The aim of this study is to decrease different wastewater contaminants using entrapped activated carbon in alginate polymer (AG-AC) by adsorption. Different phosphate concentrations were prepared in the laboratory. The effect of the operating parameter was studied by external factors (e.g., contact time, pH, concentrations, adsorbent dose, and stirring rate). The results were analyzed according to the Freundlich and Langmuir adsorption isotherm. The adsorption data are more appropriate by the Freundlich isotherm. Efficient removal percentages for COD (500 mg\L) at pH 3 using dose 30g of the adsorbent for 60min with a fixed stirring rate at 100rpm were about (79%). Efficient removal percentages for phosphates (5mg\L) at pH 4 using dose 30g of the adsorbent for 45min with a fixed stirring rate at 100rpm were about (89%). The best-operating conditions have been determined to increase phosphate removal efficiency.

Keywords: entrapped AG-AC, COD, phosphate, adsorption study

Water is the basis of life on the Earth and the most important natural resources after air for all living organisms. During the natural cycle of water, humans capture and divert a lot of the water, it returned to the environment in a different condition as wastewater.^1–3 Pharmaceutical wastewater is a source of a variety of hazardous products (e.g. COD and phosphates) are typically produced in batch processes, quantities of water used for washing or extraction or washing of equipment which has negative effect on human health, so the removal of these contaminants has scientific and political interest.^4–10 The removal of pharmaceutical contaminants from wastewater receives interest scientific and political attention.^5,11–15 Chemical oxygen demand (COD) is a measure of the amount of organic matter which presence in water. Several studies carried out to reduce the content of COD of wastewater by using different methods of treatment. Water quality has many criteria and standards which COD test is one of the limits show the water quality where a high proportion of organic matters and low water quality. Pollutants can evaluate as COD which presence of it indicates the presence of pollutants.^16–20 Phosphate, even in a very low concentration, as one of the main species responsible for eutrophication, which is a hazardous problem concerning water contamination. Because of technical and economic drawbacks, phosphate removal and recycling technologies have not yet been widely adopted.^20–27 Therefore, current researchers have been focused on modification methods that more adequately address in contaminant removal from industrial wastewater.^28,29

Activated carbon has a high adsorption capacity,^30–34 but its cost in wastewater treatment the dispersion enough and not easy because of the dispersion of the powder. To overcome this issue and increasing the removal efficiency through entrapment activated carbon with other material^32,36 such as alginate polymer which helps in adsorption process where allows polluted aqueous medium to pass through it and be in contact with activated carbon.^32,35–40 It has become one of the best effective and economics processes for treatment of wastewater, thus this method has agitated great concern during the latest years.^41–44 The focus of this study was to examine the probability of entrapped activated carbon in alginate polymer for the removal of COD and phosphate from an aqueous medium. In supplement, the Freundlich and Langmuir isotherms are used to fit the data and it noted that adsorption is better explained by the Freundlich model.

Materials and reagents

All chemicals used were of the analytical reagent grade and of the highest purity. The pH acidity of the aqueous solution was adjusted using 0.1M NaOH and 0.1 M HCl solutions.

Preparation of Adsorbent

Alginate beads prepared by adding 1g of activated carbon to 2% of sodium alginate solution, stirred well. The mixture solution added drop by drop to a 5% CaCl₂ solution. The alginate beads formed taken out and washed several times to be used in the adsorption method.

Preparation of standard solutions

The standard concentrations were prepared according to Standard Methods for the Examination of Water and Wastewater, 22 editions, 2013. Required concentrations of phosphate solutions were prepared from the stock solution.

Adsorption isotherm models

Freundlich isotherm: The Freundlich isotherm⁴⁵ is an empirical equation employed to describe heterogeneous adsorption surface and is given by:

Where K_f ((mg/g) (mg/L)^-1/n) and n (dimensionless) are Freundlich constant related to the adsorption capacity and adsorption intensity, respectively. (K_f) and (n) evaluated by plotting ln q_e and ln C_e.

Langmuir isotherm model

Langmuir isotherm model assumes monolayer coverage of adsorbate over a homogeneous surface of adsorbent.⁴⁶ The Langmuir linearized model is given by the equation:

Where q_e (mg/g) is, the mass of phosphate adsorbed per mass of adsorbent used, C_e (mg/L) is equilibrium concentration of phosphate, q_max (mg/g) is the maximum monolayer capacity of adsorption, and K_L (L/mg) is the Langmuir constant related to binding sites affinity and adsorption energy. The plot of C_e/q_e versus C_e employed to generate the values of q_max and K_L.

Procedure of adsorption experiment

Alginate beads with entrapped activated carbon added to a solution of chemical oxygen demand (COD) and phosphates solution containing different concentrations. Under various conditions which include: Effect of (pH, contact time, dose, concentrations and stirring rate) and mixing the adsorbent with solution, filter solution through glass fiber filter paper (WHATMAN 1441-125) and take specific amount of filtrate and reagents carefully in test tube, then concentrations evaluated according to reference method SM4500 from 22^nd Edition of Standard Methods for the Examination of Water and Wastewater. Then the adsorption capacity and isothermal studies conducted for Freundlich and Langmuir equations.

Calculation of adsorption capacity

The efficiency of removal determined by calculation of the uptake percent (Uptake %) and the amount of adsorbed per gram of the adsorbent (Q_e), as the following equations: -

The uptake percent measured as: -

The uptake per gram calculated from a metal mass balance yielding: -

Where, (Q_e) is mg metal ions per g adsorbent; (V) is the reaction volume (L), (C₀) and (C_e) are the initial and residual metal concentrations (mg/L), respectively, and (m) is the number of ads sorbent (g).⁴⁷

Chemical oxygen demand (COD) removal

Effect of pH: The influence of pH value on the amount of COD removed by entrapped activated carbon in alginate from the aqueous solution was estimated by carrying out experiments with different solution pH (1, 3, 5, 7 and 9) at different contact time (30, 60 and 120min), and plots of the pH against the percentage of COD that was removed from the solution are shown in Figure 1. The conditions used were: the initial COD concentration was 500mg\L, the adsorbent dosage 30g and the stirring rate fixed at 100rpm. The optimum pH for the removal was 3. The pH of the aqueous solution plays a decisive role in affecting COD adsorption. Similar results were reported in the scientific literature for the removal of COD.^48,49 At lower pH levels, the removal sharply increased because the positively charged functional groups of organic molecules bind through electrostatic attraction to the negatively charged of the surface of the adsorbent. On another hand, at higher pH the reduction in adsorption due to the increase of hindrance to organic ions diffusion because of that the abundance of (OH)^- ions leads to repulsion between the organic molecules and the surface of the adsorbent.^50–55 

Figure 1 Effect of pH on COD removal.

Effect of contact time: The contact time is important operational factor affect removal efficiency. Figure 2 depicts COD removal as a function of contact time. From the Figure 2, it is shown that an increase in contact time increased removal efficiency (Table 2). As shown, the COD uptake by entrapped activated carbon in alginate was very rapid within the first 15min. After 15min, the uptake of COD progressively decreased with time. Similar results were reported in the scientific literature for the adsorption of COD.^29,49–57 As the treatment time proceeded, the adsorbent sites had the inclination toward saturation. Equilibrium was established at 60min; other factors such as pH, initial COD concentration, stirring rate, and adsorbent dose were 3,500mg\L, 100rpm, and 30g, respectively. Increasing in time leads to increase in the contact between the solution to the larger surface area of adsorbent as there are many adsorption sites.^58–61

Figure 2 Effect of contact time on COD removal.

Effect of adsorbent dose: Figure 3A depicts COD removal efficiency as a function of adsorbents dosage. The adsorbent doses were varied between (10 and 50)g/L, other operational factors (pH, contact time, initial COD concentration and stirring rate) were 3, 60min, 500mg\L, and 100rpm. The optimum adsorbent dose for COD removal was about 30g, as shown in Figure 3B. Similar results were reported in the scientific literature for the adsorption of COD (Table 3).^62,63 As expected, at high adsorbent does the removal increased because of the increased adsorbent surface area and the number of available adsorption sites increased.^8,58,63,64

Figure 3A Effect of adsorbent dose on COD removal.

Figure 3B The optimum effective dose for COD removal.

Effect of stirring rate: Figure 4 depicts COD removal efficiency by entrapped activated carbon in alginate as a function of stirring rate (Table 4). The stirring rate was varied between 100 and 500rpm, other operational factors (pH, contact time, initial concentration and adsorbents dosage) were 3,60min, 500mg\L, and 30g\L, respectively. The optimum stirring rate for COD removal was 100rpm. By increasing in stirring speed resulting to increasing in COD percentage removal, was due to the fact that, increasing in stirring rate enhanced the contaminants diffusion of COD content on the surface of the adsorbent.^63,65

Figure 4 Effect of stirring rate on COD removal.

Effect of the initial concentration: The effect of concentration of the aqueous solution on the percent COD reduction by entrapped activated carbon in alginate was studied at various initial concentrations, as shown in Figure 5A, other operational factors (pH, contact time, stirring rate and adsorbents dosage) were 3,60min, 100 rpm, and 30g\L, respectively. It can be observed that adsorption was lower at higher concentrations of COD and vice versa (Table 5A).⁶³ 

Figure 5A Effect of initial concentration on COD removal.

Adsorption isotherm study on COD removal

The sorption capacity of the adsorbent predicted and evaluated by Adsorption isotherm study (Table 5B). The adsorption equilibrium data obtained at different initial COD concentrations were described using four different isotherm models, such as the Freundlich and Langmuir equations are the most two common isotherms applications used for wastewater treatment, under predefined conditions of pH, initial concentrations, adsorbent dose, contact time and stirring rate).^65–70 The acceptability and suitability of the isotherm equation to the equilibrium data were based on the values of the correlation coefficients, R² estimated from linear regression of the least square fit statistic on Micro Math Scientist software. Figure 5B&C represents the adsorption isotherms for the two models. The adsorption data were fitted well with the Freundlich Isotherm model with the highest R² in their categories as shown in Table 1. The n value >1 from Freundlich and maximum adsorption capacity of 38.96 (mg/g) from Langmuir with K_L<1 obtained are the indication that the adsorption is favorable on the investigated adsorbent.

Figure 5B Freundlich adsorption isotherm for COD contributing component.

Figure 5C Langmuir adsorption isotherm for COD contributing component.

Phosphate removal

Effect of pH: The influence of pH value on the amount of phosphate ions removed by entrapped activated carbon in alginate from the aqueous solution was estimated by carrying out experiments with different solution pH (2, 4, 6, 8, 10 and 12) at different contact time (30, 45 and 60min), and plots of the pH against the percentage of the phosphate ions that was removed from the solution are shown in Figure 6A. The conditions used were: the initial phosphate ions concentration was (5mg\L), the adsorbent dosage (30g) and the stirring rate fixed at 100 rpm. The optimum pH for the removal was 4. The pH of the aqueous solution plays a decisive role in affecting ascorbic acid and lactose adsorption (Table 6). Similar results were reported in the scientific literature for the adsorption of.^71–74 In most systems, the absorption of anions like phosphate decreases with the increase of pH and surface charge becomes more negative.^75,76 The surface of the adsorbent became positively charged in acidic medium and negatively charged in alkaline medium. Phosphate ions are negatively charged and it appears that there are repulsion forces between ions and the negatively charged adsorbent surface in alkaline medium because of that abundance of (OH)^- ions and it appears that there are repulsion forces between phosphate ions and the negatively charged adsorbent surface, as shown in Figure 6B.^72–75

Figure 6A Phosphate sorption by on (AG-AC) under various pH conditions.

Figure 6B Effect of pH on phosphate removal.

Effect of contact time: The contact time is important operational factor affect removal efficiency. Figure 7 depicts phosphate removal as a function of contact time (Table 7). From Figure 7, it is shown that by increasing in contact time lead to increase removal efficiency. As shown, the phosphate uptake by entrapped activated carbon in alginate was very rapid within the first 15min. After 15min, the uptake of phosphate progressively decreased with time. As the treatment time proceeded, the adsorbent sites had the inclination toward saturation. Equilibrium was established at 45min; other factors such as pH, initial phosphate concentrations, stirring rate, and adsorbent dose were 4, 5mg\L, 100rpm, and 30g, respectively. Similar results were reported in the scientific literature for the adsorption of phosphate.^26,77 By Increasing in time leads to increase in the contact between the phosphate ions to the larger surface area available of adsorbent as there are lots of free active sites for the adsorption.^{40,58,59,63,75}

Figure 7 Effect of contact time on phosphate removal.

Effect of adsorbent dose: Figure 8A depicts phosphate removal efficiency as a function of adsorbents dosage. The adsorbent doses were varied between 10 and 50g/L, other operational factors (pH, contact time, initial phosphate concentrations and stirring rate) were 4, 45min, 5mg\L, and 100rpm, respectively. The optimum adsorbent dose for phosphate ions removal was found to be about 25g (Figure 8B). As expected, at high adsorbent does the removal increased because of the increased adsorbent surface area and the number of available adsorption sites increased (Table 8).⁸⁰

Figure 8A Effect of adsorbent dose on phosphate removal.

Figure 8B The optimum effective dose for phosphate removal.

Effect of stirring rate: Figure 9 depicts phosphate removal efficiency by entrapped activated carbon in alginate as a function of stirring rate. The stirring rate was varied between 100 and 500rpm, other operational factors (pH, contact time, initial concentration, and adsorbents dosage) were 4, 45min, 5mg\L, and 30g\L, respectively. The optimum stirring rate for phosphate ions removal was found to be 100 (rpm). Similar results were reported in the scientific literature for the adsorption of phosphate.^78,79 Increasing in stirring rate, enhanced the phosphate ions diffusion to the surface of the adsorbent (Table 9).

Figure 9 Effect of stirring rate on phosphate removal.

Effect of the initial concentration: The effect of concentration of the aqueous solution on the percent phosphate reduction by entrapped activated carbon in alginate was studied at various initial concentrations, as shown in Figure 10A, other operational factors (pH, contact time, stirring rate and adsorbents dosage) were 4, 45 min, 100rpm, and 30g\L, respectively (Table 10A). At the beginning of the adsorption process, the removal efficiency was higher because of the great number of available adsorption active sites of adsorbate molecules but it decreased with time gradually due to the saturation and diminished of this sites, also caused a reduction in removal ratio.

Figure 10A Effect of initial concentration on phosphate removal.

Adsorption isotherm study for phosphate removal

The sorption capacity of the adsorbent predicted and evaluated by Adsorption isotherm study (Table 10B). The adsorption equilibrium data obtained at different initial phosphate concentrations were described using two different isotherm models, such as the Freundlich and Langmuir equations are the most two common isotherms applications used for wastewater treatment, under predefined conditions of pH, initial concentrations, adsorbent dose, contact time and stirring rate). The acceptability and suitability of the isotherm equation to the equilibrium data were based on the values of the correlation coefficients, R² estimated from linear regression of the least square fit statistic on Micro Math Scientist software. Figures 10 B&C represents the adsorption isotherms for the four models. The adsorption data were fitted well with the Freundlich Isotherm model with the highest R² in their categories]. The n value >1 from Freundlich and maximum adsorption capacity of 0.34mg/g from Langmuir with K_L<1 obtained are the indication that the adsorption is favorable on the investigated adsorbent.^81,82

Figure 10B Freundlich adsorption isotherm for phosphates contributing component.

Figure 10C Langmuir adsorption isotherm for phosphates contributing component.

In this study, the results showed that entrapped activated carbon in alginate polymer is capable of COD and phosphate removal from aqueous solution. Various operating parameters on phosphate and COD removal efficiency investigated and optimized. Removal affected by the experimental parameters such as contact time, dosage, pH, stirring rate, initial concentration. The entrapped activated carbon in alginate polymer be a cost-effective alternative and can lead to success in wastewater treatment and produce high-quality treated effluent.

None.

Author declares there is no conflict of interest towards the manuscript.

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