Capacity of humic substances to complex with iron at different salinities in the Yangtze River estuary and East China Sea

Abstract

The iron binding capacities (IBC) of fulvic acid (FA) and humic acid (HA) were determined in the salinity range from 5 to 40. The results indicated that IBC decreased while salinity increased. In addition, dissolved iron (dFe), FA and HA were also determined along the Yangtze River estuary's increasing salinity gradient from 0.14 to 33. The loss rates of dFe, FA and HA in the Yangtze River estuary were up to 96%, 74%, and 67%, respectively. The decreases in dFe, FA and HA, as well as the change in IBC of humic substances (HS) along the salinity gradient in the Yangtze River estuary were all well described by a first-order exponential attenuation model: y(dFe/FA/HA, S) = a₀ × exp(kS) + y₀. These results indicate that flocculation of FA and HA along the salinity gradient resulted in removal of dFe. Furthermore, the exponential attenuation model described in this paper can be applied in the major estuaries of the world where most of the removal of dFe and HS occurs where freshwater and seawater mix.

Figure 1

Locations of the seawater sampling stations in the Yangtze River estuary (July, 2015; left panel) and the ECS (October, 2011; right panel). (Ocean Data View (ODV) software, version 4.5.6, Schlitzer, R., http://odv.awi.de, 2013).

Figure 2

Vertical section of salinity, Fe concentration (nmol/L), FA (μg/L) and HA (μg/L) concentration along the Yangtze River estuary. (Ocean Data View (ODV) software, version 4.5.6, Schlitzer, R., http://odv.awi.de, 2013).

Figure 3

Vertical section of salinity and Fe (nmol/L), FA (μg/L) and HA (μg/L) concentrations along section P in the ECS. The Fe concentrations were obtained from Su et al. (2015), and the salinity data were obtained from Li et al. (2014). (ODV software, version 4.5.6, Schlitzer, R., http://odv.awi.de, 2013).

Figure 4

Non-linear curve fit to dFe, FA and HA concentrations along the salinity gradient, and the relationship to removal of dFe, FA and HA along the salinity gradient. “x” represents the salinity of the YRE.

Figure 5

Iron binding capacity of FA and HA in the UVSW in the presence of 1 mg/L HS.

Figure 6

Effect of salinity on the iron binding capacity of FA/HA of the SRFA/HA (A) and Yangtze River estuarine samples (B). The dashed line represents the fitting curves fit to first-order exponential equations. (A) The error bars around the IBC of SRFA/HA at every salinity value of standard seawater were standard deviations of the average values of IBC.