Iron(III) Modification of Bacillus subtilis Membranes Provides Record Sorption Capacity for Arsenic and Endows Unusual Selectivity for As(V) Ting Yang, † Ming-Li Chen, † Lan-Hua Liu, † Jian-Hua Wang, †, * and Purnendu K. Dasgupta* ,‡ † Research Center for Analytical Sciences, Box 332, Northeastern University, Shenyang 110819, China ‡ Department of Chemistry and Biochemistry, University of Texas at Arlington, Arlington, Texas 76019-0065, United States * S Supporting Information ABSTRACT: Bacillus subtilis is a spore forming bacterium that takes up both inorganic As(III) and As(V). Incubating the bacteria with Fe(III) causes iron uptake (up to ∼0.5% w/w), and some of the iron attaches to the cell membrane as hydrous ferric oxide (HFO) with additional HFO as a separate phase. Remarkably, 30% of the Bacillus subtilis cells remain viable after treatment by 8 mM Fe(III). At pH 3, upon metalation, As(III) binding capacity becomes ∼0, while that for As(V) increases more than three times, offering an unusual high selectivity for As(V) against As(III). At pH 10 both arsenic forms are sorbed, the As(V) sorption capacity of the ferrated Bacillus subtilis is at least of 11 times higher than that of the native bacteria. At pH 8 (close to pH of most natural water), the arsenic binding capacity per mole iron for the ferrated bacteria is greater than those reported for any iron containing sorbent. A sensitive arsenic speciation approach is thus developed based on the binding of inorganic arsenic species by the ferrated bacteria and its unusual high selectivity toward As(V) at low pH. ■ INTRODUCTION Arsenic pollution problems are common worldwide. South Asia (Bangladesh and the Gangetic delta in particular) has been so afflicted with natural groundwater arsenic poisoning (>600,000 with diagnosed arsenicosis, >20 million at risk) that the World Health Organization has labeled it the greatest environmental calamity in recorded history. 1 The US National Academy of Engineering targeted a sustainable solution to remove arsenic from drinking water for the first Grainger Challenge Award. 2 The $1 M winning entry utilized a composite iron matrix as the active element to remove the arsenic. 3 The exact form in which arsenic occurs in the Gangetic delta in the aquifers is debated. However, arsenic is putatively present as/with arsenopyrite, 4 biotite, 5 clay minerals, iron hydroxide-coated sand grains, 6 etc.; all contain iron: arsenic is always highly correlated with iron in these waters. 7 Iron has a high affinity for arsenic. Iron-based sorbents, e.g., hydrous iron oxide (HFO), 8-12 or HFO on matrix sorbents, e.g., activated carbon, 13 fibrous ion ex- changers, 14 cellulose, 15 etc., are commonly used to remove waterborne arsenic. When sorbents are treated with aqueous Fe(III) and extensively washed, 21-23 Fe likely precipitates as HFO on the matrix. 16,17 As a green and sustainable alternative to conventional metal remediation techniques, biosorption has gained an increasing role in the removal of metals, especially arsenic. 18 Chitosan, 19 macrofungi, 20 ferns, 21 algae, 22 waste biomass, 23 , etc. have all been tried with differing degrees of success. The authors’ research groups have had independent long-standing interest in trace arsenic determination; 24-32 more recently, we jointly embarked on the characterization of biosorbents for the measurement and removal of arsenic. We found a live HeLa cell, a kind of human cervical cancer cell, could take up arsenic; both surface and intracellular accumulation are involved. 32 Interestingly, while both As(III) and As(V) were taken up at high pH, at low pH, As(V) was taken up with a 40:1 selectivity over As(III). The sorbed arsenic was readily eluted by strong acid; HeLa cell-packed beds were used for arsenic analysis. As a safer alternative with respect to the HeLa cell, we explored Bacillus subtilis (B. subtilis), a spore forming gram- positive rod-shaped bacterium that naturally occurs in soil and many vegetations. B. subtilis is easily grown, and its culturing is well controlled under laboratory conditions. More importantly, its pathogenic potential is generally regarded as low or absent. 33 Indeed, it is presently marketed as a probiotic. 34 In sorbent- immobilized form it has been studied for copper and cadmium uptake. 35 We observed that native B. subtilis can remove even very low concentrations of inorganic arsenic of either oxidation states. Incubating the bacteria with iron(III) enhanced significantly the selectivity for As(V) at acid condition and in the meantime achieved record sorption capacity for arsenic at a higher pH. Received: August 12, 2011 Revised: December 31, 2011 Accepted: January 19, 2012 Published: January 19, 2012 Article pubs.acs.org/est © 2012 American Chemical Society 2251 dx.doi.org/10.1021/es204034z | Environ. Sci. Technol. 2012, 46, 2251-2256