Application of a Terephthalate and Pyrazine-Based MOF in Cr Adsorption †

: In this study, a terephthalate and pyrazine-based metal–organic framework (MOF) was prepared using the oxygen and nitrogen donor ligands through the hydrothermal method. In the MOF, cobalt and nickel ions were selected as metal nodes which are connected by terephthalate and pyrazine linkers . The as-prepared MOF was utilized as Cr adsorbent in water by an ultrasonic method. The MOF capacity towards chromium ion adsorption was obtained about 96% in 50 ppm initial concentration. In order to characterize and determine the morphology of the title MOF, the FT-IR and XRD methods were applied, while the chromium concentration before and after adsorption was determined by the ICP method.


Introduction
Today, water contamination has become a worldwide challenge due to the growth of various industrial activities such as food processing, mining, oil, agricultural, textile, leather, pharmaceutical industries and so on [1]. Hexavalent chromium, Cr(VI) is highly toxic and mutagenic when inhaled. The ingestion of chromium(VI) through water has been linked to stomach tumors, moreover, it may cause allergic contact dermatitis (ACD). Chromium is naturally present in the environment in trace amounts, but it is used in rubber and stainless steel manufacturing, chrome plating, dyes for textiles, tanneries, etc. Irrigation water standards for chromium are 0.1 mg/L [2]. Chromium, especially hexavalent chromium, is highly toxic to fish, because it is easily absorbed across the gills, readily enters blood circulation, crosses cell membranes, and bioconcentrates up the food chain. In contrast, the toxicity of trivalent chromium is very low, attributed to poor membrane permeability and little biomagnification [3].
The adsorption capacity (AC) and removal efficiency (RE) of heavy metal ions by the title MOF as an adsorbent could be calculated as follows: In these equations, C0 (mol/L) is the initial concentration of heavy metal ions and Ci (mol/L) is the concentration after a determined adsorption time ti, respectively. V (mL) is the solution volume of heavy metal ions, and Wg (mg) is the dosage of MOF. When the adsorption reaches to the equilibrium point, Ce is equal to Ci and at the same time, AC is equal to Qe, namely, equilibrium adsorption amount.

XRD Pattern of [CoNi(μ3-tp)2(μ2-pyz)2]
The XRD pattern coincided with the simulated XRD of the similar previously reported MOF [4] confirming the correction of the solvothermal method, as shown in Figure 1.

FT-IR Spectrum
As shown in Figure 2, each therephthalate linker joins three Co 2+ and three Ni 2+ centers with two different coordination modes of bidentate and bridging bis-monodentate. Bridging modes of two tp linkers leads to the formation of planar 10-membered rings of Ni2Co2C2O4 units. These 2D planes containing rectangular grids are pillared by pyrazine to create the 3D network.

Cr Adsorption Results at Different pH
Adsorption of chromium ion on the synthesized MOF in different pH was studied.

Effect of pH
As mentioned in the literature, absorption process is correlated with the pH. At low pH, H + ions compete with analyte cations to occupy the active sites of adsorbent. On the other hand, at highly basic pH, OH − causes that the analyte precipitate in the solution and incorrect data will be received. Therefore, finding an optimum pH is necessary in the adsorption process. First, a 50 ppm solution of chromium was prepared and then, 0.01 g of adsorbent was added to 50 mL of it in separate flasks. As can be seen in Figure 3, at pH 5 the amount of adsorbed chromium reaches to the maximum value. Consequently, other tests were followed in this pH.  Adsorption % pH

Conclusions
During this visit, the substrate level of Cr from aqueous solution can be controlled using the title metal-organic framework. The optimum conditions for Cr ion surface display were as follows, the Cr ion coated by Co/Ni MOF, pH 5, absorbent amount = 0.01 g and initial concentration = 50 ppm.