Structural Fe in ferruginous smectite (sample SWa-1, Source Clays Repo
sitory of the Clay Minerals Society) was reduced by a mixture of five
Pseudomonas species of bacteria in a defined Fe-free medium to determi
ne the effect of microbial reduction on clay swelling. Iron(II), total
Fe, and gravimetric water content (m(w)/m(c)) were determined in clay
gels equilibrated at applied pressures of 0.1, 0.3, and 0.5 MPa. The
water content of microbially reduced SWa-1 decreased at all three appl
ied pressures as the Fe(II) content approached about 0.8 mmol Fe(II)/g
-clay. As Fe(II) increased from 0.8 mmol/g-clay, however, further chan
ge in m(w)/m(c) was negligible. Concurrent with microbial reduction of
structural Fe was a significant decrease in the swelling pressure (PI
) of SWa-1: for example, when m(w)/m(c) = 1.2 (g/g), PI changed from 0
.47 MPa at Fe(II) = 0.2, to 0.19 MPa at Fe(II) = 0.9 mmol/g-clay. Both
biologically and chemically reduced smectites displayed lower values
of m(w)/m(c) and a concurrent decrease in II as Fe(II) content increas
ed, but the effect of Fe(II) on m(w)/m(c) was greater for the microbia
lly reduced smectites at all applied pressures.