Pyridine based cross-linked chitosan: a biopolymer adsorbent for the green removal of toxic metals from water

Herein we report the green recovery of toxic metals [namely: Cd2+, Cr3+, Mn2+, Pb2+, and Ni2+] from water utilizing a biopolymer: 2,6-pyridine dicarboxylic acid cross-linked chitosan (PDCCCS) as the adsorbent. Adsorption studies were performed at a previously determined optimum adsorption conditions for Cu(II) [i.e temperature = 30 °C, pH of about 7.5, contact time = 60 mins and initial metal ion concentration of 2.5 mM]. At the RI-PB/def2-SVP level of theory, the Density Functional Theory (DFT) approach has been used to evaluate adsorption energy for metal ions. Selectivity studies were performed at pH 4.20, 5.56, 6.65 and 7.61. While Mn(II), Cd(II) and Ni(II) were strongly adsorbed at higher pH (7.5), Cr(III) and Pb(II) were seen to be strongly adsorbed at lower pH (around 4.0). Selectivity studies revealed that PDCCCS can be utilized for simultaneous removal of the metals at pH 4.2; selective adsorption of Mn(II) at pH 5.56 as well as simultaneous-selective removal of Ni(II) and Mn(II) near neutral pH. The maximum adsorption limit of PDC-CCS for Mn(II), Cd(II) and Ni(II), were found to be 1258.79, 1118.70 and 829.62 mmol/g respectively. When compared with some relevant previously used adsorbent, PDC-CCS shows an exceptional adsorption capacity. Consequently, a successful biopolymer adsorbent for the treatment of water contaminated by hazardous metals.

. Despite the fact that the utilization of chitosan in its modified form for the expulsion of noxious metals from water has pulled in a great deal of interests as of late , the utilization of pyridine based cross-linked chitosan has not been accounted for as far as we could possibly know. Recently, we reported a new pyridine based cross-linked chitosan (figure 1d) (2,6pyridine dicarboxylic acid cross-linked chitosan) as a non-toxic biopolymer adsorbent for the recovery of Cu(II) ions from water [46]. In furtherance to this, the pyridine based biopolymer has been utilized in this study in order to extract other toxic metals from water, including; cadmium, chromium, manganese, lead, and nickel with the end goal of investigating/researching the selectivity of this adsorbent with regard to the solution pH and the interaction time of the adsorbent at the optimum temperature and the ideal initial metal ions concentration. Additionally, the Density Functional Theory (DFT) approach has been employed to justify the the adsorbent's adsorption limit/capacity for each of the metals under scrutiny.
In accordance with the updated literature procedure of Sailakshmi et al [50], pyridine-2,6dicarboxylic acid crosslinked chitosan (PDC-CCS) was prepared and the cross-linking degree was determined using the bradford assay. The presence of pyridine-2,6-dicarboxylic acid in the PDC-CCS was revealed by 13 C NMR and UV-visible spectroscopy through peaks due to aromatic carbons and carbonyl carbons. FT-IR confirmed the interaction of the cross-linker with chitosan at the -NH2 functional group. Elemental analysis showed an increase in the C/N ratio after cross-linking indicating a successful incorporation of the crosslinker. The result of the Bradford assay confirmed that the cross-linking is 100% complete. After crosslinking, Xray diffraction spectroscopy revealed a reduction in the crystallinity of the biomaterial. Thermal analysis suggested a decrease in stability upon cross-linking. N2 adsorption isotherm and SEM analysis indicated an increased surface area as well as increased porosity of the synthesized cross-linked chitosan [46].

Figure 1: Structure of (a) chitin (b) chitosan (c) fully deacetylated chitosan (d) crosslinked chitosan showing possible binding sites
Following the Thien et al literature approach, we used PDC-CCS from previous research to recover Cu(II) from water. [47]. In addition, to obtain an optimal adsorption state, the impact of temperature, the solution pH, adsorbent time of contact along with initial concentration of Cu(II) ions were examined. In fact, the adsorption limit/capacity Q has been evaluated according to equation 3.
Where Q, C and Co are, individually, the adsorption limit/capacity (mmolg -1 ), the final equilibrium concentration of metal ions (mmoll -1 ) and the initial concentration of metal ions.
Likewise, the solution volume (l) and sorbent mass (g) are V and W, respectively. Additionally, evaluation of the experimental data has been performed with kinetic models (pseudo-first-order and second-order kinetic models according to equation 4 and 5 respectively) and models of Isothermal Adsorption (Langmuir and Freundlich adsorption isotherms according to equation 6 and 7 respectively) Where the quantity of Cu(II) ion adsorbed (mgg -1 ) at equilibrium and time t is qe and qt respectively; The first-order and second-order adsorption rate constants (min -1 ) are respectively expressed by k1 and k2; Ce is the equilibrium Cu(II) ion concentration in solution (mgl -1 ), whereas the equilibrium adsorption limit/capacity (mgg -1 ) is denoted as Qe; for a single-layer coverage (mgg -1 ), qm represents the saturated adsorption limit, while kf, n and KL are taken as Thus, the optimum adsorption capacity of PDC-CCS for Pb(II) and Cr(III) would be over 859.05 mmol/g and 519.26 mmol/g in the absence of competitive cations as seen in figure 3a and b respectively. Hence, the optimum pH of 7.5 can only be considered for the adsorption of Cd 2+ , Mn 2+ , Ni 2+ and Cu 2+ .

Figure 3: Competitive adsorption capacities and selectivity of PDC-CCS at pH 4.2 for Mn 2+ , Cr 3+ , Ni 2+ , Cd 2+ , Pb 2+ , and Cu 2+ within; (a) five mins (b) ten mins (c) fifteen mins and (d) twenty mins.
Additionally, the selectivity studies show that the adsorbent (PDC-CCS) has the tendency to adsorb all the competing metal ions within 15 minutes of contact time at a pH of 4.2 ( figure 3).
The capacity of adsorption and adsorbent's selectivity towards the metal ions are in the following order: Cu 2+ > Pb 2+ > Cr 3+ > Mn 2+ > Cd 2+ > Ni 2+ . However, at pH 5.56 ( figure 4), the tendency of the adsorbent to adsorb the metal ions in solution decreases but with high selectivity (100%) towards Mn(II). The observed reduction in adsorption capacity at the pH of 5.56 may be due to the strong competition among the metal ions for uptake by the adsorbent.
In essence, as one metal ion is adsorbed, it is replaced by another metal ion; the process continues until an equilibrium is attained when small amount of Mn(II) is successfully adsorbed without replacement after 15 minutes of contact time. Interestingly, PDC-CCS shows exceptional adsorption capacity towards metal ions compared to some of the applicable adsorbents previously published as shown in table 2.

Conclusion
The adsorption of Cd 2+ , Cr 3+ , Mn 2+ , Pb 2+ and Ni 2+ utilizing 2,6-pyridinedicarboxylic acid crosslinked chitosan (PDC-CCS) has been discussed. The capacity of adsorption by PDC-CCS was investigated at pH 7.5 while adsorption selectivities were examined at pH 4.2, 5.56, 6.65 and 7.61. Density functional theory approach has been used to support the trend in adsorption capacities of PDC-CCS for the metal ions. Results obtained indicate that PDC-CCS is a novel biopolymer adsorbent which can be employed for the simultaneous removal of toxic metals and selective removal of Mn(II) from water.

Conflicts of interest
There are no disputes to report.