August 4, 2008

Summary

A refinery waste water sample was supplied to Steward.  The sample had a strong odor, appeared brown-yellowish in color, and had a pH of 11.  The selenium concentration was about 1 ppm.  The kinetics of selenium reduction with Thiol SAMMS® (THSL) was enhanced by decreasing the pH.  At pH 11 and 0.05% THSL concentration, THSL was able to reduce selenium concentration by 30% within 45 minutes.  Decreasing the pH to 4.3 resulted in a further 30% selenium reduction for a total of a 60% reduction.  This is possibly due to the change in selenium speciation with pH levels. 

Increasing the THSL concentration had a dramatic effect on the selenium adsorption.  Selenium reduction was increased from 60% to 90% (pH 4.3) when THSL concentration was increased from 0.05% to 0.25%.  Adding more THSL or step additions of THSL did not show any apparent further benefit.

Background & Problem Statement

Four containers of a waste water containing selenium were received.  SAMMS THSL was investigated as an adsorbent for selenium.  The as-received waste water had a strong odor and a brown-yellowish color with a pH of 11.

Experiment

Treatment tests for the waste water for selenium removal were performed as follows:

1.  Contact solution:  as-received refinery waste water
2.  Precondition required amount of Thiol-SAMMS material (preconditioned with 20% ethanol water solution).
3.  Add preconditioned Thiol-SAMMS into contact solution.
4.  Equilibrate the mixture on the shaker, and then use 0.2 µm syringe filters sample solution phase periodically.
5.  Analyze selenium concentrations in the filtered samples by ICP-MS.
6.  Data validation.

 Results

The speciation of ionic selenium changes with pH.  The as-received refinery waste water had about 1 ppm of selenium concentration (we did not do a speciation analysis) and pH of about 11.  This is in agreement with information previously supplied by the customer.  Since Thiol-SAMMS works across a broad range of pHs, it was speculated that the best results would be obtained at a lower pH range rather than a high pH range due to the nature of the functional groups of THSL and selenium speciation at different pH ranges.  Figure 1 below clearly shows such a trend.  Selenium reduction was increased from ~30% to 60% when pH was reduced from 11 to 4.3.  The kinetics behaved quite differently under different pHs.  Under high pH, the repulsion force is suspected to have reduced the reaction rate dramatically.

 Figure 1.  pH Effect on THSL Kinetics of Selenium Reduction

Compared with mercury adsorption kinetics, selenium adsorption on THSL was quite slow.  Typically the majority of the mercury would be captured by THSL within the first few minutes; in contrast, only about 10% of the selenium was removed within 6 min, and the selenium concentration was being continuously reduced over time.  At 45 minutes residence time the selenium concentration was reduced by 60%.  Therefore, it was anticipated that increasing THSL concentration would dramatically increase the selenium adsorption rate.  Figure 2 below indicates once the THSL concentration was increased from 0.05% to 0.25%, the reaction rate in the first few minutes increased dramatically with the increased THSL concentration.  Reduction in pH further to 2.5 apparently resulted in sluggish kinetics compared with pH of 4.3.  The same trend was observed when the target element is mercury; but the difference in selenium adsorption between those two pHs is much larger than that in mercury adsorption.

 Figure 2. THSL Concentration and pH Effect on Selenium Adsorption Kinetics

 
Figure 3 below shows that at a pH of 4.3, further increasing the THSL concentration from 0.25% to 0.5% did not show any reaction rate increase.  After 15 min residence time, the ongoing reduction in selenium concentration has leveled off.  Step additions of THSL show a very slight, if any, benefit.  Adding more SAMMS did not help to further reduce the selenium concentration. 

Figure 3. THSL Concentration and Step Addition Effect on Selenium Adsorption

 Data validation was performed in two ways. The calibration was confirmed using NIST certified water containing trace metals. A repeated test was conducted to ensure the repeatability, as shown in Table 1 below.

Table 1.  Repeated Test Results

Conclusions

The effectiveness of Thiol-SAMMS for selenium reduction was enhanced by decreasing the pH.  At a pH of 11 and using a 0.05% THSL concentration, THSL was able to adsorb ~30% of the selenium within 45 minutes.  Decreasing the pH to 4.3 resulted in a further 30% selenium concentration reduction for a total of 60% reduction.    Increasing the THSL concentration had a dramatic effect on the selenium adsorption; the reduction in selenium concentration was increased from 60% to 90% when THSL concentration was increased from 0.05 % to 0.25%.  Adding more THSL or step additions of THSL did not show any more apparent benefit.