Synthesis of metal–organic framework immobilized bromelain complex and its application in corn starch wastewater treatment

Pineapple leaves and stems are often treated as agricultural waste during pineapple harvesting. Disposing of or incinerating pineapple leaves and stems contributes to environmental pollution and results in the waste of recyclable resources. Currently, a viable approach is to extract bromelain from pineapple leaves and stems. Bromelain exhibits broad applicability, capable of catalyzing the hydrolysis of numerous plant proteins, hence it can be effectively utilized in the treatment of wastewater with high protein content. In this study, bromelain isolated from pineapple leaves and stems was immobilized within a zeolitic imidazolate framework (ZIF-8) via a one-step co-precipitation method, synthesizing the bromelain@ZIF-8 complex. The results show that (1) the immobilization of bromelain did not alter the morphology or crystalline structure of ZIF-8, while the thermal stability of the enzyme was significantly enhanced; (2) when take bovine serum albumin and corn starch wastewater as substrates, the bromelain@ZIF-8 complex exhibited better performance in protein hydrolysis applications under the synergistic effect of bromelain and ZIF-8, demonstrating great potential in removing proteins from industrial wastewater; (3) when treating corn starch wastewater by coagulation, the optimal treatment effect of the wastewater was achieved by sequentially adding 300 ppm flocculant polyaluminum chloride (PAC), 5 ppm bromelain@ZIF-8 complex, and 667 ppm coagulant aid polyacrylamide (PAM), resulting in a COD reduction of 25.22%, and reductions in color and suspended solids by 97.80% and 90.84%, respectively. This work not only reduces the environmental pollution, but also provides a new possibility for the treatment of high plant protein wastewater.