Effect of ammonium hydroxide modification of ZIF-8 nanoparticles on hexavalent chromium removal from aqueous solution

Owing to the extraordinary properties of zeolitic imidazolate framework (ZIF)-8, it finds myriad of applications ranging from sensing to wastewater treatment. Herein, ZIF-8 and ammonia solution treated ZIF-8 (ZIF-8/NH3) were synthesized, characterized, and then employed to remove hexavalent chromium Cr(VI) from aqueous solutions. ZIF-8 with crystalline and distinct structures was successfully formed, as evidenced by characterization methods like XRD, FTIR, BET, and FESEM. Response surface methodology-optimized batch adsorption experiments showed that the performance of adsorbents was strongly impacted by significant factors such as pH, adsorbent dose, and initial Cr(VI) concentration. Further, ZIF-8/NH3 showed a maximum adsorption capacity of 4.69 mg/g in comparison to ZIF-8 of 3.84 mg/g according to Langmuir isotherm analysis, with R2 values of 0.9300 for ZIF-8/NH3 and 0.9060 for ZIF-8. In addition, the kinetic analyses revealed pseudo-second-order behaviour for both adsorbents, indicating chemisorption as the predominant mechanism. In summary, it is concluded that after the ZIF-8 modification with NH3, a slight increase in improvement of adsorption capacity was observed.