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THE EFFECT OF MICROBIAL FE(III) REDUCTION ON SMECTITE FLOCCULATION

¹ Naval Research Laboratory, Seafloor Sciences Branch, Stennis Space Center, MS 39529, USA
² Department of Geology, Miami University, Oxford, OH 45056, USA

View larger version (97K): [in a new window]   Figure 1. Bench-top experiment of settling behavior of bioreduced smectite (BS) in Tube A and non-reduced control (NC) suspensions in Tube B. Following 48 h incubation of S. oneidensis with nontronite (NAu-1), a substantial color change of BS occurred in TubeA as compared to NC in Tube B, suggestingmicrobial reduction of structural Fe(III) in the clay. Clay suspensions in Tubes A and B were observed for 80 min after being shaken homogeneously. The BS suspensions are flocculated and settled faster while the NC suspensions remained dispersed in the water column (see arrows showing the boundary of suspension).

View larger version (14K): [in a new window]   Figure 2. Change of HCl-extractable Fe(II) with time in the non- reduced control (NC), bioreduced smectite (BS), and aerobically inoculated smectite (AIS). The measured HCl-extractable Fe(II) concentration (mM) was normalized to milligrams of smectite in each tube, and reported as mmol/g. The extent of reduction by the end of incubation reached 18%, based on the measured Fe(III) concentration and the amount of smectite used in the tubes. The vertical errors are typically <10%.

View larger version (19K): [in a new window]   Figure 3. Aggregate size distribution of non-reduced control (NC), bioreduced smectite (BS) suspension (variable incubation time from 3 to 48 h) and aerobically inoculated smectite (AIS) suspensions using a Micromeritics Sedigaph. The average aggregate size was measured at 50% cumulative mass % and settling velocity was calculated using Stokes’ law.

View larger version (25K): [in a new window]   Figure 4. (a) The packet-size distribution and (b) aspect ratio of bioreduced smectite (BS) and non-reduced control (NC) showing an increase in mean packet size (11.4 nm to 23.9 nm) and aspect ratio (0.11 to 0.18). The errors of the measurements are typically ±5%. The measurements were made on TEM lattice-fringe images using a microfiche reader.

View larger version (115K): [in a new window]   Figure 5. Direct measurement of smectite packet thickness and length on the TEM lattice-fringe images of (a) non-reduced control (NC) and (b) bioreduced smectite (BS). The boundaries of packetswere defined by small angles, concentrations of edge dislocations, tapering fringes in sequences of layers, or strong image contrast. The outlined areas were enlarged to measure the packet thickness. The big arrows indicate the lengths of various smectite packets.

View larger version (77K): [in a new window]   Figure 6. TEM images of (a) non-reduced control (NC) suspensions, (b) bioreduced smectite (BS) suspensions, and (c) aerobically inoculated smectite (AIS) suspensions showing the evolution in floc architecture associated with microbial Fe(III) reduction: note pore areas (p) in NC and AIS and clusters of biopolymers (see arrow) secreted by S. oneidensis (S) and smectite particles.