Organic Matter and Mineral Interactions Modulate Flocculation Across Arctic River Mixing Zones

Abstract

The focus of this study is organic matter changes and suspended particle and colloid behavior in riverine and marine water mixing zones. Laboratory experiments demonstrated that salinity, clay mineral type, and the nature of dissolved organic matter (DOM) have a strong influence on mineral suspension stabilities (clays, carbonates, and ferric hydroxide), which represent the main fractions of particulate and colloidal matter riverine export. Clay mineral modification by humic acids stabilizes suspensions under increasing salinity, resulting in greater export of particles to coastal zones. Simultaneously, modified particles are more efficiently aggregated by a flocculant than nonmodified particles. Field investigations were conducted in Arctic river mixing zones, providing new information about colloid matter and colloid organic carbon distributions in the mixing zones, which agreed well with laboratory studies. The ratio between colloid organic carbon and suspended matter may prove a useful criterion to distinguish effective flocculation zones. Arctic River DOM fluorescence suggests dominance of terrigenous humic substances in mixing zones as well as coprecipitation by coagulation and flocculation. Our findings may reconcile reports of nonconservative behavior of DOM previously reported in the mixing zone at salinities of less than 12. We suggest that there is a flocculation mechanism in the estuarine mixing zone, which implies the presence of natural active substances, released during microorganism viral lysis (viral shunting).

Sharp changes in properties of aqueous sodium chloride solution at “critical” concentrations of the salt

“…It is important to understand the mechanisms of physical-chemical processes occurring to riverine matter when in contact with salt water [45]. It is known that the dominant saltinduced changes occur at salinity 3 -15 g/L and are connected with coagulation and flocculation of the major riverine matter in the mixing zone [1,2,13]. Satellite data in the fluvial-marine transition zone in the Laptev Sea and colored dissolved organic matter fluorescence data of the surface water from estuarine regions of Khatanga, Kolyma and Indigirka rivers in the East Siberian Sea demonstrated rapid removal of dissolved organic carbon (DOC) in coastal waters [46,47].…”

“…Biological species undergo osmotic stress at critical salinity and release soluble organic substances due to osmotic lysis or viral shunt. This newly formed DOM may contribute to the aggregation process, leading to the flocculation of suspended matter [10,13,50].…”

“…Salinity is the main environmental factor thatplays a decisive role in transport of riverine runoff in estuaries. The data obtained in the water of the mixing zone of the Northern Dvina River [12,13,16] showed that elevated concentrations of particulate suspended matter (PM) were revealed at salinities between 2.9 and 6.4 g/L. Alongside a decreasing concentration of particulate organic carbon (POC) at increasing salinity, elevated concentrations of POC were determined at 2.9, 11.9 and 18.5 g/L.…”

“…The range of 5-8 g/L is the critical salinity for river living organisms when marine biota replaces river biota at increasing salinity and the major biotic and abiotic processes demonstrate non-linear dynamics of change in rates and directions [8][9][10]. Simulation of flocculation of minerals (clays, carbonate, ferric hydroxide) which are similar to riverine particulate matter and colloids at increasing salt concentrations showed nonlinear behavior of their aggregative stability in the same range of salinity [11][12][13]. Non-conservative behavior of dissolved organic carbon (DOC), that means an increase in DOC concentrations at low salinity in an estuarine area, was found in some studies [14][15][16].…”

Aggregation of soil clay particles by organic polyelectrolytes

“…The proposed adsorption-flocculation mechanism of soil formation due to combine action of different types of organic polyelectrolytes may be applied to other fields of investigation, for instance, the study of aggregation of colloid and particulate riverine matter in the mixing zone river-sea (delta, estuary) where settled aggregates may be a beginning of soil formation [1], water purification and preparation of sand-clay pastes for various applications.…”