Microbial stability evaluation of cement-based waste forms at different waste to cement ratio.

An evaluation of the effect of differences in chromium nitrate to cement ratio on the microbial stability of a chromium nitrate/cement waste form, as reflected in the leaching of chromium, calcium, magnesium and aluminum; was carried out in this study. An increase in the proportion of chromium in the waste form from 4.8 to 8.7% had no noticeable effect on microbial stability, with the total chromium leached essentially unchanged. Further increases in the proportion of chromium in the waste form from 8.7 to 10.7%, and from 10.7 to 15.9% resulted in a substantial decrease in microbial stability, with 3-fold and 1.3-fold increase in the total chromium leached, respectively, observed. For calcium, increases in the chromium proportion were accompanied with increases in the total calcium leached even though the increases were not in direct proportion to the increases in chromium proportion. For magnesium and aluminum, increases in the proportion of chromium within the range 4.8-10.7% were accompanied with increases in the total respective metals leached, with minor variation for each metal. On the whole, the maximum percentage chromium leached from the different waste forms was substantially lower than those of the other metals.

Evaluation of microbial stability of simulated solid and liquid waste forms using a refined biofilm formation method.

A refined biofilm formation method was used to evaluate the stability of a simulated liquid waste form containing a simulated liquid waste (salts) and cement in three different proportions, and a simulated solid waste form containing a simulated solid waste (resin) and cement in three different proportions. The experimental samples of all the simulated liquid waste forms showed evidence of microbial growth on them after 3 days of evaluation as indicated by substantial increase in sulfate production, and exhibited considerable instability to microbial degradation as indicated by substantial leaching of calcium. The experimental samples of all the simulated solid waste forms showed evidence of inhibition of growth of Thiobacillus thiooxidans for about 18 days, after which the growth of the microbe became evident in two out of three. Within the growth inhibition period, the differences between experimental and control samples were minor. After the growth of T. thiooxidans became evident, comparatively higher degradations were observed for the experimental samples of the resin containing solid waste forms.