Synthesis of citric acid-modified resins and their adsorption properties towards metal ions

Types of resins anchored on citric acid were synthesized and identified. The citric acid-modified resins PS-CA, PS-O-CA and PS-N-CA were synthesized by anchoring citric acid on PS-Cl, PS-OH and PS-NH2, respectively. The PS-CA, PS-O-CA and PS-N-CA were used to adsorb Fe3+, Al3+, Cu2+, Pb2+, Cd2+ and Hg2+. The influences of pH, adsorption time and metal ion concentration on the adsorption capacities of the resins were investigated. After optimization, PS-CA was a good adsorbent to Fe3+, Cu2+, Pb2+ and Cd2+ with qm values of 143.9 mg g−1, 77.4 mg g−1, 18.9 mg g−1 and 119.9 mg g−1, respectively. PS-N-CA was a good adsorbent to Al3+ and Hg2+ with qm values of 176.6 mg g−1 and 114.9 mg g−1, respectively. The adsorption kinetics and adsorption isotherm experiments indicated that the pseudo-first-order rate equation was more appropriate for characterizing the kinetic data and the Langmuir model was more suitable for fitting the equilibrium data. The reusability of the citric acid-modified resins was also evaluated and these resins exhibited considerable reusability.


Introduction
The presence of metal ions in water resources is one of the most common environmental pollutions in developing countries [1]. Metal ions are steadily accumulated in the living environment and cause serious damage to the nervous system, reproductive system, kidney, liver and brain of animals and human beings [2,3]. In order to remove metal ions, several methods, such as oxidation, precipitation, ion-exchange and adsorption, are widely adopted in water treatment [4][5][6][7]. Among them, the adsorption method is  of great interest because of its easy operation and low cost [8]. Chelating resins acting as adsorbents have been widely studied recently for their good combining capacities and adsorption capacities towards metal ions [9]. Furthermore, metal ions adsorbed on chelating resins could also be efficiently desorbed, which makes resins recyclable. Functional groups, such as -OH, −COOH and −NH 2 , in resins are the main acting sites that could combine with metal ions. For instance, Mohammad et al. synthesized a new chelating resin functionalized with 2,3-dihydroxybenzoic acid and found that this new chelating resin exhibited good adsorption capacity towards Fe 3+ [9]. Jermakowicz-Bartkowiak et al. have synthesized aminoguanidyl group-modified resins and these resins were good adsorbents for gold [10]. These works revealed that anchoring appropriate functional groups on resins is the key procedure to obtain chelating resins with high adsorption capacities towards metal ions.
Citric acid is a common organic acid which has good chelating ability towards metal ions. It has been widely used as a cleaning agent for removing metal ions. The chemical structure of citric acid is shown in figure 1. The hydroxyl group and carboxyl groups in the structure of citric acid are the active groups which account for its good metal-chelating ability. These groups could also be used as linking groups for loading citric acid on an insoluble macromolecule supporter. In recent years, some citric acid-modified plant substrates were synthesized and used to adsorb heavy metal ions. For instance, citric acid-modified Ceiba pentandra hulls, soya bean straw and pine sawdust exhibited good abilities to adsorb Cd(II), Cu(II), Ni(II), Pb(II) or Zn(II) ions [11][12][13][14]. These results revealed that citric acid is an ideal molecule that could be used as functional group for the synthesis of new adsorbents with considerable adsorption capacities towards metal ions. Besides plant substrates, polystyrene resins are other common support materials which have the advantages of stable mechanical properties, controllable pore structure, good permeability, low cost and easy recovery. However, to date, loading citric acid on resins has not been reported. Compared with traditional weak acid resins, the citric acid-modified resins may have several advantages. The three carboxyl groups in the structure of citric acid would be conducive to enhance metal-chelating ability towards highly charged metal ions. Furthermore, the lone pair electrons in linking atoms (O or N) are also conducive to enhance metal-chelating ability towards metal ions. Thus, loading citric acid on resins is promising to obtain new chelating resins with considerable adsorption capacities towards metal ions.
In this work, new types of chelating resins were synthesized by anchoring citric acid on polystyrene resins. The adsorption behaviours of citric acid-modified resins towards metal ions (Fe 3+ , Al 3+ , Pb 2+ , Cu 2+ , Cd 2+ and Hg 2+ ) were also investigated. The results revealed that PS-CA was a good adsorbent to Fe 3+ , Cu 2+ , Pb 2+ and Cd 2+ with q m values of 143.9 mg g −1 , 77.4 mg g −1 , 18.9 mg g −1 and 119.9 mg g −1 , respectively. PS-N-CA was a good adsorbent to Al 3+ and Hg 2+ with q m values of 176.6 mg g −1 and 114.9 mg g −1 , respectively. The desorption experiments indicated that these resins exhibited considerable reusability.

Calculation of adsorption capacity
The adsorption capacities of the citric acid-modified resins towards metal ions were calculated according to the following equation: where q e is the adsorption capacity (mg g −1 dry resin) of resin towards metal ions at adsorption equilibrium. C 0 and C e are the initial and equilibrium concentrations of metal ion solutions (mg ml −1 ). V 0 is the volume of the metal ion solutions. W is the weight of dry chelating resins used (g).

The influence of pH on adsorption capacities
The influence of pH on adsorption capacities of resins towards metal ions was determined as follows: 50 ml of metal ion solutions (C 0 = 200 µg ml −1 ) with different pH was shaken with pretreated 0.10 g dry chelating resins in a 100 ml stoppered conical flask in an SHA-B incubator (100 r.p.m.) for 24 h. Subsequently, the concentration of metal ions in the adsorption solution was determined by an atomic absorption spectrophotometer (AAnalyst300).

Adsorption kinetics of citric acid-modified resins towards metal ions
The evaluation of kinetics is of benefit for the prediction of adsorption time and sample concentration. In this study, the adsorption kinetic curves of metal ions on the citric acid-modified resins were obtained as follows: 50 ml of metal ion solutions (C 0 = 200 µg ml −1 ) was shaken with pretreated 0.10 g dry chelating resins in a 100 ml stoppered conical flask in the SHA-B incubator (100 r.p.m.). Subsequently, the concentration of metal ions in the adsorption solution was determined at different times until equilibrium.

Adsorption isotherms of chelating resins towards metal ions
The equilibrium adsorption isotherms of metal ions on the chelating reins were obtained as follows: solutions (50 ml) with different concentrations of metal ions (C 0 = 25, 50, 100, 150 and 200 µg ml −1 ) were contacted with 0.10 g dry chelating resins in conical flasks. The flasks were continually shaken for 2 h (100 r.p.m.). Then, the concentrations of metal ions in the adsorption solutions were determined.

Characterization of the modified resins
Curves a1 and a2 in figure 3 are the Fourier transform infrared (FTIR) spectra of PS-Cl and PS-CA, respectively. In the spectrum of PS-Cl (a1), there was an absorption band in the vicinity of 678 cm −1 that was the stretching vibrations of the C-Cl bond [16]. In the spectrum of PS-CA (a2), the carboxyl groups were disclosed by the absorption peaks at 1721 cm −1 and 3426 cm −1 , which were the stretching vibration bands of -C=O and -OH, respectively. This result revealed that citric acid was successfully anchored on resins. In addition, the absorption band in the vicinity of 678 cm −1 still existed in curves a1, which indicated that the -Cl groups were not fully substituted by citric acid during the reaction [17,18]. Curves b1, b2 and b3 in figure 3 are the FTIR spectra of PS-Cl, PS-OH and PS-O-CA, respectively. Comparing with the curve of PS-Cl (b1), the new absorption peak of 3408 cm −1 in curve b2 was the stretching vibration band of -OH. This result indicated that PS-OH was successfully obtained on the basis of PS-Cl. The absorption peak at 1724 cm −1 in curve b3 was the stretching vibration band of −C=O revealing that citric acid was successfully anchored on resins through esterification. Curves c1 and c2 in figure 3 are the FTIR spectra of PS-NH 2 and PS-N-CA, respectively. Comparing with curve c1, the absorption peak at 1718 cm −1 was the stretching vibration band of −C=O. This result implied that citric acid was successfully anchored on resins through amidation. Based on the gravimetric method, the contents of citric acid in PS-CA, PS-O-CA and PS-N-CA were 0.84 mmol g −1 , 0.79 mmol g −1 and 0.93 mmol g −1 , respectively. Physical properties of citric acid-modified resins were determined by N 2 adsorption/desorption isotherms at 77 K using an ASAP 2020 automatic surface area and porosity analyser. The Brunauer-Emmett-Teller (BET) surface area was obtained by the BET method. The pore volume and average pore diameter were obtained by the Barrett-Joyner-Halenda method. Figure 4

The influence of pH on adsorption capacities
The influence of pH on adsorption capacities of resins towards metal ions was noted and the results are shown in figure 5. Considering that metal ions would be hydrolysed in high pH, the experiments were carried out at low pH values where metal hydroxide chemical precipitation does not occur. Thus, the pH of Fe 3+ , Al 3+ , Pb 2+ , Cu 2+ , Cd 2+ and Hg 2+ was evaluated in ranges of 0.5-2.5, 1.0-3.7, 2.0-5.6, 2.0-5.3, 1.0-7.0 and 1.0-6.0, respectively. From figure 5, it is obvious that the adsorption capacities of resins towards metal ions increased with the increase of pH. This is mainly for the reason that carboxyl groups in citric acid-modified resins existed as carboxylic ions in high pH. Carboxylic ions have good combining abilities towards metal ions. Thus, high pH is conducive to increase the adsorption capacities   After evaluation, the optimal values of pH for Fe 3+ , Al 3+ , Pb 2+ , Cu 2+ , Cd 2+ and Hg 2+ were 2.5, 3.7, 5.6, 5.3, 7.0 and 6.0, respectively. These optimal values of pH were adopted in the following adsorption experiments.
In order to quantify the individual metal adsorption ability for different functional groups (−OH, −COOH and amine) towards various metals, the adsorption capacities of metal ions on PS-Cl, PS-OH, PS-NH 2 and PS-COOH were also investigated at their optimal pH. The results are tabulated in table 2. There were no functional groups and heteroatoms with lone pair electrons in PS-Cl. Thus, PS-Cl exhibited poor ability to adsorb Fe 3+ , Al 3+ , Pb 2+ , Cu 2+ , Cd 2+ and Hg 2+ and the adsorption capacities were about 4.16 mg g −1 , 1.38 mg g −1 , 1.93 mg g −1 , 0 g g −1 , 0 mg g −1 and 0.37 mg g −1 , respectively. In PS-OH and PS-NH 2 , the lone pair electrons in oxygen and in nitrogen have good ability to combine with metal ions. Thus, the adsorption capacities of PS-OH and PS-NH 2 were higher than those of PS-Cl. In PS-COOH, the carboxyl groups have good ability to combine with metal ions. Thus, the adsorption capacities of resins showed marked improvement after functionalization with -COOH. Comparing the adsorption capacities between PS-Cl, PS-OH, PS-NH 2 , PS-COOH and citric acid-modified resins, it is obvious that the adsorption capacities of metal ions on resins were enhanced measurably after introducing carboxyl groups. In citric acid-modified resins, there are more carboxyl groups and atoms (O or N) with lone pair electrons. Thus, citric acid-modified resins showed the highest adsorption capacities towards metal ions.    Overall, most citric acid-modified resins were good for the adsorption of Fe 3+ , Al 3+ , Cu 2+ , Cd 2+ and Hg 2+ but poor for the adsorption of Pb 2+ . In order to better illustrate the adsorption mechanisms of metal ions on modified resins, pseudofirst-order and pseudo-second-order kinetic models were employed to fit the above experimental data. The model that performed best was selected on the basis of the linear regression correlation coefficient   values (R 2 ). These two kinetic equations are presented as follows. Equation of pseudo-first-order kinetic model [19]: ln(q e − q t ) = ln q e − k 1 t.

Adsorption kinetics of citric acid-modified resins towards metal ions
(3.1) Equation of pseudo-second-order kinetic model [20]: where q e and q t are the adsorption capacity of metal ions on the citric acid-modified resins at equilibrium and at any time t (mg g −1 dry resin), respectively. The parameters k 1 (min −1 ) and k 2 (g mg −1 min −1 ) are the rate constants of the pseudo-first-order and pseudo-second-order models for the adsorption process, respectively. The kinetic data in figure 6 were fitted by the above two models. The calculated results of the pseudofirst-order and pseudo-second-order rate equations are shown in table 3. The parameter R 2 was adopted to evaluate the models. The results showed that the values of R 2 for pseudo-first-order and pseudosecond-order rate equations were mostly above 0.98, which indicated that these two models could be used to fit the adsorption processes. The values of R 2 for the pseudo-first-order rate equation were mostly higher than 0.99, which showed the good quality of linearization. Thus, the pseudo-first-order kinetic equation was more appropriate for the adsorption. Based on the q e calculated by pseudo-first-order rate equation, it can be concluded that PS-CA was a good adsorbent to absorb Fe 3+ and Cd 2+ , PS-N-CA was a good adsorbent to adsorb Al 3+ , Cu 2+ and Hg 2+ . PS-CA, PS-O-CA and PS-N-CA all showed poor abilities to adsorb Pb 2+ with q e below 10 mg g −1 .    where q e and q m are the equilibrium and maximum adsorption capacity (mg g −1 dry resin), respectively. C e is the equilibrium concentration of metal ion solution (mg ml −1 ). K L is the parameter related to the adsorption energy (ml mg −1 ). K F reflects the adsorption capacity of resins ((mg g −1 )(ml mg −1 ) 1/n ). The parameter n represents the adsorption affinity of the adsorbent for resins. The equilibrium data in figure 7 were fitted by the above two isotherm models. The corresponding parameters of Langmuir model and Freundlich model were calculated and are tabulated in table 4. The values of R 2 were employed to evaluate the models. The results showed that the values of R 2 for the Langmuir model were all above 0.98, indicating that the Langmuir model was more appropriate for the adsorption.

Comparison of adsorption capacities between different citric acid-modified adsorbents
A comparison of the results of citric acid-modified resins obtained in this work with those of some other adsorbents reported in the literature is given in table 5 [11][12][13].  Table 5. Citric acid-modified adsorbents used for the adsorption of metal ions. CA-SD, CA modified sawdust; CA-BWSS, CA modified base washed soya bean straw; CA-WWSS, CA modified water washed soya bean straw; CA-PS, CA-modified pine sawdust. lower than those on citric acid-modified plant substrates. The above results indicated that the insoluble carrier in citric acid-modified adsorbents also has great influence on adsorption. Polystyrene resins have large surface area and good permeability. Thus, the adsorption capacities of Cu 2+ and Cd 2+ on citric acidmodified resins were much higher. However, plant substrates also have some advantages. For instance, plant fibre is a highly polar substance which contains very high levels of -OH groups. This property benefits the adsorption behaviours of adsorbents towards metal ions. It is probably for this reason that the adsorption capacities of Pb 2+ on citric acid-modified plant substrates were much higher.

Desorption of metal ions adsorbed on citric acid-modified resins
Desorption tests of removing metal ions on citric acid-modified resins were also conducted. According to the results in figure 5, it can be concluded that reduction of pH would make carboxyl group nonionized and decrease the combining capacities between resins and metal ions. Thus, metal ions adsorbed on resins might be desorbed by shaking with acidic solutions. In our experiments, metal ions adsorbed on resins were desorbed by shaking with 20 ml acidic solution for 24 h. The results showed that most of the metal ions could be desorbed efficiently with desorption ratios above 98% in 0.5% HCl solution. However, the desorption ratio of Fe 3+ on PS-CA was 74.6%. With the enhancement of acidity, Fe 3+ on PS-CA was desorbed efficiently (greater than 98%) in 3.0% HCl concentration. Overall, metal ions adsorbed on citric acid-modified resins could be desorbed efficiently by shaking with HCl solution. This result also suggested that citric acid-modified resins could be regenerated using 0.5% or 3.0% HCl solution.

Conclusion
In summary, citric acid-modified resins PS-CA, PS-O-CA, PS-N-CA were synthesized by anchoring citric acid on PS-Cl, PS-OH and PS-NH 2 , respectively. These citric acid-modified resins exhibited good abilities to adsorb Fe 3+ , Al 3+ , Cu 2+ , Cd 2+ and Hg 2+ , while poor abilities to adsorb Pb 2+ . The adsorption capacities of metal ions on resins were significantly influenced by pH. The ideal values of pH for Fe 3+ , Al 3+ , Pb 2+ , Cu 2+ , Cd 2+ and Hg 2+ were 2.5, 3.7, 5.6, 5.3, 7.0 and 6.0, respectively. The adsorption kinetics and adsorption isotherm experiments indicated that the pseudo-first-order rate equation was more appropriate for characterizing the kinetic data and the Langmuir model was more suitable for fitting the equilibrium data.  HCl solution with desorption ratios above 98%. After desorption, these resins still exhibited considerable adsorption abilities.
Data accessibility. All data are provided in the main text. Authors' contributions. X.L. designed the experiment and wrote the manuscript. L.X. and Y.L. performed the experiments.
W.Z. performed the characterization of citric acid-modified resin. All the authors gave their final approval for publication.