The Effects of Sulfide on Pulp and Paper Wastewater Color Reversion
Type of DegreeThesis
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More and more concern is being given to the aesthetics of wastewater, specifically the color of the resulting effluent. The underlying causes of colored wastewater suggest incomplete pollutant removal and unsuccessful overall treatment. Therefore, monitoring color will soon become a more commonly regulated parameter. Yet, in order to accomplish decolorization as effectively and efficiently as possible, understanding the mechanisms of color reversion, and thereby pin-pointing the color culprit, needs to be achieved first. But, minimal research has been conducted on the generation of color in pulp and paper wastewaters thus far. Therefore, a study of color reversion was conducted in order to observe the effects of sulfide dissolved in pulp and paper wastewater. Color reversion is the yellowing of pulp on exposure to air, heat, certain metallic ions, and fungi due to modification of residual lignin-forming chromophores. The rate and degree of color reversion has been hypothesized to be related to four main mechanisms: 1) anaerobic color reduction, 2) anaerobic color generation via sulfide reaction, 3) aerobic color reversion, and 4) aerobic decolorization. The major goal of this thesis is to elucidate the role sulfide has on color reversion of treated and untreated paper mill wastewater. Experimentation involving sulfide exposure, aeration, and individual lignin compounds all encompass the efforts of this research. Sulfide exposure yielded as great as 100% color increase in some cases; whereas, in other tests, the results suggested sulfide had relatively no effect on color reversion. The differences appeared to be associated with the initial color of the wastewater—lighter colored samples yielded higher color increases upon sulfide exposure, while the darker samples displayed generally no relationship to sulfide concentration, except at unrealistically high doses. Aeration appeared to have a reverse effect on sulfide color reversion. However, following this color reduction, other mechanisms became dominant which caused color reversion to recur. These results support the color phenomenon seen across ponds where at times color remains consistent while during other times, it has actually been reported to increase, suggesting another color generation mechanism is involved. As aforementioned, the underlying differences seen in color reversion appear to be related to the wastewater composition, given the extreme variability of pulp and paper wastewater. The concentrations of lignin derivatives (specifically humic substances) were hypothesized to have direct influences on sulfide color reversion. When different humic functional groups were tested, the catechol and anthraquinone solutions portrayed the greatest effects on sulfide color reversion. The results of this research increase the overall understanding of the color reversion phenomenon, but do not solve this problem in its entirety.