Microbe-mineral interactions:Mechanisms and immobilization effect toward heavy metals
Microbial-mineral interactions have recently demonstrated to play a critical role in affecting the migration,transformation characteristics and environmental impacts of heavy metals,via formation of"microbial-mineral"binary complexes or"microbial-mineral-organic matter"multiple complexes.Microbes have evolved a series of adaptation mechanisms and survival strategies under heavy metal stress,during which minerals are modified and heavy metals immobilized.In such situation,minerals not only provide nutrients for microbial growth,but could also act as an important protective umbrella for microbes against toxicity of heavy metals or other pollutants.In this review paper,the interaction mechanisms between microorganisms and minerals were first introduced.Subsequently,the synergistic effect and mechanisms of microbe and mineral in heavy metal immobilization were highlighted via microbial-silicate mineral and microbial-iron mineral systems.Mechanisms of microbial interaction with minerals are quite complex,and there are two synergistic actions by which microorganisms can decompose mineral substrates:physical and biochemical.Biophysical weathering of minerals commonly occurs in the interaction of fungi and actinomycetes with minerals.Fungi and actinomycetes can break minerals through fungal hyphae penetration and growth along crystal planes,cleavage,cracks and grain boundaries,mechanically boring and denudating the rocks at the nanoscale,leaving the cracks and tunnels inside.As for the majority of heterotrophic microorganisms,the biochemical route for mineral dissolution is dominated by leaching,which is enforced by three main mechanisms.That is,the secretion of siderophores,organic/inorganic acids and redox actions.These mechanisms strongly accelerate dissolution of mineral,causing the formation of"metal sink"around the microbial biomass.The"metal sink"in return,works as nutrient sources to promote the growth of microorganisms.Importantly,the intimate interaction between microbial and soil mineral significantly alters the surface reactivity of mineral and microbial activity,and therefore leads to the immobilization and changes of speciation and bioavailability of heavy metals.Microbial-silicate mineral and microbial-iron mineral systems were employed to ascertain why microbe-mineral systems possess such merits in heavy metals remediation than single microbe or mineral system.In microbial-silicate mineral system,the interaction of microbe and silicate promotes heavy metal remediation depends primarily upon the following mechanisms:1)provide additional adsorption sites such as hydroxyl,carboxyl,sulfhydryl,phosphorus acyl.2)increase the utilization rate of inherent active sites.3)change the binding characteristics of heavy metals with minerals,for example,negatively charged microorganisms can act as intermediates to bridge heavy metal ions with minerals,forming stabilized monodentate or multidentate complexes.4)affect the dispersion or agglomeration status of mineral particles and microorganisms.5)change the original structure of mineral,like the exfoliation of silicate layers of 2D silicate minerals,or even destroy the lattice structure.6)make adjustments on the type and content of secretion and excretion products to regulate the microbial activities under the stimulation of direct contact with minerals.While the microbial-iron mineral systems tend to promote the metal or nonmetallic pollutants with variable valence states from toxic forms to less toxic ones by accelerating the electron transfer.This paper aims to provide a solid theoretical and scientific basis for microbial-mineral remediation of heavy metal pollution.
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