FAN Jing-yu, HE Xiao-yan, WANG Dao-zeng

Shanghai Institute of Applied Mathematics and Mechanics, Shanghai University, Shanghai 200072, China, E-mail: jyfan@shu.edu.cn

Experimental study on the effects of sediment size and porosity on contaminant adsorption/desorption and interfacial diffusion characteristics*

FAN Jing-yu, HE Xiao-yan, WANG Dao-zeng

Shanghai Institute of Applied Mathematics and Mechanics, Shanghai University, Shanghai 200072, China, E-mail: jyfan@shu.edu.cn

(Received June 6, 2012, Revised August 16, 2012)

The joint effects of the sediment size and porosity on the contaminant adsorption/desorption and interfacial diffusion characteristics were experimentally investigated. The adsorption of Phosphorus (P) on the natural and artificial sediment suspensions was measured with respect to the P adsorption isotherms and kinetics in the experiment. The obtained adsorption isotherms for different grain-sized sediment suspensions fit well with the Langmuir equation, dependent on the initial aqueous concentration and sediment content. The P kinetic adsorption behaviors for cohesive fine-grained and non-cohesive coarse-grained sediment suspensions clearly show the size-dependent feature. On the other hand, the P kinetic release feature of a porous sediment layer is affected by not only the direct desorption of the uppermost sediments, but also the diffusivity in the pore-water within the underlying sediment layer, characterized by the sediment size and porosity, respectively. Furthermore, the temporal contaminant release from the permeable sediment layer into the overlying water column increases with the increasing flow velocity, while this enhancement in mediating the interfacial diffusion flux is somewhat insignificant in an immediate release stage, largely due to the resistance of the diffusive boundary layer on the hydrodynamic disturbance.

sediment, contaminant, adsorption, desorption, interfacial diffusion

Introduction

The complex exchange processes of substances like nutrients, Persistent Organic Pollutants (POPs), heavy metals and other potentially harmful materials between the contaminated sediments and the overlying water column in rivers, lakes, reservoirs, and estuaries are of considerable importance in understanding the impacts of the contaminated sediments on the aquatic environments[1-6]. It is well known that the transport process across the sediment-water interface in the case of the static release is dominated by the direct diffusive process, and there exists a diffusive boundary layer within which the mass transfer of the contaminants occurs largely due to the molecular diffusion or turbulence diffusion[7,8]. This diffusive process is likely to cause an increasing contaminant concentration of the overlying water column even after those externally-supplied contaminant sources have been removed or heavily reduced, and the associated benthic diffusion flux can commonly be estimated based on the measured concentration gradient across the sediment-water interface[8,9].

For the adsorptive contaminants, the adsorption/ desorption behaviors of the sediments play an important role in the transport process across the sedimentwater interface, and the physical properties of the porous sediment layer may be particularly pronounced for the mass transfer across the diffusive boundary layer. In this regard, the sediment size is an important property that affects the contaminant adsorption and desorption. Prior research has been performed to investigate the size-specific adsorption/desorption of the sediment suspensions[10,11]. However, even in the case of the static release from the porous sediment layer, the joint influence of the grain size and porosity of theporous sediment layer on the interfacial diffusive process has not been adequately considered in previous research. In particular, the concentration gradient of the pore-water in the underlying active sediment layer is subjected to unsteady conditions due to not only the limited capacity to retain the contaminant but also the temporal concentration variation caused by both the diffusive release and the sediment desorption. Therefore, the static release from the porous sediment layer into the overlying water column is a time-dependent diffusive process, and the temporal concentration variation of the overlying water column becomes considerable in this case.

Considering that the natural sediments are composed of different grain-sized components, and it is significant to characterize the size-specific contaminant adsorption/desorption behavior, which is essential for estimating the adsorption/desorption of the natural sediments by the linear additivity assumption with respect to their size fractions[10]. In this article, the Phosphorus (P) adsorption isotherms and kinetics of the natural and artificial sediment samples are firstly examined for different grain-sized sediment suspensions in the experiment. The P desorption and release features from the porous sediment layer are then experimentally investigated under quiescent condition, and special attention is paid to the effect of various physical properties of the porous sediment layer, such as the grain size and porosity, on the kinetic release process. In addition, the associated hydrodynamic mechanisms that can cause the time-dependent interfacial diffusive characteristics from the permeable sediment layer into the overlying water column under different flow velocity conditions are further analyzed.

1. Experimental facilities and measurement techniques

The natural sediments used in the experiment were collected in situ from Dianshan Lake in Shanghai, China. The median grain size of the natural sediments was about 30 μm, and besides, a series of the artificial sediments samples were prepared and sieved into six size fractions with the grain sizes of D1 (<50 μm), D2 (50 μm-100 μm), D3 (100 μm-200 μm), D4 (200 μm-300 μm), D5 (300 μm-600 μm), and D6 (600 μm-1 000 μm). All the sediment samples were subjected to a pre-treatment procedure to remove the adsorbed P and other nutrients and organics. The obtained clean sediment samples were dried and used in the experiment.

For the natural and artificial sediment suspensions, the P adsorption isotherms were firstly determined in a small vessel of 300 ml capacity at various initial aqueous concentrations C0less than 12 mg/l for the natural sediments or less than 2.5 mg/l for the artificial sediments (at the sediment content of 10 g/l) and the sediment content ranging from 35 g/l to 550 g/l (at the initial aqueous concentration of 20 mg/l). The final aqueous concentration in the sediment suspensions was measured as the equilibrium concentration Ceafter 24 h, and the adsorption quantity per unit weigh sediments Cswas calculated by the difference between the initial and final aqueous concentrations in the sediment suspensions. All aqueous P concentrations of the sampled solutions were measured by using the spectrophotometer according to the standard procedure.

The kinetic adsorption of the natural and artificial sediment samples was determined in a stir chamber with the capacity of 2.5×10–5m3×2.5×10–5m3×3.0× 10–5m3. The sediment/water ratio was set to 1:2 in this kinetic adsorption for the sediment suspensions and the following release from the porous sediment layer. Two initial aqueous concentrations were adopted at the low concentration of 23 mg/l and the high one of 100 mg/l. An electric stirrer was continually and steadily operated to maintain the fully mixing in the sediment suspensions, and the sampled solution was used to determine the aqueous concentration variation with time. It should be noted that due to high sediment content in the kinetic adsorption experiment, the adsorption equilibrium of the sediments in suspension was not always attained.

Table 1 The physical properties of the porous sediment layers

After the kinetic adsorption experiment for the sediment suspensions, the sediments in suspension were deposited under quiescent condition till forming a distinct stratification, and the upper layer solution was taken out from the chamber by siphoning method. The P-free distilled water was injected into the chamber to investigate that desorption and release from the porous sediment layer composed of different grain-sized sediments under quiescent condition, and these unconsolidated sediment layers had different porosities mainly dependent on the sediment size. The porosity of the porous sediment layer could be directly measured from the weight difference between the saturated and dried sediment samples. For the artificialsediments with the increasing grain sizes ranging from D1 to D6, the corresponding porosities of the porous sediment layers basically had a decreasing trend ranging from 47.0% to 36.8% with an exception of D6 case corresponding to 41.0%, as listed in Table 1. It should be also noted that the initial amount of the contaminants adsorbed on the sediments and dissolved in the pore-water were not identical, depending on the P partitioning between the particular and aqueous phases in the kinetic absorption experiment. In the course of desorption and release experiments, the concentration variation with time of the overlying water column Cwwas determined by measuring the sampled solution.

Finally, the temporal release from the permeable sediment layer under different flow velocity conditions were experimentally studied in an open water channel with a rectangular test section of 6.0 m in length, 0.25 m in width and 0.45 m in height. The average flow velocity (u =0.05 m/s, 0.1 m/s) and water depth (h =0.5 m) of the flume could be obtained by suitably adjusting both the rotating speed of a frequency-controlled centrifugal water pump and the opening of a tail gate located at the end of the flume. The artificial sediments with the median grain size of 350 μm were used in the experiment, and the permeable sediment layer was placed in a cavity with the length of 2.0 m and the thickness of 0.045 m in the middle of the test section. More details concerning the measurement method and sample preparation can be found in Refs.[12,13]. During the course of the temporal release experiment, the time-dependent release from the permeable sediment layer could be examined from the measured concentration variation with time of the overlying water column.

Fig.1 The measured adsorption isotherms for natural sediment suspensions

2. Results and analyses

2.1 The adsorption isotherms and kinetics of different grain-sized sediment suspensions

In general, the contaminant adsorption/desorption characteristics of the sediments depends on numerous interacting physical, biochemical and environmental factors, among which the physical properties of the sediments are the main influencing factors under consideration in this article. The adsorption/desorption isotherms and kinetics for different grain-sized sediment suspensions are the fundamental properties that greatly affect the sediment-associated contaminant transport process. In the present adsorption experiment, the P adsorption isotherms for the natural and artificial sediment suspensions were measured and obtained at specific initial aqueous concentrations C0, as shown in Fig.1 and Fig.2, respectively.

Fig.2 The measured adsorption isotherms for artificial sediment suspensions of D2 fraction

The P adsorption isotherms for natural and artificial sediment suspensions exhibit similar overall trend, indicating that the increasing initial concentration leads to the increasing adsorption quantity of the unit weigh sediments. The obtained data in terms of the adsorption quantity and the aqueous equilibrium concentration for all cases fit well with the Langmuir equation, and the measured adsorption quantity Csof the natural sediments is shown to be higher in magnitude than that of the artificial sediments. This is predominantly attributed to the finer grain size and consequently larger specific surface area for the natural sediments.

Fig.3 The measured adsorption quantities for the sediments in suspension under different sediment contents

The effect of the sediment content on the adsorption quantity Csis taken into account in a wide range of the suspended sediment concentrationS , and the measured adsorption quantities for different grain-sized sediments in suspension are shown in Fig.3. For the fine-grained sediment fraction (D2), there exists a maximum adsorption quantity under the experimental conditions, and with the continuously increasing S for dilute and dense sediment suspensions, the corresponding Cstends to increase and subsequently cline down, respectively. While for the relatively coarse sediment fractions (D3 and D4), there also exists a maximum adsorption quantity in magnitude with the appreciably lower peak as comparing to that in D2 fraction, and with the continuously increasing S for dense sediment suspension, the corresponding Cstends to flatten out till up to a constant adsorption quantity which is to some extent insensitive to the grain size. Therefore, it can be inferred that it is indispensable to comprehensively take the effect of the grain size and sediment content on the adsorption property into consideration.

Fig.4 The measured adsorption kinetics for different grainsized sediment suspensions

The measured adsorption kinetics for different grain-sized sediment suspensions at low initial concentration are shown in Fig.4. It can be seen from Fig.4 that the kinetic adsorption quantities of different grain-sized sediment suspensions show a clear sizedependent feature, and the fine-grained sediments have larger adsorption quantities than the coarse-grained sediments. There is a notable increase in the P adsorption quantity in D1 sediment fraction, as expected, attributing to the larger specific surface area for the finer grain size. With the increasing grain size, the P adsorption quantities in other sediment fractions tend to decrease in magnitude, and the slopes of the adsorption kinetics curves in each sediment fraction become more and more flat, indicating the gradually decreasing adsorption rate in the time-dependent adsorption process.

2.2 The desorption and release features from the porous sediment layer under quiescent condition

The contaminant release from an undisturbed sediment layer into the overlying water column under quiescent condition is a typical mass transfer process purely due to the molecular diffusion across the sediment-water interface, and as a result, the concentration profile of the overlying water column changes abruptly within the region adjacent to sediment-water interface in an initial release stage and tends to more and more uniform in an equilibrium stage. In this case, the contaminant concentration variation of the overlying water column is directly related to the supply of the contaminants from the porous sediment layer, including the direct desorption from the uppermost sediments and the release from the pore-water within the underlying sediment layer. Thus, it is of particular interest to characterize that the relative importance of the direct sediment desorption and the pore-water release in the interfacial diffusion process.

Fig.5 The comparison of the kinetic release from the porous sediment layers composed of D2 and D3 fractions at low initial concentration

Fig.6 The comparison of the kinetic release from the porous sediment layers composed of D4 and D6 fractions at low initial concentration

Figure 6 shows the comparison of the kinetic release from the porous sediment layers composed of D4 and D6 fractions at low initial P concentration, and it is verified that the relative importance of the co-existent contaminant supplies, namely those adsorbed onto the sediments and dissolved in the pore-water. For particulate contaminants, only the uppermost sediments can desorb the contaminants directly into the overlying water column, and the subsurface sediments may desorb the contaminants into the pore-water within the porous sediment layer. Therefore, the dissolved contaminants in the pore-water involving the initial part and the subsurface sediment desorption part may exert more pronounced effect on the concentration variation of the overlying water column via the diffusivity within the porous sediment layer, which is a relatively slow diffusive process gauged by the porosity, and thus this contribution to the interfacial diffusion is dominated mainly in the posterior release stage.

Fig.8 The comparison of the kinetic release from the porous sediment layers composed of D1 and D5 fractions at high initial concentration

The contaminant amount adsorbed onto the sediments and dissolved in the pore-water can also substantially influence the interfacial diffusion process, and this can be observed from the comparison of the kinetic release from the porous sediment layers composed of D1 and D5 fractions at both low and high initial concentrations, as shown in Fig.7 and Fig.8, respectively. At low initial concentration, the contaminant amount adsorbed onto the sediments are considerably limited in the case of D5 fraction, and the direct desorption from the uppermost coarse sediments is negligible as comparing to the release from the pore-water in the case of D1 fraction. While at high initial concentration, it is shown from Fig.7 that the important determinant of the interfacial diffusion in the experiment duration is the same as the above-mentioned transformation from the direct desorption of the uppermost sediments to the release from the pore-water within the porous sediment layer.

2.3 The temporal release from the permeable sediment layer under different flow velocity conditions

The hydrodynamic condition of the overlying water column plays an important role in the interfacial transport process, which is not solely due to the molecular diffusion. In general, the flow velocity of the overlying water column produces several additional hydrodynamic mechanisms that alter the interfacial momentum exchange or mass transfer fluxes across the sediment-water interface even without the resuspension of the sediments, among which the shear velocity near the sediment-water interface causes the dispersion like transport. Other influencing mechanisms that are likely to drive the interfacial transport, either diffusion or advection, process include the small scale turbulence structure, such as the viscous sublayer bursts and sweeps, the surface roughness, and the pressure fluctuations around surficial sediment grains or bed forms[9,14-16]. In particular, the flow over the permeable sediment layer may induce the momentum andmass exchange across the sediment-water interface, and the no-slip surface boundary condition is no longer applicable. Thus, the increasing flow velocity of the overlying water column commonly gives rise to the intensified interfacial diffusive flux, no matter whether the resuspension of the sediments occurs. However, most of prior research commonly concern the hydrodynamic effect on the interfacial transport process that is thought to be independent on time or based on the quasi-steady assumption, and the temporal release feature from the permeable sediment layer composed of the sediments with the median grain size of 350 μm under different flow velocity conditions is examined in the present experiment.

Fig.9 The temporal concentration variation of the overlying water column under different flow conditions

The temporal concentration variation of the overlying water column during the release from the permeable sediment layer is shown in Fig.9. For the case of the relative small velocity (u =0.05 m/s), it can be observed that a major trend for the concentration of the overlying water column exhibits a somewhat monotonously increased variation with time, approximately in a logarithmic curve that can be fitted using both the theoretical analysis and the measured data. With the increasing flow velocity (u =0.1 m/s), the release from the permeable sediment layer into the overlying water column is intensified in the course of the whole experimental duration. Nevertheless, this enhancement trend in mediating the interfacial diffusion flux is somewhat insignificant in an immediate release stage, and yet it is more detectable and remarkable in a posterior release stage. Note that the concentration gradient across the sediment-water interface in the immediate release stage should be sharper than that in the posterior release stage, it would be expected that the enhancement trend of the temporal interfacial diffusive flux should be more pronounced in the immediate release stage, due to those additional hydrodynamic mechanisms. Therefore, the cause as to this disagreement with the time-independent interfacial diffusive process in the case of an impermeable sediment layer is likely to be the comprehensive interaction between the molecular diffusion and the additional hydrodynamic disturbance, and it can be inferred that the molecular diffusion within the initiallyformed thin diffusive boundary layer results in the resistance to the intensified interfacial flux driven by the additional hydrodynamic disturbance, which is also revealed in prior research[5,8]. While in the posterior release stage, the concentration profile within the thicker diffusive boundary layer becomes more and more uniform, and accordingly, the transport process across the sediment-water interface is gradually dominated by the hydrodynamic disturbance, instead of the molecular diffusion.

3. Conclusion

The joint effects of the sediment size and porosity on the contaminant adsorption/desorption and interfacial diffusion characteristics have been experimentally investigated. The adsorption of P on the natural and artificial sediment suspensions has been measured with respect to the P adsorption isotherms and kinetics in the experiment. The obtained adsorption isotherms for different grain-sized sediment suspensions fit well with the Langmuir equation, dependent on the initial aqueous concentration and sediment content. The P kinetic adsorption behaviors for cohesive fine-grained and non-cohesive coarse-grained sediment suspensions clearly show the size-dependent feature, and the fine-grained sediments have larger adsorption quantities than the coarse-grained sediments. The P kinetic release feature of the porous sediment layer is affected by not only the direct desorption of the uppermost sediments, but also the diffusivity in the pore-water within the underlying sediment layer, and the important determinant of the interfacial diffusion process in the immediate and posterior stages transforms from the direct desorption of the uppermost sediments to the release from the pore-water within the porous sediment layer. Furthermore, the temporal contaminant release from the permeable sediment layer into the overlying water column increases with the increasing flow velocity, and this enhancement in mediating the interfacial diffusion flux is somewhat insignificant in the immediate release stage, largely due to the resistance of the diffusive boundary layer on the hydrodynamic disturbance. While the transport process across the sediment-water interface is gradually dominated by the hydrodynamic disturbance in the posterior release stage.

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10.1016/S1001-6058(13)60334-0

* Project supported by the National Natural Science Foundation of China (Grant Nos. 10972134, 11032007), the Shanghai key Laboratory of mechanics in energy Engineering and the Shanghai Program for Innovative Research Team in Universities.

Biography: FAN Jing-yu (1968-), Male, Ph. D., Associate Professor

WANG Dao-zeng, E-mail: dzwang@staff.shu.edu.cn