Efficient removal of lead(II) ions in water using functionalized poly(styrene) oligomers

.


Introduction
Adsorbent materials based on waste polymers such as polypropylene (PP)', polyethylene terephthalate (PET)?, poly(vinyl chloride) (PYC)?, polycarbonate (PC)*, polyethylene (PE) and poly(methyl-methacrylate) (PMMA)® have received great attention due to its low cost, high -adsorption -capacity, relatively easy regeneration, and the possibility to shape them into most suitable morphology-like spheres, fibers, films or membranes".According to recently literature on the removal of heavy metals, various adsorbent based on organic and mineral structures, biological and polymeric materials have been used so far.In fact, we have recently demonstrated that introduction ofacrylamide units inside of PS oligomer chains provide an interesting adsorbent material with a strong affinity for binding lead®.In spite of their demonstrated efficacy, the most of studies have been focused mainly on the study of adsorption properties and elucidation of adsorption mechanism while that optimization of variables that affect this process have been scarcely explored".In this regarding, optimal -adsorption conditions are -prerequisite, particularly in the case of scaled up applications, to minimize cost and maximize the adsorption efficiency towards heavy metals.Classical optimization, in which only one factor is changed at a time with the purpose to measure its effect, takes a lot of time and requires a large number of experiments.Conversely, the response surface methodology, -in -which key -parameters -are simultaneously optimized, overcomes the deficiencies of single factor optimization.It is important to remark that using RSM substantially reduces the number of experiments necessary to predict the optimal adsorption conditions!".Moreover, modeling the process refines the interpretation of complex phenomena and provides a basis for process scaling.In this research, the adsorption process of Pb(Il) from aqueous solution using an adsorbent material based on PS oligomers was simultaneously optimized using factorial design with the percentage removal as variable response.With the aim to select the best conditions for the removal of Pb(II), the 241 factorial design with replicates at the central point was complemented with the central composite design with axial points to build a quadratic polynomial model and to investigate the response surface space.

Materials and methods
Synthesis and characterization of chemically modified PS oligomers and their preliminary adsorption studies  1), where the independent variables under analysis were pH (A), initial adsorbate concentration (B), adsorbent dosage (C) and temperature (D) (See table 2).Optimal conditions for adsorption experiments based on the Batch mode were determinate by using the optimal predictor quadratic model ( 1): Where Y is the predicted response, b, is the constant coefficient, by; are the quadratic coefficients, b;; are the interaction coefficients, x; and x; are the coded factors of the independent variables considered for optimal conditions for adsorption of Pb(ID) ions using chemically modified PS oligomers.
On the other hands, the analysis of variance was used to measure the magnitude of the influence of the independent variables on the response factor.The adjusted determination coefficient (RÁ4;) was used as a measurement of the proportion of the total observed variability described for 1. Quadratic models were used to build the surface response of the adsorption of Pb(II) ions and to find the maximum value of response by using the statistical software Design Expert 9.0.6.2 (Stat-Ease Inc., USA).

Adsorption experiments
The adsorption experiments were carried out in a batch method'3, which consists of shaking 20.0 mL of Pb(11) in stoppered glass tubes according to the adsorbent dosage, pH, temperature, and initial adsorbate concentration indicated in table 1.The coded factors and their actual levels are given in Table 2.The percentage of removal (%) was calculated by equation (1) as follows: Where C, and Cy are the concentration at initial and equilibrium states (mg L), respectively.
The equilibrium amount qe (mg g) adsorbed per unit mass of adsorbent material was evaluated using equation @) 3 Where qe is the equilibrium amount of Pb(II) adsorbed per unit mass, V (L) is the volume of solution and W (g) is the mass of chemically modified PS oligomers.percentage adsorption increase by increasing pH from 2.00 to 5.80.This behavior could be attributed to the fact that at higher pH, the acrylamide units linked to PS oligomers would favored the protonated form, which will increase the number of protonated species and generates the electrostatic repulsion forces among the adjacent protonated terminal amide groups (See Scheme 1) .A similar tendency has been reported by Sarkar et al.,'5 for removal of malachite green dye using biodegradable graft copolymer derived from amylopectin and poly(acrylic acid).The author found that at pH > pHyze of adsorbent, the electrostatic attraction between negatively charged surface of adsorbent and positively charged adsorbate is enhanced, resulting in high malachite green dye removal.given in fig. 3. The micrograph analysis revealed that PS oligomers (Fig. 3a) exhibit many pores throughout the surface, while functionalized PS oligomers (Fig. 3b) show the formation of spherical aggregates with a wide distribution of sizes, and an apparent low porosity due to introduction of acrylamide units inside oligomer chains.

33
Statistical analysis With the aim to determine the ideal conditions for the removal of Pb(ID), the effect of four adsorption variables were studied using central composite design (CCD) and response surface methodology (RSM).The analysis of variance (ANOVA) used to determine the significance of curvature for absorption of Pb(II) ions at a confidential level of 95% is given in table 3. The analysis of these data revealed that the curvature is significant, which means that there is an inflection point on variable set under study.Hence, the linear model is not able to represent the design space and, for this reason, FCCD design was selected to fit the quadratic model considering the 18 experimental runs.
According to the ANOVA data for removal of Pb(1I), the pH resulted to be the most important quadratic term as well as the most significant individual factor while that adsorbent dosage (p = 0.0768) and initial adsorbate concentration (p = 0.0863) exhibited both less significance for adsorption of Pb(II) ions in comparison with the pH (Eq.( 3)).This is consistent with the results reported by Meenakshi ef al. '5 for adsorption of Cd(Il) and Pb(II) from aqueous solutions using poly(aniline) grafted chitosan copolymer as an adsorbent.In the case of other independent variables, double interactions as well as quadratic terms, the ANOVA analysis showed a non-significant with p-value in the 0.11-0.89range.On Enero -Marzo, 2023 María Concepción García López, er al base of these findings, we can assume that the binding of Pb(ID) with chemically functionalized PS oligomers is dependent of pH, adsorbent dosage, and initial adsorbate concentration as was suggested by analysis of variance.
Percentage ofremoval % = 23.13+ 3034 + 632x, + 4.14x,x; + 0.92x,x, +62402x5 + 11.652x) + 38725%, +33397 -( The adjust determination coefficient (R;) for removal Efficient removal of Iead(II) ions in water using functionalized poly(styrene) oligomers On the other hand, the mathematical model was used to build response surface plots and investigate the interactions among the independent variables as well as determine the optimal condition of each factor for the maximum adsorption of Pb(II).Fig. 5a) shows that the percentage removal of Pb(II) is increased at pH values above 5.26 when the adsorbent dosage and imitial adsorbate concentration are fixed.Moreover, fig. 5 b) and c) show that at adsorbent dosage below of 10 g and temperatures above 35°C, the percentage adsorption of Pb(I1) ions exhibits remarkable decrease.34 Optimization of Batch adsorption mode To determine the optimal conditions, equation ( 3) was used to maximize the percentage removal of Pb(II).Under these conditions, the maximum percentage removal of Pb(IN) (93.12 %) was predicted to occur at 38 °C, pH 5.80, an initial adsorbate concentration of 36.40 mg L', and an adsorbent dosage of 10.77 mg with a desirability function of 0.926.The analysis of response variable revealed that the percentage adsorption of metal ions is enhanced above pH 5.63 due to increase of electrostatic interactions between the surface of functionalized PS oligomers and positively charged ions.But if the temperature is lesser than 35°C, a decrease on the percentage adsorption of Pb(II) could be attributed to low mobility of heavy metals ions.On the contrary, at temperatures greater than 35°C, the percent of adsorption is reduced because increasing the kinetic energy decreases the efficiency of the removal process.Likewise, the effect of the initial adsorbate concentration on the percentage adsorption at concentrations below to 10.74 mg L' shows that the active sites on the adsorbent material are mot completely occupied.Thus, the interaction of heavy metal ions through the surface is not enhanced, decreasing percentage removal.To validate the predicted response, experimental assays were carried out at the optimal conditions giving a percentage removal of Pb(I) equal to 91%, which is similar or greater than other adsorbent materials derivatives from waste polymers (Table 4).

Conclusions
In summary, this study demonstrated the usefulness of a central composite design to determine the optimal conditions to enhance the percent of removal of Pb(Il) from aqueous solutions.
After the simultaneous optimization by quadratic model, the optimal conditions were obtained at 37.71%, an initial adsorbate concentration of 36.37 mg mL"', a pH of 5.80 and an adsorbent dosage of 10.75 g.At these conditions, the predicted response for the removal of Pb(II) was 93.12%, which is similar to the experimental value (91.23%).The percentage of removal determined under the optimal conditions by the simultaneous optimization showed a high percentage, which is similar or higher compared to with other adsorbent materials based on waste polymers.Lastly, the surface charge distribution, the structural and morphological -features -corroborate its -successful obtention and confirm it is use as an inexpensive, efficient, and available adsorbent for removal of heavy metal from aqueous systems.
Fig. 1 pH drift method to obtain pHrzc value for functionalized PS oligomers.

Fig. 4
Fig. 4 Predicted against experimental data plots for adsorption of Pb(II).

Fig. 5
Fig. 5 Response surface plots for adsorption of Pb(II) aqueous solutions.

Table 2
Independent variables and coded levels used in the experimental design