Chemical vs. Biological Crystals, all the Same?

Abstract

Using microorganisms to mediate crystallisation of metals and minerals in open-culture bioreactors has potential to recover recyclable materials from dilute aqueous streams, but also to prevent their emission to the environment. Although this potential is already exploited in practice to some extent, biological crystallization for metal recovery is still largely a black box technology with limited understanding of the role of the microorganisms in the crystallization, and the differences with chemical crystallisation. Using biocrystallisation of scorodite (FeAsO4.2H2O) and sphalerite (ZnS) as examples we propose that the role of microorganisms strongly depends on established saturation state of the solution. For scorodite, microorganisms are used to exert control over the crystallization as their ferrous iron-oxidizing activity keeps the solution slightly oversaturated. Also, the oversaturation level is kept homogeneous because of continuous biological formation of the reactant ferrous iron throughout the solution. In continuous bioreactor experiments on which we reported previously, scorodite crystal sizes still increased after 72 days of bioreactor operation indicating that indeed crystal growth was favored over nucleation. On the other hand, in our experiments with zinc sulfide, crystallization proceeded in highly oversaturated solutions in a continuous sulfate reducing bioreactor fed with a zinc sulfate solution and H2/CO2 as electron donor and carbon source. The high oversaturation likely resulted in dominant primary nucleation in the bulk solution, with little or no control over crystal growth, even though agglomeration may still have occurred. This was exemplified by particle sizes which decreased in the bioreactor experiment and remained stable after already about 2 weeks of operation.