Valorization of heavy metal-adsorbed microalgae from a membrane bioreactor for levulinic acid production

Abstract

Though widely used in many industrial activities, heavy metals are known to cause serious pollution problems in the environment. Many studies have indicated that microalgae is an effective adsorbent for treating heavy metal pollution. Furthermore, research on the use of biomass as a substitute for petroleum raw materials has seen increased importance in recent years. The carbohydrates in microalgae render such raw materials a potential biomass substance for obtaining platform compounds. Therefore, in addition to being able to use microalgae as an adsorbent, the reuse of discarded microalgae shows much research potential. In this study, Chlorella vulgaris was used as an adsorbent in conjunction with an immersed microalgae membrane bioreactor to adsorb chromium, nickel, and cooper in synthetic groundwater. The heavy metal-adsorbed microalgae waste was then collected via coagulation using chitosan. The microalgae biomass could produce platform compounds via liquid acid catalysis under microwave heating. The levulinic acid (LA) yield was affected by temperature, reaction time, and catalyst concentration. The heavy metal-containing microalgae achieved LA yields of 6.54% with 0.5 M sulfuric acid, at 180 °C and one hour reaction time. Under this condition, 99.80% of total copper remained in the solid residue, while nearly all the nickel and chromium entered the liquid phase after conversion. These results demonstrate the potential of using metal-containing microalgae biomass to produce platform compounds through acid-catalyzed conversion under microwave heating, and heavy metal distributions in the liquid and solid phases from microalgae waste might influence further separation and purification of LA.