Effect of solar radiation on Dissolved Natural Organic Matter

Summary

Lakes and rivers are the main sources of drinking water in Nordic countries. For aesthetic and hygienic reasons high concentration levels of dissolved natural organic matter (DNOM) are not acceptable in drinking water. An increasing amount of DNOM in raw water sources for drinking water in Fennoscandia is thus raising our concerns. Moreover, the DNOM constitute an important transport mechanism for the flux of allochthonous nutrients to surface water. Once in the lake the DNOM photo-degrades due to adsorption of energy from sunlight. This causes its larger molecular weight (MW) compound to decompose into a variety of smaller organic photoproducts. Moreover, some of the DNOM is mineralized completely to inorganic compounds. Mineralization of DNOM causes a release of aluminum (Al) and iron (Fe) ions along with the release of orthophosphate (PO43-). The metal ions rapidly hydrolyze and form oxy-hydroxides which co-sorb phosphate and precipitate. Due to the removal of mineralized PO43- by subsequent precipitation with Fe- and Al-oxy hydroxides it is difficult to detect how much orthophosphate becomes mineralized by solar radiation. This is needed in order to assess the amount of this limiting nutrient becoming available for autotrophs. Thus the essence of this study is to test the option of using algae growth response as an indication of phosphorus release. Considerable research has been conducted comparing the changes in characteristics of the matrix that DNOM is in and structural characterization of DNOM, before and after solar exposure. A continuous increase in the relative contribution of lower to higher MW DNOM compounds along with a decrease in DOC concentration has been found with the extension of exposure time. This implies that the photochemical mineralization of DNOM by the photo-oxidation transforms the higher MW DNOM compounds into lower MW compounds, and that some of the DNOM is completely mineralized. This is further corroborated with fluorescence spectroscopy, indicating a decreased ratio of Humic to Fulvic acids in the DNOM as a response to sunlight exposure. This would imply that when sunlight is absorbed by DNOM, the average molecular weight is reduced. Results from assays showing increased algal growth imply that PO43- has been mineralized from DNOM due to photochemical reaction. On the other hand the growth response is not always significant. This may partly be due to concurrent heterotopic mineralization by bacteria.