Simulation and parameter determination of persistent organic pollutants sorption
process in sediment
Donglin Yu, Jie Shi
Ocean University of China, Qingdao, 266100, China
1 Purposes
The distribution of persistent organic pollutants (POPs) in the ocean is affected by thesorption-desorption process of sediment particles, which is determined by the concentration of POPsin seawater, water temperature, and organic carbon content of sediment particles. Quantitative
relationships between the net sorption rates and these factors have not been established and used inPOPs transport models. This study aimed to accurately simulate the net sorption processes ofphenanthrene, a representative POPs, in marine environments. To achieve this, the parameters of thetwo-compartment mass-transfer model was determined by fitting the model results to theexperimental data. Quantitative relationships were then established between the model parameters
and the initial concentration of phenanthrene, water temperature, as well as organic carbon content
of the sediment particles.
2 Methods
2.1 Two-compartment mass-transfer model
In the two-compartment mass-transfer model, the net sorption process is divided into fast andslow processes (Ding et al., 2008). Consequently, the pollutant concentration in the solid phase (�� ,�mg g-1) also have two components: the concentrations of fast (�� , mg g-1) and slow processes (��� ,mg g-1). The net sorption rate is the sum of the two processes and can be expressed as���
��

=�

���
��

+�

����
��

�

�

= (�� ∙ �� − �� ∙ ��) + (��� ∙ �� − ��� ∙ ��� )(1)�

where �� (mg L-1) are the concentrations of the aqueous phase. �� and ��� (L g-1 min-1) are the��sorption rate coefficients, and �� and ��� (min-1) are the desorption rate coefficients, �� ∙ �� and����� ∙ �� are sorption rates for the fast and slow processes, and �� ∙ �� and ��� ∙ ��� are desorption
rates for the fast and slow processes.

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2.2 Experimental data and simulations
The experimental data used in this study were obtained from the measured net sorption processesof phenanthrene between sediment particles and seawater. Three groups of experiments (Groups I, II,and III) were performed to examine the effects of the initial phenanthrene concentration in seawater,water temperature, and organic carbon content of the sediment particles on the net sorption process.
These three groups of experiments can be used to determine model parameters under different
phenanthrene concentrations in seawater, water temperatures, and organic carbon contents of thesediment particles. The experimental data of the three groups were the results of previous studies andthe specific experimental methods are referred in the corresponding papers (Yang and Zheng, 2010;Zheng, 2010; Xu and Huang, 2011).
The parameters of the model were determined by fitting the solutions to Eq. (1) to theexperimental data. The correlation coefficients (�2 ) and sums of squares of error (SSE) between themodel results and the experimental data were calculated to evaluate the effects of the simulation.
Based on these results, the dependences of the parameters in Eq. (1) on the initial phenanthrene
concentration in seawater, water temperature, and organic carbon content of the sediment particles
were examined.
3 Results
3.1 Influence of the initial concentration in seawater on net sorption
The net sorption experimental data for Group I were obtained for three different initial pollutant
�

concentrations. In the two-compartment mass-transfer model, �� and ��� represent the migration
�

rates of phenanthrene from seawater to sediment particles and �� and ��� represent the migration��

rates from sediment particles to seawater. �� and �� determine the net sorption rate of the fast��process, and ��� and ��� determine the net sorption rate of the slow process. �� , ��� , �� , and ���

showed little change at the three different initial concentrations. In other words, the initial��concentration of phenanthrene in seawater had little impact on the �� , ��� , �� , and ��� values. Thus,��

a fixed set of parameters was used in all three experiments (Fig. 1). �� , ��� , �� , and ��� were tuned
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to obtain a set of optimal values to reduce the bias between the model results and the experimental
data, which were 0.0399 L g-1 min-1, 0.00331 L g-1 min-1, 0.475 min-1, and 0.0766 min-1, respectively,obtaining an �2 of up to 0.9931.

The results show that the model can reproduce the experimental data using a set of parameters fordifferent initial concentrations. Therefore, it is feasible to use a set of parameters for ocean
simulations with variable phenanthrene concentrations.

Fig. 1 Simulation curves of the two-compartment mass-transfer model for phenanthrene at different
initial concentrations with the same set of parameters.
3.2 Influence of the water temperature on net sorption
The experiments in Group II tested the dependence of the net sorption curves on the watertemperature. Based on the fitting results of these three experiments quantitative relationships
between the parameters in the two-compartment mass-transfer model was established, which are
expressed as follows:�(2)�� =− 0.0006 ∙ � + 0.0553

��� =− 0.0001485 ∙ � + 0.0075(3)��� = 0.0022 ∙ � + 0.034

(5)

�(4)

�� =− 0.0008 ∙ � + 0.4943

where T is the water temperature (℃).
When only one set of parameters was used, the maximum value of �2 was 0.8903. When thelinear relationship between the parameters and the water temperature in Eqs. (2-5), the mean �2reached up to 0.9954, with an SSE of 7.66×10-6, which indicated a better performance of the model(Fig. 2).

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The accuracy of the simulations was improved by considering the dependence of the model
parameters on water temperature. Moreover, it has become more reasonable and feasible to apply
this model to real oceans with spatially and temporally changes in the water temperature.

Fig. 2 Simulation curves of the two-compartment mass-transfer model considering the dependenceof the parameters on the water temperature.
3.3 Influence of the organic carbon content of sediment particles on net sorption
The experiments in Group III showed the net sorption curves of the sediment particles with threedifferent organic carbon contents. The parameters were assumed to be linear functions of the organiccarbon content. In the two-compartment mass-transfer model, the linear relationships between
parameters and the organic carbon content are expressed as follows:�(6)�� = 0.3318 ∙ ��� + 0.00036445

��� = 0.0495 ∙ ��� − 0.00016960(7)��� =− 0.0258 ∙ ��� + 0.0033

(9)

�(8)

�� = 0.2554 ∙ ��� + 0.0271

where ��� is the organic carbon content (%).

Fig. 3 Simulation curves of the two-compartment mass-transfer model considering the dependence

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of the parameters on the organic carbon contents.
When considering the dependence of the parameters on the organic carbon content of the sedimentparticles (Fig. 3), the fitting results showed that �2 was up to 0.9612 and SSE was 1.35×10-4.
However, if the dependences of the parameters on organic carbon content is ignored, the optimal �2

was only 0.7600. The results showed that the simulation accuracy significantly improved bychanging the model parameters for different organic carbon contents.
4 Future Challenges
Further improvements are required in this study. Estimations of the parameters of thetwo-compartment mass-transfer model largely depended on the three selected laboratory
experiments. Thus, additional sorption kinetics experiments must be conducted to verify theaccuracy of the parameters in the model.
In future studies, similar methods can be used to determine the sorption-desorption parameters ofother POPs. In addition, the results of this study make it possible to simulate the distribution and fateof POPs in both seawater and sediment by embedding a net sorption module into the transport modelof POPs, which improves the accuracy of the simulation. Moreover, the parameters can be adjusted
according to the comparison between the simulated results and the observations, and thesorption-desorption parameters suitable for other POPs can be determined.
Reference
Ding, H., Li, X. G., Xu, S. M., Sun, Y. C., Shao, X. L., 2008. HCB adsorption- desorption behavior
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sediments. Environ. Toxicol. Chem. 29(10), 2169-2176. https://doi.org/10.1002/etc.270.
Zheng, X., 2010. Studies on the sorption behaviors of phenanthrene and methylene blue on marine
sediments. master’s thesis, the Ocean University of China, Shandong, Qingdao.
https://doi.org/10.7666/d.y1830235.
Xu, X. W., Huang, S. L., 2011. Adsorption-desorption behaviors of phenanthrene on sediments fromtheHaiheRiver.

ActaSci.

https://doi.org/10.7666/d.y1813877.

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Circumstantiae

31(1),

114-122.