Materials and reagents

All chemicals used were of the analytical reagent grade and of highest purity, such as activated carbon, sodium alginate ([C₆H₇NaO₆]n, Qingdao Bright Moon), ascorbic acid (C₆H₈O₆, Shandong Luwei), lactose (C₁₂H₂₂O₁₁, Dfe pharma), sodium hydroxide (NaOH, Riedel–de Haën), hydrochloric acid (HCl, Scharlau), nitric acid (HNO₃, Columbus), potassium dichromate (K₂Cr₂O₇, Prolabo), ammonium molybdate ([NH₄]₆Mo₇O₂₄, P.O.C.H., Polska, Poland), potassium hydrogen orthophosphate (KH₂PO₄, Fluka Garantie), stannous chloride (TiCl₂, Lobacheme), sulfuric acid (H₂SO₄, sever biotech), ethanol 96% (C₂H₅OH, World co. for sub & med industries), mercuric(II) sulfate (HgSO₄, Lobachemie), silver sulfate (Ag₂SO₄, LABSCAN), and calcium chloride (CaCL₂, Fisher Scientific). The pH value of the aqueous medium was adjusted using 0.1 M of NaOH or HCl solutions.

Methods

Preparation of adsorbent

Adsorbent beads were prepared by adding 1 g of activated carbon to 2% of sodium alginate solution, stirred well. The mixture was added to a 5% CaCl₂ solution; the solution was filtrated and the adsorbent beads were taken out and washed for many times to be used in the adsorption, as shown in Figures 1 and 2.

Figure 1.

Preparation of alginate beads with entrapped activated carbon (AG-AC).

Figure 2.

The characteristic and morphology of beads.

Adsorption isotherm models

Langmuir and Freundlich isotherm models were used for describing the equilibrium adsorption data.^25,26 Langmuir assumes monolayer coverage of adsorbate over a homogeneous adsorbent surface. The linearized Langmuir isotherm model is given by the equation:

where q_e (mg/g) is the mass of solute adsorbed per mass of adsorbent used, C_e (mg/L) is equilibrium concentration of solute, 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.

The Freundlich isotherm 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 constants related to the adsorption capacity and adsorption intensity, respectively. K_f and n are evaluated by plotting ln q_e and ln C_e.

Procedure of adsorption experiment

When entrapped activated carbon in alginate polymer is placed in a solution containing ascorbic acid or lactose which agitated for an adequate time, adsorption occurs. Aqueous solutions (100 mL) were used and the adsorption capacity was determined at different environmental conditions. According to the standard method for wastewater analysis ASTM 2005, a withdrawn sample from the solution after filtration was analyzed for dissolved ascorbic acid and lactose concentrations by spectrophotometry. Then the adsorption capacity and isothermal studies were conducted for Freundlich and Langmuir adsorption isotherm.

The percentage of removal was calculated using the following equation:

where C_o is the initial concentration (mg/L) and Ce is the equilibrium concentration (mg/L) in solution. The amount of organic content adsorbed by entrapped activated carbon in alginate polymer was calculated using the following equation:

where q_e is the equilibrium adsorption capacity (mg/g), V is the volume of aqueous solution (L) and m is the weight of the adsorbent (g).

Effect of pH value

The influence of pH value on the amount of ascorbic acid and lactose 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 120 min), and the conditions used were as follows: the initial ascorbic acid and lactose concentration was 500 mg\L, the adsorbent dosage was 30 g\L, and the stirring rate was fixed at 100 rpm; with ascorbic acid removal efficiencies of 62%, 61%, 46%, 27%, and 21%; 71% 70%, 48%, 39%, and 34%; and 72%, 71%, 48%, 40%, and 34%; and lactose removal efficiencies of 45%, 44%, 26%, 18%, and 15%; 52%, 50%, 33%, 22%, and 19%; and 53%, 51%, 35%, 23%, and 19%, respectively, and plots of the pH against the percentage of the ascorbic acid and lactose that was removed from the solution is shown in Figure 3. The optimum pH for the removal was 3. The pH of the aqueous solution plays a decisive role in affecting ascorbic acid and lactose adsorption. 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 surface of the adsorbent. On the contrary, 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.^{27282930313233–34}

Figure 3.

Effect of pH on ascorbic acid and lactose removal.

Effect of contact time

The contact time is another important operational factor that affects removal efficiency. Figure 4 depicts ascorbic acid and lactose removal as a function of contact time. From Figure 2, it is shown that an increase in contact time increased removal efficiency. As shown, the ascorbic acid and lactose uptake by entrapped activated carbon in alginate was very rapid within the first 15 minutes. After 15 minutes, the uptake of ascorbic acid and lactose progressively decreased with time. As the treatment time proceeded, the adsorbent sites had the inclination toward saturation. Equilibrium was established at 60 minutes; other factors such as pH, stirring rate, and adsorbent dose were 3, 100 rpm, and 30 g, respectively, with ascorbic acid removal efficiencies of 57%, 61%, 70%, 71%, 72%, and 72%) and lactose removal efficiencies of 41%, 44%, 50%, 51%, 52%, and 53%, as shown in Figure 4. Increased in time leads to increase in the contact between the solution to the larger surface area of adsorbent as there are many adsorption sites.^3536–37

Figure 4.

Effect of contact time on ascorbic acid and lactose removal.

Effect of adsorbent dose

Figure 5 depicts ascorbic acid and lactose removal efficiency as a function of adsorbents dosage. The adsorbent doses were varied between 10 to 50 g/L, other operational factors pH, contact time, initial concentration, and stirring rate were 3, 60 min, 500 mg\L, and 100 rpm, respectively, with ascorbic acid removal efficiencies of 57%, 63%, 70%, 72%, and 76% and lactose removal efficiencies of 34%, 41%, 50%, 56%, and 60%, as shown in Figure 5. As expected, at high adsorbent does the removal increased because of the increased adsorbent surface area and the number of available adsorption sites increased. The optimum adsorbent dose for ascorbic acid and lactose removal was 30 g, as shown in Figures 6 and 7, respectively.^{35,37,3839–40}

Figure 5.

Effect of adsorbent dose on ascorbic acid and lactose removal.

Figure 6.

Optimum effective dose for ascorbic acid removal.

Figure 7.

Optimum effective dose for lactose removal.

Effect of stirring rate

Figure 8 depicts ascorbic acid and lactose removal efficiency by entrapped activated carbon in alginate as a function of stirring rate. The stirring rate was varied between 100 and 400 rpm, other operational factors such as pH, contact time, initial concentration, and adsorbents dosage were 3, 60 min, 500 mg\L, and 30 g\L, respectively, with ascorbic acid removal efficiencies of 70%, 71%, 72%, and 72%; and lactose removal efficiencies of 50%, 51%, 51%, and 52%, as shown in Figure 8. The optimum stirring rate for ascorbic acid and lactose removal was 100 rpm. The increase in stirring speed resulting in increase in ascorbic acid and lactose percentage removal was due to the fact that increase in stirring rate enhanced the diffusion of ascorbic acid and lactose contaminants on the surface of the adsorbent.⁴¹

Figure 8.

Effect of stirring rate on ascorbic acid and lactose removal.

Effect of the initial ascorbic acid and lactose concentration

The effect of concentration of the aqueous solution on the percent ascorbic acid and lactose reduction by entrapped activated carbon in alginate was studied at various initial concentrations varying from 100 to 1000 mg\L, other operational factors such as pH, contact time, stirring rate, and adsorbents dosage were 3, 60 min, 100 rpm, and 30 g\L, respectively, with ascorbic acid removal efficiencies of 79%, 75%, 73%, 71%, 70%, 67%, 66%, 64%, 62%, and 60%; and lactose removal efficiencies of 62%, 58%, 55%, 51%, 50%, 48%, 46%, 44%, 42%, and 39%, as shown in Figure 9. It can be observed that adsorption was lower at higher concentrations of ascorbic acid and lactose and vice versa.^32,42,43

Figure 9.

Effect of initial concentration on ascorbic acid and lactose removal.

Adsorption isotherm study

Adsorption capacity of the adsorbent was predicted and evaluated by adsorption isotherm study.⁴⁴ The Freundlich and the Langmuir equations are the most common isotherms applications used for water and wastewater treatment.^19,45 The most commonly used model for measuring the sorption of organic compounds from wastewater is the Freundlich isotherm.^19,46 Freundlich and Langmuir isotherm has acquired vogue because of their ability to fit a variety of sorption data.⁴⁷ Freundlich isotherm model for ascorbic acid with correlation coefficient (R²) around 0.996, n around 1.421, and K_f around 3.12 showed better fit of adsorption data than Langmuir isotherm model with R² around 0.983, q _max around 34.722, and KL around 0.0037, as shown in Figures 10 and 11, respectively. Freundlich isotherm model for lactose with R² around 0.992, n around 1.485, and K_f around 5.22 showed better fit of adsorption data than Langmuir isotherm model with R² around 0.991, q _max around 21.692, and KL around 0.0025, as shown in Figures 12 and 13, respectively. Freundlich model indicates that it is more appropriate than the Langmuir isotherm model.

Figure 10.

Freundlich isotherm of ascorbic acid removal.

Figure 11.

Langmuir isotherm of ascorbic acid removal.

Figure 12.

Freundlich isotherm of lactose removal.

Figure 13.

Langmuir isotherm of lactose removal.

Statistical analysis

The effect of the variables pH, contact time, dose, concentration, and stirring rate on the removal process of ascorbic acid and lactose has been studied using the entered method, where it was found that R² = 0.966 and 0.947, and the studied variables profane occupy more than 96% and 94% of the total of the variables affecting the removal process as that the standard error of the estimate are 3.1264 and 4.34099, respectively, which means that the percentage of error in this study is very low.

Analysis of variance was applied and the data were given in Tables 1 and 2. The tables show the sum of squares and the effect of the whole model. It was observed that the P value is .000, where the model is considered successful if P value is less than .05. From Tables 3 and 4, it can be inferred that all variables had an effect on the removal process except stirring rate, where the P value is larger than .05, which means that it can be neglected during the removal process.

Table 1.

Ascorbic acid analysis of variance.^a

Table 2.

Lactose analysis of variance.^a

Table 3.

Ascorbic acid coefficients.^a

Table 4.

Lactose coefficients.^a

Analysis of statistical and practical results correlation

The effect of different operating parameters was investigated using Linear regression analysis using SPSS Statistics, where the obtained results support the practical results. The results showed that the effect of pH, adsorbent dose, stirring rate, and concentration has a significant effect on the removal process where the P value is less than .05. By applying the B values shown in coefficient tables, the removal equation can be deduced:

where X1 is the effect of pH (2, 4, 6, 8, 10, and 12), X2 is the effect of contact time (15, 30, 60, 120, 180, and 240 minutes), X3 is the effect of adsorbent dose (10, 20, 30, 40, and 50 g/L), X4 is the effect of stirring rate (100, 200, 300, 400, and 500 rpm), and X5 is the effect of concentration (100, 200, 300, 400, and 500 mg/L).

Application to a real sample

Sample has been analyzed before and after treatment according to Egyptian law No. 93\1962 and its decree No. 44\2000 concerning discharge final effluent to public sewer system, as shown in Table 5. The sample was collected from Chemical Industries Development Company where pharmaceuticals wastewater is characterized by high values of COD and BOD. The removal efficiency on the sample by using about 30 g\L of entrapped activated carbon in alginate, initial pH 3.4, contact time 60 minutes, and the stirring rate fixed at 100 rpm is shown in Table 6.

Table 5.

Maximum effluent concentrations set by law 93/1962 and decree 9/1989.

Table 6.

Analysis of the sample.