Study On The Controlling Of Nutrient Phase Transformation By Adsorption-desorption In The Changjiang Estuary

Estuarine mixing zone between the land and the ocean has multiple functions.Water properties like the concentration of suspended particle matter, Salinity,temperature, DOC, DO and pH in the estuary vary in time and space, and alter thenutrients partitioning between solid phase and solution via adsorption and desorption.Seaward nutrient fluxes change considerably in the estuary and it is important tounderstand the effects of estuarine processes on the phase transformation of nutrientin order that the flux of dissolved phase can be properly made.Chinese estuaries are shallow with abundant water and high concentration ofsuspended sediments, and of particularly interested in studying particulate-solutiontransformation. The Chanjiang Estuary serves as an important land-ocean interactionregion and can be taken as a natural laboratory for studying estuarine biogeochemicalprocesses. In this study, effects of salinity, pH, temperature, dissolved oxygen, organicmatter and SPM on the solid-liquid phase partitioning for nutrients (PO₄^3-, NH₄⁺,SiO₃^2-, NO₃^-) were examined with the samples from Changjing Estuary.Based on the data obtained from in-situ observation and laboratory simulation,some of the results can be summarized below.1. The concentration of suspended particle matter in the Chanjiang Estuary variesconsiderably. Within the maximum turbidity zone, concentration of suspended particlematter reaches up to m ore than 5×10³ mg L^-1. The pH range within the Changjiangestuary is limited to 7.7-8.3.2. Simulation results show that adsorption edges for phosphate adsorption on toSPM under different conditions illustrate the upended sigmoid curve characteristic oftransition phosphate. Changes in pH for the samples have a significant effect on thesolid-liquid partition of phosphate. The adsorption percentage of the phosphate wasincreased with pH-specifically increasing the pH of the water samples inducedpartitioning of phosphate to solid phases. The percentage of the adsorption andpartitioning coefficients for phosphate in the Changjiang Estuary deceasedsignificantly with higher concentrations of phosphate.3. For phosphorus, desorption was higher in seawater than that in fresh water. Asharp increase in percent desorption with an increase in salinity was obviouslyobserved in low salinity(S＜15) water. The amount of phosphorus released increased with an increase pH was previously observed for phosphorus in mixtures. All theprocesses discussed here play a major role in the retention of phosphate in particlesand ultimately in estuaries. Based on the sorption K_d values presented here, we wouldexpect that phosphate would be trapped in the estuarine environment as the metals aremostly associated with the particulate phase in estuaries. The model developed here isin accordance with the observed distribution patterns of phosphate in the ChangjiangEstuary.4. The adsorption edges for NH₄-N adsorption to SPM under different conditionsillustrate the sigmoid curve characteristic of transition NH₄-N. Changes in pH for thesamples from the Changjiang Estuary have a significant effect on the solid-liquidpartition of NH₄-N. The adsorption percentage of the NH₄-N in the experiments wasincreased with pH of the water samples induced partitioning of NH₄-N to solidphases.5. The adsorption percentage of the NH₄-N in the experiments was increasedwith the increasing of SPM, DO of the water samples, while the adsorptionpercentage of the NH₄-N in the experiments was decreased with the increasing ofsalinity, temperature and the concentration of NH₄-N in the samples.6. The desorption percentage of the SiO₃^2- in the experiments was increasedsalinity. When S=15, there was a maximum in the desorption percentage of the NO₃^-.The desorption percentage of the NO₃^- in the experiments was increased when S＜15,while S＞15, the desorption percentage of the NO₃^- in the experiments was decreased.The desorption percentage of the SiO₃^2- in the experiments was increased pH, also.While the desorption percentage of the NO₃^- would be constant with the change of thepH. Dissolved oxygen would not affect the desorption percentage of the SiO₃^2- OffSPM, and the desorption percentage of the NO₃^- was increased with DO7. The Changjiang estuary is known to contribute significantly to theeutrophication that has caused drastic changes to the ecosystem of the East China Sea.However, evidence for historical changes in nutrient concentrations and compositionand the consequent effects on the ecosystem in the coastal water is sparse. Somelong-term data for nutrient concentrations and Si: N: P ratios in the freshwater and theestuary and the long-term response of the ecosystem structure in the estuary. Thesedata reveal increases in the dissolved inorganic nitrogen and phosphate concentrationsin the Changjiang freshwater by a factor of five from the1950s to the end of the 2000sand a reduction in dissolved silicate by two thirds over the same period. Concomitantly, an increase in DIN concentration and a reduction in silicateconcentration both by a factor of two were observed in the surface water of theChangjiang estuary.