Factorial experimental design applied to adsorption of cadmium on activated alumina

The effective removal of heavy metals from industrial wastewater is a very important issue for many countries. This paper examines the removal of cadmium ions from aqueous solutions and industrial ef ﬂ uents byadsorption on activatedalumina. TheBrunauer – Emmett – Teller(BET) speci ﬁ c surfacearea, pore diameter and pore volume of the activated alumina were 156.7 m 2 /g, 58.4 Å and 0.23 cm 3 /g, respectively. Factorial experimental design was applied to evaluate the main effects and interactions among dose of activated alumina, initial cadmium concentration, pH of the solution and temperature. Analysis of variance, the F-test and the Student ’ s t -test shows that dose of activated alumina, initial cadmium ion concentration and temperature are the most signi ﬁ cant parameters affecting cadmium ion removal and pH is the least signi ﬁ cant parameter. Under optimal conditions, cadmium removal from industrial ef ﬂ uent samples was > 98%. Furthermore, desorption and regeneration studies were carried out in order to evaluate the cost-effectiveness of activated alumina.


INTRODUCTION
Heavy metals released into the environment pose a significant threat to the environment and human health because of their toxicity and persistence.
Cadmium is one of the most toxic heavy metals affecting humans, animals and plants; it has no known metabolic role and does not seem biologically essential or beneficial to the metabolism of living beings (Bhattacharyya ). Cadmium is mostly introduced into natural water resources by wastewater discharged from industrial effluents. The most common industries releasing cadmium in their effluents are metal plating; manufacture of cadmium-nickel batteries, plastic stabilizers, paints and pigments, and petrochemicals; and mining (Krika et al. ). When it enters the human body, most cadmium goes directly to the kidney and liver and persists for many years causing serious damage to these organs. Itai-itai, renal damage, emphysema, hypertension and testicular atrophy are all harmful diseases occurring in people exposed to cadmium (Lalor ; Recovered water is now a part of Tunisia's overall water resources balance. It is considered as an additional water resource and as a potential source of fertilizing elements; as a result, the legislation on industrial wastewater discharges has become increasingly strict. According to the Tunisian NT106.002 standard, the limit on concentration of cadmium to release into sewerage systems is 0.1 mg/L. Therefore, it is a great challenge to remove cadmium ions from wastewater. Removal of cadmium ions from aqueous solutions has been traditionally carried out by chemical precipitation. However, chemical precipitation is usually used to treat wastewater containing high concentrations of heavy metal ions and it is ineffective when the metal ion concentration is low. In addition, chemical precipitation can produce large amount of sludge which can be treated only with great difficulty. Membrane processes such as ultrafiltration, reverse osmosis and nanofiltration can remove cadmium ions with high efficiency, but problems such as process complexity, membrane fouling and low permeate flux have limited their use in cadmium removal. Flocculation-coagulation involves chemical consumption and generation of increased sludge volume. Electrocoagulation has also been used for the removal of cadmium from wastewater; however, the disadvantages of this method are the high cost and generation of toxic sludge (Fu & Wang ).
Adsorption, on the other hand, is considered as an ideal process because of its convenience, ease of operation, low operational cost and simplicity of design. One of the goals of this study is to apply a factorial design at two levels in order to determine the influence of various parameters and their interactions on the removal efficiency of cadmium and then assess the importance of the AA as adsorbent to remove cadmium from industrial effluent in Tunisia. In addition, regeneration studies were performed to estimate the potential of this process in industrial applications.

Materials
The granular AA used was provided by Sigma-Aldrich. It was dried at 110 W C for 24 hours in order to eliminate impurities and to prepare it. An aqueous stock solution of cadmium ions (1 g/L) was prepared using reagent grade Cd(NO 3 ) 2 .4H 2 O. Different initial concentrations of cadmium ions (Cd(II)) (10 mg/L to 100 mg/L) were prepared by dilution from the stock solution in distilled water.

Batch adsorption experiments
Adsorption experiments were carried out in a stirred thermostatic bath (Grant ® ) to study the effects of pH, initial were analysed to determine residual Cd(II) concentration.

Chemicals and analytical methods
The residual concentration of cadmium was determined by the potentiometric method using a specific electrode (Thermo Scientific, Orien 9448SC).
The solution pH was measured by a pH-meter (Metrohm, 708 pH meter). Chloride and nitrate ions were analysed by anion chromatography using a Metrohm 761 compact ion chromatograph. The analyses of Ca and Mg were conducted by the titrimetric method.
The percentage removal of cadmium (%Cd) was calculated using Equation (1): where C 0 and C e are the initial and equilibrium concentrations of Cd(II) respectively (mg/L).

Characterization of adsorbent
The total pore volume was determined from the adsorption of N 2 at 77.37 K on an ASAP 2020 apparatus. The surface area was calculated using the Brunauer-Emmett-Teller (BET) method. The pore size distribution was obtained from the desorption branch of isotherms using the Barrett-Joyner-Halenda (BJH) method. Detailed textural characteristics of AA are summarized in Table 1.
The textural properties were determined from the N 2 adsorption-desorption isotherms, which are given in Figure 1 together with the pore size distribution. According to the IUPAC classification, the corresponding isotherm can be classified as type IV which is characteristic of a mesoporous material. This is a good model for AA because of its tight pore size distribution and its massive surface areas, providing a vast number of sites where adsorption processes can occur.
The BJH pore size distribution also shows that AA has the pore size distribution in the mesoporous range (2-50 nm).
From the steepness of the adsorption isotherm, it can be seen that the mesopore structure is not well ordered and has a broad pore size distribution. Moreover, evidence of the appearance of open pores in the AA is shown by the presence of the hysteresis loop (Diallo et al. ).

Validation of the analytical method
Several parameters have been taken into account in order to validate the method for determining residual Cd(II) concentration by the potentiometric method using a specific electrode. We have evaluated linearity, specificity and fidelity (repeatability and reproducibility). In the whole validation, the calibration curve for the measurements was always prepared with at least six points, as recommended by the French standard XPT 90-210. Table 2 gives experimental validation of the analytical method.
According to the values of Table 2, the analytical method by specific electrode is valid and appears as an efficient method.

Effect of contact time
The effect of contact time was determined by studying adsorption of Cd(II) at initial concentrations of 10 mg/L and 100 mg/L with 1 g/100 mL of AA. As clearly seen in Figure 2, at up to 50 minutes of initial contact time, the Specific surface area (m 2 /g) 156.7 Total pore volume (cm 3 /g) 0.23 Average pore diameter (Å) 58.4

Effect of pH
The effect of pH was studied in the range 3-8 for both initial concentrations of 10 mg/L and 100 mg/L with 1 g/100 mL of AA. It was essentially found that the removal yield increases with pH. A decrease in the pH value (from 5 to 3) involves a decrease in the removal yield from 97% to 41% and from 65% to 30% for an initial concentration of 10 mg/L and 100 mg/L, respectively. This may be due to the modification of adsorbent surface below pH 5 (Kasprzyk-Hordern ).
Similar results have been reported in the literature.    Table 4, were executed in a random order to avoid systematic errors.
The effect of a factor is defined as the change in response produced by a change in the factor level.
The codified mathematical model employed for the factorial design was: where b 0 represents the global mean, b i represents the estimation of the principal effect of the factor i for the response %Cd and b ij represents the estimation of interaction effect between factor i and j. X 1 , X 2 , X 3 and X 4 are the dimensionless coded factors of the following parameters: dose of AA, initial Cd(II) concentration, pH and temperature, respectively. The results were analysed with MINITAB 16 software.

Effect of process variables
The main effects of each parameter on the Cd(II) removal efficiency are shown in Figure 5. From the analysis of the  graphs and the coefficients of Equation (3), we can conclude that the initial cadmium concentration is the most important variable on the cadmium removal efficiency since its coefficient is the largest in absolute value (20.23). The negative sign of this coefficient means that the intensification of this parameter decreases the amount of Cd(II) removal.
However, the effects of adsorbent dose (AA) and temperature are positive since an increase in percentage removal of cadmium is observed when these factors change from low to high. With an increase in adsorbent dose, the number of sites available for cadmium adsorption increases, which facilitates an increase in the percentage removal of Cd(II).
The pH of the solution is the least significant variable since its coefficient is the lowest in absolute value (5.17).
The interaction effects were also studied and are shown in Figure 6. The parallel lines in this figure indicate that there are no significant interactions between the studied factors. However, the most important interaction is observed between pH and adsorbent dose. This indicates that decreasing pH from 8 to 5 enhances the cadmium removal efficiency at low adsorbent dose (0.5 g). This interaction remains non-significant since the pH of the solution has no significant effect in the range of values studied.
For a better understanding of the relationship between factors and a response, a cube plot was produced (Figure 7).
The cube plot shows that increasing adsorbent dose from 0.5 to 1.5 g enhances significantly the Cd(II) removal (from 52.08% to 94.37%) at low temperature (10 W C), while at higher temperature (40 W C), changes in adsorbent dose do not have a greater effect (an increase of only 7.14%). In addition, increasing initial Cd(II) from 10 to 100 mg/L, at higher adsorbent dose (1.5 g), diminishes the percentage removal from 94.37% to 32.80% at lower temperature. A change of only 9.88% is observed at higher temperature.
This means that both the effect of variation of initial Cd(II) concentration and adsorbent dose are higher when the temperature is low.
The highest percentage removal in this study was 97.29%, obtained at higher temperature (40 W C), adsorbent dose of 1.5 g and initial cadmium concentration of 10 mg/L.

Application studies on industrial effluents
The real application of Cd(II) removal by adsorption on AA was performed on wastewater, containing cadmium ions, which was collected from a battery manufacturing plant in Tunisia. The sample was stored in a polyethylene container   in a refrigerator at a temperature below 4 W C until analysed.
To reduce the cadmium concentration and to possibly reuse the wastewater, batch adsorption studies using the sample were carried out under the optimum conditions found previously (adsorbent dose of 1.5 g and stirred in a thermostatic bath for 150 minutes at 40 W C). The physicochemical characteristics of the effluent before and after the adsorption process were measured. The results are summarized in Table 6. These results suggested that AA has an excellent potential application for the removal of cadmium from wastewater.   Desorption and re-adsorption studies Desorption and reusability of AA is an important step for practical application in wastewater treatment technology.
After Cd(II) adsorption with initial concentrations of 10 mg/L and 1.5 g of AA, the adsorbent was filtered and oven-dried at 80 W C, and the adsorbed Cd(II) was desorbed with 0.1 mol/L HCl. The desorbed Cd(II) was separated by filtration and analysed. The spent adsorbent after filtration was washed several times with deionized water to remove residual acid, and dried for repeated Cd(II) adsorption from aqueous solutions. Four successive cycles of adsorption and desorption of Cd(II) were carried out in the batch system to assess the reusability of AA for Cd(II) adsorption.
As shown in Figure 8, more than 94% Cd(II) removal is possible after four cycles of adsorption-desorption. The highest percentage removal of cadmium in this study was obtained at higher temperature (40 W C), adsorbent dose of 1.5 g and initial cadmium concentration of 10 mg/L.
The adsorption studies on industrial effluent under optimal conditions indicate that AA has good potential to remove cadmium from wastewater samples since Cd(II) concentration in the treated effluents were 0.07 mg/L thereby meeting the Tunisian NT106.002 standard. Moreover, the high surface area of AA (156.7 m 2 /g) and the advantage of recycling and reuse make it an attractive wastewater treatment option.