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

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.     

Experimental design of diffusion and desorption of contaminant in heterogeneous media

Storage of contaminants in low permeability media (LPM) presents a great challenge for prediction of remediation effectiveness and efficiency. The reason lies in the contaminants' complex behaviors within heterogeneous media. Both interparticle and intraparticle diffusion contribute to the difficulty of precise site assessment. Sorption of contaminants--especially within LPM--may sequester the contaminants from active treatment, while desorption over a long period of time leads to contaminant release from storage and consequent re-contamination. Research has been conducted toward better understanding of contaminant diffusion and sorption/desorption processes to better predict contaminant response to site treatment. However, most of the research has been carried out within homogeneous media, while real scenarios in environmental problems feature media whose permeability and other characteristics vary significantly over the treatment volume. Further, few efforts have combined the interparticle/intraparticle diffusion and sorption/desorption processes together. This research aims at a feasible experimental design of diffusion and desorption of contaminant in heterogeneous media to address the gaps in previous research. A 2-D experimental system was designed to evaluate interparticle/intraparticle diffusion processes of trichloroethylene (TCE) in heterogeneous media. The 2-D system was modified to include organic matter in media for simulation of sorption/desorption processes. Results of the research will improve the understanding of how these different transport processes act together within heterogeneous media. Results will also allow for the evaluation of the impact of contaminant mass transport from within low permeability media at a potential treatment site and can support the development of mathematical tools/models combining interparticle/intraparticle and sorption/desorption processes. Such a model will promote more accurate site assessment and provide more confidence in the choice of an effective, economically optimized remediation strategy.     

地下水源热泵回灌过程颗粒迁移-脱离模型及堵塞区判定

开展多孔介质中悬浮颗粒的迁移-脱离过程研究对地下水回灌工程、注浆工程和石油工程等具有重要意义。以地下水源热泵工程为依托,提出了描述颗粒迁移-脱离过程的波浪状曲面模型,研究不同的粒径组合以及不同压力对多孔介质中颗粒迁移以及脱离特性的影响。在此基础上,采用Comsol对典型地下水源热泵工程进行模拟,揭示了不同粒径及压力分布条件下回灌堵塞区分布规律。结果表明:在相同颗粒粒径组合(相同临界速度)下,压力越大,可以脱离的悬浮颗粒的范围越大;在相同压力下,临界速度越大,可以脱离的悬浮颗粒的范围越小;其他条件相同时,位置离抽水井和回灌水井越近,压力越大,渗流速度越大,悬浮颗粒越容易脱离。     

地表水力特性对植被过滤带中胶体迁移的影响

地表坡面流通过植被过滤带(VFS)时,植物的茎与枝会把水流的动能转化为湍流的动能,改变地表水的水力特性,进而影响胶体颗粒物的迁移.为了探讨不同水力特性的地表坡面流对胶体在VFS中的迁移和去除机制产生的影响,进而为利用VFS去除胶体污染物的工程设计提供理论依据,通过室内试验,观测不同雷诺数地表坡面中胶体在VFS的迁移过程...     

立方排列球体层渗流特性数值模拟研究

在多年冻土区块石夹层和块石护坡路基温度场的计算中,可以将块石层看作为多孔介质,但是块石的不规则给其渗透特性的研究带来了很大的麻烦,通过对立方排列球体层渗透特性的研究以期能为块石层的渗透特性研究做出铺垫。基于连续性方程、动量方程以及RNG湍流模型对直径为0.20 m、0.24 m和0.30 m立方排列球体层渗透特性进行模拟研究。对比和分析计算结果发现:球体层内部空气压力梯度与渗流速度之间呈良好的非线性幂函数关系;球体层的阻力系数与球体直径也显现出幂函数关系,球体直径越大,阻力系数越小。     

细颗粒悬沙浓度分布的影响因素分析

从颗粒运动的PDF(概率密度分布函数)输运方程出发建立颗粒相的质量、动量和脉动速度二阶矩方程。对明渠二维恒定均匀流，利用垂向动量方程导出了修正的泥沙扩散方程，表明颗粒脉动强度梯度、升力、有效重力和紊动扩散都影响悬沙运动。理论和数值分析了细颗粒泥沙的密度、粒径、浓度，挟沙水流的水深和剪切流速等因素对泥沙扩散系数和浓度分布形状的影响，并定量分析了细颗粒试验资料，表明传统扩散方程在粒径、剪切流速相对较大、水深和水沙密度比较小的流动中存在较大误差，需要修正。     

填埋场黏土衬垫中起始水力梯度对污染物运移影响

垃圾填埋场黏土衬垫对防止渗沥液的渗漏和运移起到至关重要的作用,其中有些黏土渗流呈现非达西渗流现象,存在起始水力梯度i_0,然而起始水力梯度对黏土衬垫防污性能的影响尚未明确。为此,开展了起始水力梯度对污染物运移的影响研究。首先,在不同围压作用下,采用去离子水对黏土进行渗透试验;然后,在围压σ=200 kPa的条件下,分别采用水力梯度为0,12.5,25.0和375.0对4个平行黏土试样进行Na~+溶液渗透,定期测定渗出液Na~+浓度,并最终分层测定试样中Na~+的含量。结果表明:随着固结压力的增大,起始水力梯度越大;当水力梯度分别0,12.5和25.0时,渗出口无渗透液产生,并且Na~+在黏土中的迁移规律基本相同,即分子扩散系数基本相等;当水力梯度为375.0时,水动力弥散系数远远大于分子扩散系数。因此,可以认为当水力梯度小于i_0时,渗沥液不渗透或渗透很小,污染物运移以分子扩散为主;当水力梯度大于i_0时,污染物运移逐渐转为以对流为主。     

室内空气环境的数值模拟与通风模式的评估

采用CFD技术对存在隔板及持续释放二氧化碳的污染源的开放式办公室进行了数值模拟,计算分析了两种通风模式下污染物在室内的分布状况以及空气年龄的变化情况,其中空气年龄的计算是利用Fluent软件提供的自定义函数接口技术引入了对应的输运方程来实现。根据模拟结果对开放式办公室内如何选择通风模式提出了建议。结果显示当污染源位置发生变化时,置换型通风模式与混合型通风模式在通风率和换气率的表现上也会发生变化。当室内隔板数目增加的时候,置换型通风模式在房间的整体通风效果上要稍优于混合型通风模式。     

Impact of consolidation pressure on contaminant migration in clay liner

Consolidation deformation occurs in clay liners under the self-weight of wastes at a simple garbage dump or dredged sediment dump, which leads to a decrease in the porosity. However, the migration of contaminants in clay liners is influenced by the porosity. Thus, the impact of consolidation deformation of clay liners on the migration of contaminants cannot be ignored. Based on Biot’s consolidation theory, the contaminant migration theory, and consideration of the three kinds of migration mechanisms of convection, diffusion, and adsorption, a one-dimensional migration model of contaminants in deforming porous media was established, and the finite difference method was adopted to obtain the numerical solutions for an established initial-boundary value problem. The impact of consolidation pressure on the migration law of a contaminant was studied. The results show that, regardless of adsorption modes, different consolidation pressures have similar impacts on the migration law of the contaminant. Namely, over a certain migration time, the greater the consolidation pressure is, the smaller the migration depth of the contaminant. The results also show that, while the migration time increases, the impact of a certain increment of consolidation pressure on the variation of contaminant concentration with the depth increases gradually and, while the migration depth increases, the impact of a certain increment of consolidation pressure on the variation of the contaminant concentration with time increases gradually.     

Relative roles of resuspended particles and pore water in release of contaminants from sediment

Sediment layers containing contaminants play a significant role in environmental hydrodynamics. Experiments were conducted in order to characterize the relative roles of resuspended particles and pore water under different flow and sediment conditions. A conservative tracer （NaC1） and a reactive tracer （phosphate） were used as contaminants in the bottom sediment in a laboratory flume. The mixing between the overlying water and pore water occurred over a short time while the desorption of contaminants from fine-grained resuspended particles lasted a relatively long time. The effects of resuspended particles and pore water on the variations of release flux and concentration of contaminants in water with time under different hydrodynamic conditions were quantified. The results show that pore water dominated the initial release flux, which could be several orders of magnitude greater than the flux due to molecular diffusion. Flux contribution of desorption from sediment particles in the latter release could be equal to what was seen from pore water in the initial stage.     

EFFECT OF SOLID PARTICLES ON STABILITY FLOW IN THE BOUNDARY LAYER ON A FLAT PLATE

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感潮河段盐水入侵的河口水质模型

盐潮入侵将严重影响内河水质。本文考虑强混合型河口潮汐的特点,采用同步不同格式对河口盐潮入侵问题的流场及浓度场进行数值计算。采用欧拉-拉格朗日混合解法将对流-扩散方程中的扩散项和对流项分裂求解,应用高精度时间序列插值的特征差分隐格式求解对流项,扩散项采用传统的有限差分法。在汊口处,将描述河网汊口水质变化的三维对流扩散方程在汊口控制体体积积分,变量间的插值分布采用Panankar提出的幂函数,得到新的汊口离散方程。计算与实测结果比较表明,本文提出的方法有效可行。     

Characteristics of phosphorus adsorption by sediment mineral matrices with different particle sizes

The particle size of sediment is one of the main factors that influence the phosphorus physical adsorption on sediment. In order to eliminate the effect of other components of sediment on the phosphorus physical adsorption the sediment mineral matrices were obtained by removing inorganic matter metal oxides, and organic matter from natural sediments, which were collected from the Nantong reach of the Yangtze River. The results show that an exponential relationship exists between the median particle size (D₅₀) and specific surface area (S_g) of the sediment mineral matrices, and the fine sediment mineral matrix sample has a larger specific surface area and pore volume than the coarse sediment particles. The kinetic equations were used to describe the phosphorus adsorption process of the sediment mineral matrices, including the Elovich equation, quasi-first-order adsorption kinetic equation, and quasi-second-order adsorption kinetic equation. The results show that the quasi-second-order adsorption kinetic equation has the best fitting effect. Using the mass conservation and Langmuir adsorption kinetic equations, a formula was deduced to calculate the equilibrium adsorption capacity of the sediment mineral matrices. The results of this study show that the phosphorus adsorption capacity decreases with the increase of D₅₀, indicating that the specific surface area and pore volume are the main factors in determining the phosphorus adsorption capacity of the sediment mineral matrices. This study will help understand the important role of sediment in the transformation of phosphorus in aquatic environments.     

感潮河段盐水入侵的水质模型

盐潮入侵严重影响内河水质。盐度的纵向分布受制于上游径流和海洋潮汐，其水动力学特性非常复杂。数值求解不恒定盐度入侵，主要困难在于对流占优时对纯对流算子的恰当处理，否则将会产生严重的数值耗散和伪振荡等非物理效应。考虑到强混合型河口潮汐的特点，本文采用同步不同格式对盐潮入侵问题的流场及浓度场进行了数值计算。采用欧拉—拉格朗日混合解法（ELM），将对流—扩散方程中的扩散项和对流项分裂求解，应用高精度时间序列插值的特征差分隐格式求解对流项，扩散项采用传统的有限差分法。在汊口处，将描述河网汊口水质变化的三维对流扩散方程在汊口控制体积积分，变量间的插值分布采用Panankar提出的幂函数，得到新的汊口离散方程。计算与实测结果比较表明，本文提出的方法有效可行。     

基于逆向概率密度函数的一维污染源排放重构

应用逆向概率密度函数进行一维对流扩散方程污染释放过程重构,给出了采用逆向概率与传统优化过程的流程图和主要区别。采用逆向概率密度函数进行污染源重构,其微分方程求解次数等于观测数据量,有效地降低了数值计算量。应用逆向概率密度函数结合优化算法针对一维对流扩散方程中正态波式和矩形波式的污染源排放过程重构的仿真结果显示:对于1～2个污染物正态函数波释放过程,采用一到两个监测点即可准确得辨识出主要参数;3个正态函数波释放过程,采用3个监测点,辨识结果仍有一定误差,可能与模型的非线性特征或释放过程波的相互干扰有关。矩形波释放过程,采用三个监测能准确辨识出所有参数。     

Experimental and theoretical study of coupled influence of flow velocity increment and particle size on particle retention and release in porous media

Experimental and theoretical studies were carried out to investigate the coupled influence of flow velocity increment and particle size on the retention and release of particles in porous media. Particle release was examined through measurement of changes in effluent particle concentrations, and particle retention was assessed through measurement of the final spatial distribution of particles remaining in the soil columns after the experiments. Particle release curves were simulated using a convection-dispersion model that includes the instantaneous release of the line source. Fitted model parameters were used to gain insights into the mechanisms that control particle retention and release. When the flow velocity increment was 0.0435 cm/s, the peak concentration of particles decreased with increasing flow velocity until the latter approached a critical level, beyond which the particle concentration increased. Particle wedging and fouling were considered the primary mechanisms that controlled particle retention and release beyond the critical particle velocity. In experiments with large flow velocity increments, small particles exhibited lower particle mass fraction than large particles as particle wedging and fouling increased with particle size. The range of longitudinal dispersivity decreased with an increasing particle size and flow velocity increment. Moreover, the mean particle velocity increased with the mean interstitial fluid velocity. The mean particle velocity profile was highly sensitive to the particle size at low velocity increments. In general,particle release rates increased with both flow velocity and velocity increment. The mass of the released particles provides further evidence that particle wedging and fouling are the major factors that control particle release in sand columns.     

一维多孔介质中气体渗流与多相气固反应分析

考虑到气体的压缩性，按渗流力学给出了多孔介质中的非均匀流场，并进一步研究多孔介质内的多相气固反应和传输。与不可压缩气体对流、反应、扩散的比较表明，两种流动的孔隙压力分布差别很小，但渗流速度的差别却较大。由于小的速度差会引起Peclet数的大的差别，最终引起气体浓度的较大差别，因而有必要考虑气体的压缩性。人口处的流速与多孔介质内由于气体膨胀产生的附加速度是有实质差别的两种对流，前者使气体浓度上升，后者使其浓度下降。对流对反应气体和产物气体传输的影响不同，但这仅是表面现象的不同，对流的本质不变。对颗粒堆积的多孔介质，颗粒半径和孔隙率越小，以Mach数作为判别气体压缩性是否可以忽略的指标就越不合理。因此对多孔介质中的气体流动，应该用压降作指标判别是否可以忽略气体的压缩性。     

悬移质泥沙输移扩散方程适用条件的讨论

悬移质泥沙的输移扩散是泥沙运动力学的基础问题之一,学者们基于不同理论对其运动规律开展了深入研究。基于已有研究,本文主要讨论了不同理论下扩散方程的适用条件。研究结果表明,传统的Rouse扩散方程及其修正式仅适用于泥沙浓度较低、颗粒惯性小至可忽略的条件;两相流理论下的扩散方程由于未考虑颗粒惯性效应,同样只适用于浓度较低、颗粒较小的条件;近期Snehasis Kundul和Koeli Ghoshal基于漂移速度建立的扩散方程由于考虑了升力、颗粒惯性等因素对悬移质分布的影响,使其适用范围相应变大,但在确定泥沙扩散系数上仍然采用的是经验公式,应用上会受到实验资料的限制。基于动理学理论的扩散模型包括基于PDF方程的扩散方程和弥散方程。两个模型中,除了重力沉降和紊流扩散作用外,由于考虑了升力、颗粒紊动、颗粒惯性等影响因素,其在浓度较高、颗粒惯性较大时仍适用,具有普适性。尤其是弥散方程,能够反映颗粒浓度、颗粒紊动、颗粒碰撞等不同机制对泥沙悬浮的影响,全面阐释了悬移质泥沙扩散背后的力学机理。     

溶剂驱油中的扩散现象研究

本文通过求一对流－扩散方程的半解析解，分析了扩散系数大小及影响扩散系数的参数对计算出的浓度分布的影响；通过将该半解析解与忽略扩散的数值解进行对比分析，讨论了数值弥散代替物理扩散的条件。结果表明，虽然随渗流速度增加，扩散系数增加，但对浓度分布几乎没有影响；而且在精度要求不高，物理扩散较小时，数值稳定性好的对流方程的数值弥散可以代替物理扩散。     

Particles under stress

The complex nature of particulate matter in natural water resources and in waste waters is characterized by the heterogeneous distribution of particle size, shape, density, and shear strength. Among these parameters, floc strength is most important in the last stages of flocculation. Experimental data on floc strength based on different methods are assessed and correlated with shear gradients in different aquatic environments. The analysis of turbulent motion reveals that the energy which affects particle agglomerates is only a small portion of the totally dissipated energy. Among the different flow fields in turbulent motion, converging/diverging flow cause strain forces which prove to be critical with respect to floc rupture. Model calculations of the surface forces on ellipsoidal particles in pure shear and strain flow fields and corresponding experiments confirm the importance of converging flow. A comparison of modeled and measured rupture forces allows to establish relationships between floc size, strain and shear rates and resulting surface forces leading to probable floc break-up. Flocs of appreciable size (200 - 2000 µm) prepared for settling are likely to be ruptured under moderate velocity gradients occurring in flocculation tanks, whereas smaller agglomerates (< 200 µm) may withstand strain forces much higher than found under practical conditions. An example of model application shows the particle stress in the entrance to porous media filters where typically high strain gradients may easily lead to a break-up of flocs larger than 200 µm.