PhD conferral Z. Hassan
Aula, Vrije Universiteit Amsterdam
Making water healthy: Microbial Ecology of Arsenic Contaminated Aquifers and Potential Bioremediation
prof.dr. H.V. Westerhoff prof.dr. S.I. Khan dr. W.F.M. Röling (†) dr. M. Sultana
Amsterdam Institute of Molecular and Life Sciences
Earth and Life Sciences
Lack of access to clean water is becoming a major public health concern throughout the world. In Bangladesh natural contamination of groundwater by arsenic has already been generating a health catastrophe. Ineffective water purification and sewage systems as well as periodic monsoons, cyclones, flooding, drought and salinity (especially in southern parts of the country) further complicate access to reliable drinking water.
A wide range of technologies have been developed for arsenic mitigation from drinking water. However, most technologies are not sustainable and affordable to the generally poor and rural population of Bangladesh, due to high maintenance costs, required logistic care and not being hygienic in respect to pathogenic microbial contamination. Subsurface Arsenic Removal (SAR) (Fig. 1) might be more sustainable. It does not require the addition of chemicals other than oxygen and water. It comprises the injection of oxygenated water into aquifers so as to oxidize ferrous iron and therewith precipitate ferric iron (oxyhydr)oxides and adsorb and co-precipitate arsenic. In a recent analysis Zaid Hassan showed that at present SAR is not effective at removal of arsenic from more than 100 litre of drinking water per well per day. While the oxidation of ferrous iron to solid ferric iron minerals, assumed to be driven by abiotic reactions, appeared to proceed almost maximally, insufficient of the arsenite co-precipitated. Zahid hypothesized that metal-cycling microorganisms might enhance arsenic removal. If so, then with respect to SAR, microbial arsenic cycling could come to play an important role in the (im)mobilization of arsenic in aquifers.
During his 4 years PhD research, Zahid Hassan focused on arsenic and iron cycling microorganisms and their potential role in bioremediation of arsenic. He first asked whether they were present below the wells. He indeed found arsenic and iron oxidizing microbes in the arsenic contaminated aquifers in Bangladesh, and in fact a vast diversity thereof. Arsenic has two forms, arsenite [As(III)] and arsenate [As(V)]. Arsenite is 60-100 times toxic and 5-10 times more mobile than arsenate. Basically, Zahid found that some of these microorganisms can transform arsenite to the less toxic and less mobile arsenate form. He concluded that, provided a sufficient density of microorganism can be achieved, the microbe-mediated arsenic oxidation should be faster than the chemical oxidation that is presently limiting arsenic removal by SAR.
Microbial oxidation of arsenite could then provide for a natural attenuation of arsenic pollution by removing arsenic from soil or water environments. Zahid Hassan is now integrating the groundwater hydrochemical and microbiological data so as to come with formulations for how various Bangladeshi and other sites could be remediated by a new microbiological SAR process, and for how drinking water quality could be improved, not only in Bangladesh but also at various European sites.
Fig. 1Scheme of a small-scale in situ subsurface iron and arsenic removal system ( SAR). The well was connected with an electric motor along with extraction and injection tubes. The SAR well was continued for up to 55 cycles (at one cycle per day) in a defined operating sequence comprising extraction, aeration and injection. A small household scale injection volume of 0.5 m3 water was used for aeration and subsequent reinjection. A total of 3.5 m3 was extracted during each cycle of which the latter 3 m3 was dumped into a nearby lake. Samples for analysis were taken after 0.1 m3 (V0), 0.5 m3 (V1), after 1.0 m3 (V2), after 1.5 m3 (V3) etc. of water had been extracted. All these samples were analyzed for hydrochemistry. Only V1 and V3 were subjected to microbiological and molecular analyses. The first 0.5 m3 of groundwater extractedwas stored in an overhead storage tank where it was aerated by use of an electric air compressor. Upon injection, the aerated water should diffuse radially and alter the near-well chemistry of the aquifer:ferric (oxyhydr)oxides should be generated via abiotic oxidation of ferrous iron. These ferric (oxyhydr)oxides should precipitate and provide a reactive surface to which ferrous iron and arsenic adsorb. Zahid Hassan is aiming to have microorganisms greatly improve the arsenic removal performance.
Hassan Z, Sultana M, Westerhoff HV, Khan SI & Röling WF (2016) Iron cycling potentials of arsenic-contaminated groundwater in Bangladesh as revealed by enrichment cultivation. Geomicrobiol J33:779–792.
Rahman M, Bakker M, Patty C, Hassan Z, Röling W, Ahmed K & van Breukelen B (2015) Reactive transport modeling of subsurface arsenic removal systems in rural Bangladesh. Science of the Total Environment 537: 277-293.
Hassan Z, Sultana M, van Breukelen BM, Khan SI & Röling WF (2015) Diverse arsenic-and iron-cycling microbial communities in arsenic-contaminated aquifers used for drinking water in Bangladesh. FEMS Microbiol Ecol91: doi: 10.1093/femsec/fiv1026.