Effect of organic matter on estuarine flocculation: a laboratory study using montmorillonite, humic acid, xanthan gum, guar gum and natural estuarine flocs

Abstract

Background: Riverine particles undergo a rapid transformation when they reach estuaries. The rapid succession of hydrodynamic and biogeochemical regimes forces the particles to flocculate, settle and enter the sediment pool. The rates and magnitudes of flocculation depend on the nature of the particles which are primarily affected by the types and quantities of organic matter (OM). Meanwhile, the OM characteristics vary widely between environments, as well as within a single environment due to seasonal climate and land use variability. We investigated the effect of the OM types and quantities through laboratory experiments using natural estuarine particles from the Mississippi Sound and Atchafalaya Bay as well as model mixtures of montmorillonite and organic molecules (i.e., biopolymers (guar/xanthan gums) and humic acid).

Results: Biopolymers promote flocculation but the magnitude depends on the types and quantities. Nonionic guar gum yields much larger flocs than anionic xanthan gum, while both of them exhibit a nonlinear behavior in which the flocculation is the most pronounced at the intermediate OM loading. Moreover, the effect of guar gum is independent of salinity whereas the effect of xanthan gum is pronounced at higher salinity. Meanwhile, humic acid does not affect flocculation at all salinity values tested in this study. These results are echoed in the laboratory manipulation of the natural estuarine particles. Flocculation of the humic acid-rich Mississippi Sound particles is unaffected by the OM, whereas that of biopolymer-rich Atchafalaya Bay particles is enhanced by the OM.

Conclusions: Flocculation is positively influenced by the presence of biopolymers that are produced as the result of marine primary production. Meanwhile, humic acid, which is abundant in the rivers that drain the agricultural soils of Southeastern United States, has little influence on flocculation. Thus, it is expected that humic acid-poor riverine particles (e.g., Mississippi River, and Atchafalaya River, to a lesser degree) may be prone to rapid flocculation and settling in the immediate vicinity of the river mouths when mixed with biopolymer-rich coastal waters. It is also expected that humic acid-rich riverine particles (e.g., Pearl River) may resist immediate flocculation and be transported further away from the river mouth.

Figure 1

The samples of natural estuarine particle were collected in the field stations near Horn Island (Station H1) in the Mississippi Sound, and near Atchafalaya Bay off Louisiana coast (Stations A3 and A5) (Satellite image courtesy of NASA MODIS).

Figure 2

The steady-state median floc size ( d ₅₀ μm) for the systems containing 250 mg L ^-1 montmorillonite and varying quantities (0 – 20 mg L ^-1 in terms of total organic carbon (TOC)) of organic matter (either xanthan gum (XG), humic acid (HA) or guar gum (GG)) was determined at four discrete salinity values (i.e., S = 0, 3.5, 7 and 17.5 psu).

Figure 3

The steady-state median floc size (d₅₀ μm) for the systems containing 250 mg L^-1 montmorillonite and varying quantities (0 – 20 mg L^-1 in terms of total organic carbon (TOC)) of organic matter (either xanthan gum (XG) or humic acid (HA)) was determined at four discrete salinity values (i.e., S = 0, 3.5, 7 and 17.5 psu). This figure shows the same xanthan gum and humic acid data presented in Figure 2, but on a different y axis scale.

Figure 4

The median floc diameter is shown as a function of salinity for three overlapping ranges of organic matter loading (i.e., TOC = 0 – 5, 2 – 11, and 5 – 20 mg L^1-). The data for the size of organic matter-free flocs are shown using red x’s for comparison.

Figure 5

The steady-state median floc size (d₅₀ μm) of untreated flocs (green circles) and H₂O₂-treated OM-free samples (pink circles) as a function of salinity. The floc samples were collected near Atchafalaya Bay (A3 and A5) and in the Mississippi Sound near Horn Island (H1) before subjected to the laboratory experiments. The solid lines represent the linear fit to the data. The size is normalized for the OM-free steady-state median size at zero salinity. There is a clear separation between untreated and OM-free flocs for the Atchafalaya Bay samples while there is no discernible difference between untreated and OM-free floc sizes for the Horn Island sample.

Figure 6

Schematic diagram showing the effect of various organic matter (GG: guar gum; XG xanthan gum; HA: humic acid) on the flocculation of aqueous montmorillonite suspension at low (~0 psu) and high (~7-17.5 psu) salinity.