) were calculated as follows (Igwegbe et al., 2016; Ahmadi and Igwegbe, 2018): where: C 0 and C t (mg/L) are the concentrations at the initial time, t=0 and time, t, respectively.
V is the volume of phenol solution (L) and W is the mass of gypsum (g).

Effect of gypsum mass
The effect of gypsum mass on the reduction of phenol concentration was studied by varying the mass at the initial phenol concentration of 0.3809 mol/L.As seen in Fig. 1 and Table 1, as the mass of gypsum was increased from 0.5 to 3 g, the percentage of phenol removed on the adsorbent (gypsum) was enhanced steeply from 29.06 to 84.25 %.Therefore, the maximum phenol removal was achieved at the highest mass of gypsum studied (3 g).The improvement in phenol removal with increasing mass of gypsum is as a result of the increase in the number of adsorption sites and adsorbent surface area (Sahu et al., 2017;Afsharnia et al., 2017).The amount of phenol per unit mass of the gypsum, q e decreased with the increasing adsorbent loading from 177.75 to 31.28 mmol/g.This is because the active adsorption sites of the adsorbent were not fully exploited (unsaturated) at a higher adsorbent loading compared to lower adsorbent mass (Gorzin and Abadi, 2018; Radnia et al., 2012).A smaller mass of adsorbent will be saturated more quickly than a larger one (Larous and Meniai, 2012).

Effect of contact time
In order to investigate the effect of time on the adsorption of phenol using gypsum, the time of contact varied from 2.5 to 20 min using an adsorbent mass of 1.5 g. Figure 2 and Table 1 show that the percentage of phenol removed and the adsorption capacity q e was increased rapidly with time.Maximum phenol removal was attained at the highest contact time studied.This may result from the fact that the frequency of collision of the adsorbate and the adsorbent was increased with retention time, which also increased the process of adsorption (Ahmadi and Igwegbe, 2018; Buhani et al., 2018).Moreover, the adsorption sites were more available with time.

Effect of solution temperature
Temperature changes the adsorption capacity of an adsorbent for a specific adsorbate (Banerjee and Chattopadhyaya, 2017).The influence of solution temperature on the phenol removal was investigated by varying the temperatures from 298 to 318 K (Fig. 3).As shown in Figure 3 and Table 1, the adsorption of phenol on gypsum was improved with increasing temperature.The highest removal of 70.39% was achieved at the highest studied temperature of 318 K.The removal of phenol was favored at a higher temperature because temperature increases the number of collisions between the adsorbent and the adsorbate, thereby enhancing the rate of adsorption (Afsharnia et al., 2017).Figure 3 also shows that the amount of phenol uptake by gypsum increased from 35.55 to 44.68 mmol/g.The rate of diffusion of the phenol molecules onto the gypsum surface was high with increased temperature.

Thermodynamics studies
The thermodynamic parameters including Gibbs free energy change (ΔG 0 ), entropy change where: K d is a thermodynamic equilibrium constant obtained using Eq. 3 (Sogut and Caliskan, 2017): where: ∆G 0 is the free energy change (kJ/mol), T is the solution temperature (K), R is the ideal gas constant (8.314J/K/ mol), C 0 is the initial phenol concentration and C e is the final phenol concentration.The values of ∆H 0 and ∆S 0 were obtained from the slope ( − ∆ 0 ) and intercept ( ∆ 0

𝑅𝑅
) of the Vant Hoff's plot of LnK d versus 1/T (Fig. 4).The calculated thermodynamic parameters are presented in Table 2.The negative value of ΔG 0 indicates the spontaneous nature of the adsorption of phenol on gypsum.The value of ΔG 0 was found to increase along with temperature, which implies that the adsorption of phenol on gypsum was favored at increasing temperature.In general, the free energy change for physical adsorption is between -20 and 0 kJ/mol, but for chemical adsorption it is in the range of -80 to -400 kJ/mol (Babakhouya et al., 2010;Atkins, 1990).This suggests that the adsorption of phenol on gypsum is a physical adsorption process.The positive value of ΔH 0 indicates the phenol adsorption process is endothermic in nature (Adeogun and Balakrishnan, 2015; Babakhouya et al., 2010).The positive ΔS 0 value obtained denotes the increase in the degree of disorderliness of the adsorbed species (AlOthman et al., 2014) and the affinity of the gypsum adsorbent for phenol.

Adsorption kinetics
Adsorption kinetic models are used to study the adsorption rate and the probable rate governing step.The kinetics of the adsorption of phenol on gypsum was studied using the pseudo-secondorder, Elovich, and intraparticle diffusion models.
The pseudo-second-order is stated as follows (Ho, 2006): where: q e and q t is the amount of phenol adsorbed per unit mass of the adsorbent at equilibrium and at time, t (mmol/g).K 2 is the pseudo-second-order rate constants (min −1 ).
The pseudo-second-order plot is shown in Fig. 5.The constants, q e , and K 2 (Table 3) were evaluated from the slope and intercept of the regression plot of t/q t versus t (Fig. 5), respectively.The value of the regression coefficient, R 2 (0.9997) implies that the adsorption kinetic data obeys the pseudo-second-order model more so than other models.A study on the removal of phenol using adsorption onto modified Pistacia mutica shells (Sarvani et al., 2018) also followed the model.
The Elovich kinetic model is expressed as (Riahi et al., 2017;Abdelkreem, 2013): where: α is the initial adsorption rate (mmol/g min) and β is related to the extent of surface coverage and the activation energy for chemisorption (g/mmol).The plot of q t versus Ln t is shown in   =    0.5 +   (8) where: c i is a constant that provides an idea about the thickness of the boundary layer, k pi is the intraparticle diffusion rate constant (mg/g min 0.5 ) and q t is the amount adsorbed (mmol/g) at time t (min).
The graph of q t versus t 0.5 is represented in Figure 7.The intraparticle constants, k pi and c i were calculated from the slope and intercept, respectively.The data conformed to the intraparticle diffusion model.The plot did not pass through the origin implying that the intra-particle diffusion is not the only rate limiting step (Ma et al., 2013).Generally, the phenol adsorption kinetic data fitted best into the pseudo-second-order kinetic model denoting a chemical adsorption process (Igwegbe et al., 2016).

Comparison with other adsorbents for phenol removal
The removal efficiencies and the adsorption capacities of phenol removal using different adsorbent materials are presented in Table 3.It indicates that gypsum is a potential material that can be applied for the reduction of phenol in aqueous environments, when compared with other materials.

CONCLUSION
The effectiveness of the adsorptive removal of phenol from its aqueous solution using gypsum (which is readily available) was investigated.The effect of gypsum mass, contact time and solution temperature on the adsorption process was studied.Phenol removal was found to increase along with adsorbent mass, contact time and temperature.The phenol adsorption process on gypsum was found to be spontaneous and endothermic in nature.The adsorption data fit into the pseudosecond-order, Elovich, and intraparticle diffusion kinetic models.The results obtained from the study showed that gypsum can act as a potential adsorbent for the removal of phenol from its aqueous solutions.

Table 1 .
Effect of gypsum mass, contact time and temperature on phenol removal at C 0 = 0.3809 mol/L

Table 2 .
Thermodynamics parameters calculated for phenol adsorption on gypsum

Table 3 .
Comparison of gypsum with other materials for phenol reduction