Zinc-based metal–organic frameworks for high-efficiency adsorption of Pb(II) and Cu(II) ions from an aqueous solution: synthesis, characterization, and performance evaluation

Heavy metal ions in water can be extremely toxic, even in trace amounts, endangering the ecosystem and posing serious threats to the natural environment. Therefore, their removal from contaminated water is of utmost importance. Among various methods, adsorption has proven to be one of the most effective techniques for treating water contaminated with heavy metals. Due to unique structural and dimensional properties, metal–organic frameworks have drawn ample research interest as effective metal ion adsorbents. This study focuses on adsorption process for the removal of lead (Pb) and copper (Cu) ions from polluted water. A new metal–organic framework was synthesized using simple, easy, cost-effective single-step synthesis method. Nanoadsorbent metal–organic frameworks exhibited excellent stability, a large surface area, and prominently exposed active sites for lead (II) and copper (II) ions. Batch adsorption experiments at 30 °C showed the removal of 98.7% of copper (II) and 99.5% of lead (II) ions at pH 4.5 and 8 in 60 and 90 min, respectively. Similarly, the maximum adsorption capacity was found to be 19 and 29 mg/g, for Cu(II) and Pb(II) ions, respectively. A 10-fold increase in crystallite size was observed after three cycles of using the material untreated for Pb(II) removal, giving a removal efficiency greater than 90%. Adsorption followed pseudo-second-order kinetics and best fit the Langmuir adsorption isotherm for both metal ions. This adsorption behavior can be easily explained by the value of the point of zero charge. Moreover, thermodynamic studies reveal that the adsorption process for heavy metal ion removal is spontaneous and entropy-driven, and involves the endothermic adsorption reaction.