Nickel biosorption by discharged biomass from wastewater treatment bioreactor: size plays a key role.

Biomass size significantly affects the characteristics of the extracellular polymeric substances, which in turn influences the biosorption mechanisms. In this work, nickel biosorption mechanisms and the capacity of excess aerobic granules (AGs) of >850 μm, 500-850 μm, 212-500 μm, and bioflocs <212 μm, discharged during wastewater treatment operation, were investigated by elemental composition and spectroscopic analysis. Ni(2+) biosorption capacity decreased apparently from 1.28-1.53 to 0.93 mEq/g with the increase of biomass size from 0-850 to >850 μm due to their different biosorption mechanisms. Chemical binding between nickel and protein was dominant for biomass less than 850 μm, whereas nickel adsorption was mainly physical when biogranule size was greater than 850 μm, because the nickel had no opportunity to interact with protein located in the core of the biogranule. Only low levels of ion exchange (<15.22 %) and precipitation (negligible) were observed for all kinds of biomass. Thus, biomass greater and less than 850 μm presented quite different biosorption mechanisms.