Browsing by Subject "flocculation"

Salt-induced flocculation is a process in which dissolved salts in seawater modify dissolved organic matter (DOM) surface charges and enable its transformation into larger fractions. These larger particles can settle to the benthos removing organic matter from the water column. Flocs formed in the flocculation process are often referred to as suspended particulate material (SPM). SPM is an important part of the biogeochemical cycles. SPM is a vector for pollutants, a food source for organisms and a habitat for microbes. Flocculation is part of the estuarine “coastal filter” -system controlling the material fluxes from land to sea. Climate change is causing water temperatures to increase in both inland and coastal waters affecting the biogeochemistry of aquatic ecosystems. Flocculation is a physicochemical process potentially impacted by temperature but the effects of temperature on flocculation are not well known. In my master’s thesis, I studied the effects of water temperature on the salt-induced flocculation of humic-rich river water. The flocculation experiments were conducted in spring 2023 using humic-rich water sampled from Vantaanjoki, Helsinki, Finland. The effects of temperature were studied in two separate experiments at temperatures ranging from 3 ºC to 20 ºC. The studied variables included quantitative suspended particulate matter as well as the optical properties of DOM including colored DOM and fluorescent DOM. Results of the experiments show that temperature did not have a significant effect on the salt-induced flocculation of DOM in humic-rich river water. However, the optical properties of DOM shifted with increasing temperatures and changing flocculation dynamics. Results show that molecular size increased as temperature increased but also when temperature was lower than 3 ºC. Explanation for the observed trends is discussed but more research is needed for more firm conclusions. DOM concentrations are expected to increase in the Arctic and sub-Arctic rivers transporting more organic matter to estuaries. Results of this thesis suggest that rising temperatures will not increase the amount of particulate matter through salt-induced flocculation meaning that a larger portion of the transported material remains in dissolved fraction. Increases in DOM concentrations may shift marine food webs by changing the food and light availability as well as biogeochemistry in estuaries.

Exchanges between DOM and POM play an important role in the carbon cycling of freshwater ecosystems. Flocculation is a process where aggregates of DOM are formed and moved from a liquid phase to a solid phase. For colloids to form flocs, the negative surface charge of the present organic colloids needs to be destabilized. This process is generally dependent on salinity, but other compounds affecting the ionic strength of the solution can influence these processes. Gypsum is applied to Finnish fields to reduce the amount of nutrient leaching from agricultural soils. Gypsum treatment effectively reduces the runoff of both particulate and dissolved phosphorus from agricultural fields. Gypsum treatments are performed in areas where the soil contains over 30% clay minerals, making gypsum usage highly relevant in the Archipelago Sea area. This thesis aimed to find out how gypsum additions influence flocculation processes and DOM characteristics of terrestrial organic matter in boreal rivers. The study focused on the implications of gypsum use in waterways from areas with agricultural activities. Three core experiments were conducted to investigate the effects of several variables on flocculation dynamics. This study provides some preliminary insights into the influence of gypsum on biogeochemical processes in rivers. Gypsum additions were found to influence the flocculation processes of terrestrial organic matter in boreal aquatic environments. The most notable effect of gypsum additions was the enhanced floc formation, creating an increased flux of organic material onto the sediment surface. This has potential implications for microbial and benthic food webs, meaning that gypsum use is something that may need to be considered when assessing the impact of agriculture on the biogeochemical processes of waterways. Gypsum additions were also found the influence the characteristics of the remaining DOM pool. With the current knowledge, the positive aspects of gypsum use heavily outweigh the negative ones.