Distinctive sorption mechanisms of soil organic matter and mineral components as elucidated by organic vapor uptake kinetics

Sorption kinetics and capacities of volatile organic compounds (VOCs) affect the remediation and fate of these pollutants in soils. The soil organic-mineral compositional heterogeneity complicates the transport and fate of VOCs in soils. The sorption kinetics of toluene vapor with two common soil components, kaolinite and humic acid, shows two distinct sorption patterns. Results with kaolinite are characteristic of surface adsorption, whereas results with humic acid are characteristic of solvation and partition effects. On soils, the kinetics of toluene vapor sorption show a two-stage sorption phenomenon. The first stage is reflective of surface adsorption (1-4 h to completion) and the second stage of much slower partitioning into soil organic matter, which was preceded by a lag phase (approximately 4 h) and took as long as 15 h for completion. The relative contributions of these two stages to soil uptake are quantifiable by two independent parameters, the soil organic fraction and the surface area. A better understanding of the effect of soil compositional heterogeneity on sorption kinetics and capacities facilitates our understanding of the prediction for the fate of organic contaminants in the environment.     

Sorption kinetics of selected volatile organic compounds in humin

Each component of the chemically heterogeneous soil exhibits a unique sorption behavior toward organic sorbates. The sorption kinetics of some volatile organic compounds (VOCs) in pressed humin disks was investigated by tracking the weight change of the disks with a microbalance. Higher sorbing capacity for more polar VOCs as well as C13 nuclear magnetic resonance data indicates that humin was more hydrophilic than Aldrich humic acid (Milwaukee, WI, USA). The apparent diffusivity of acetone, toluene, and hexane in the disks ranged from 10(-8) to 10(-10) cm2/s. The sorbed toluene in humin does not seem persistent to desorption; however, acetone and hexane, either as polar or linear compounds in humin, show persistence against desorption. On completion of the desorption experiments, there were approximately 35 and 20% sorbate residue for acetone and hexane, respectively.     

Identification of mobile aliphatic sorptive domains in soil humin by solid-state 13C nuclear magnetic resonance

Many sorption studies aim to elucidate organic matter structure and contaminant sorption relationships. Through this pursuit, a great deal of insight has been gained about contaminant interactions with humic fractions, namely the fulvic and humic acid isolates. Comparatively, less is known about the structure and environmental reactivity of the humin fraction; however, researchers have reported that the humin fraction consistently produces higher sorption coefficients than the corresponding source material and other humic fractions. In this paper, we report on a study that uses solid-state 13C nuclear magnetic resonance (NMR) to characterize six humin samples extracted from soil. In addition, 1-naphthol sorption was measured for each whole-soil and humin sample. With the exception of the peat sample, the humin samples yielded significantly higher organic carbon-normalized sorption coefficients (K(oc)) compared with the whole-soil samples. The solid-state 13C NMR analysis reveals the presence of amorphous, polymethylene-rich domains in all of the humin samples. Other researchers have indicated that these domains exhibit a high affinity for hydrophobic organic contaminants. Consequently, we hypothesize that the concentration of amorphous, polymethylene-rich domains in soil humin is responsible for the high sorption coefficients reported here and by other researchers.     

Sorption of 2,4'-dichlorobiphenyl and fluoranthene to a marine sediment amended with different types of black carbon

Recent studies demonstrate that sedimentary black carbon (BC) affects the sorption of some hydrophobic organic contaminants (HOCs) to a greater extent than sedimentary organic carbon (OC). Among HOC, polycyclic aromatic hydrocarbons (PAHs) are known to interact extensively with BC. Currently, data on the sorption of various kinds of HOCs to different types of BC are limited. In this study, we amended a marine sediment with BC from several different sources, humic acid, and inert sand. Equilibration studies with 14C fluoranthene and the polychlorinated biphenyl (PCB) 3H 2,4'-dichlorinated biphenyl were performed to determine the magnitude of sorption as a function of contaminant and BC type. The magnitude of sorption to the BC-amended sediments was greater for the PAH than the PCB as compared to the sediment alone, humic acid, and sand. For example, differences between the log partition coefficient (K(P)) for the PAH and PCB ranged from 0.41 to 0.69 log units for humic acid and sand treatments, while differences ranged from 0.88 to 1.57 log units for the BC-amended sediments. As a result, BC-normalized partition coefficients (log K(BC)) for the PAH averaged 6.41, whereas the PCB log K(BC) values averaged 5.33. These results demonstrate that PAH sorption and most likely bioavailability are influenced strongly by the presence of BC of different types, while sorption of a nonplanar PCB was affected to a lesser degree.     

Effect of molecular structures on the solubility enhancement of hydrophobic organic compounds by environmental amphiphiles

Amphiphilic molecules, such as humic substances and surfactants, are known to increase the apparent aqueous solubility of hydrophobic organic compounds (HOCs) in the aqueous phase because of their molecular structures, which consist of hydrophilic and hydrophobic moieties. In this study, we examined the effect of the structures of humic acid and HOCs on the sorption of four polycyclic aromatic hydrocarbons (PAHs) and an organochlorine pesticide, p,p'-DDT, to humic acid. As the number of aromatic rings was increased, the extent of solubility enhancement of PAHs by humic acid was increased. Although p,p'-DDT was more hydrophobic than pyrene in this study, the extent of solubility enhancement of p,p'-DDT by humic acid was lower than that of pyrene because of the molecular structures of the solutes. Anionic surfactants with and without aromatic rings also were studied for comparison, and the dianionic surfactant with two benzene rings exhibited similar results with humic acid, unlike the surfactants without and with one benzene ring. The results from this study indicate that bulky molecules, such as p,p'-DDT sorbed with more difficulty to the aggregates of amphiphiles with larger molecules, such as humic substances and the dianionic surfactants.     

Distribution of atrazine into three chemical fractions: impact of sediment depth and organic carbon content

The fate and transport of organic contaminants in aquatic sediments are impacted largely by microbial degradation and sorption to organic matter. Atrazine, a pre-emergent herbicide, has the potential to contaminate groundwater because of its slight water solubility, long half-life, and sorption to organic matter. Mineralization and distribution of 14C-atrazine into three chemical fractions were monitored over time in surface and subsurface coastal aquatic sediments of different land use. Sediments were extracted with an organic solvent followed by an alkali hydrolysis, and 14C activity was measured in the aqueous, solvent, and basic fractions (representing nonsorbed compounds, loosely sorbed compounds, and humic or fulvic acid bound compounds, respectively). Limited mineralization of atrazine occurred (< 4%). The 14C activity in the aqueous and basic fractions increased over time, was greater in surface versus subsurface sediments, and was positively correlated with sediment organic carbon (SOC) content, indicating greater biological and chemical activity. Total 14C recovered ranged from 50 to 90%, was less in surface versus subsurface sediments, and was not correlated with SOC after 80 d. These results suggest that in native aquatic surface sediments, atrazine sorption plays a major role, whereas in subsurface sediments atrazine may be available for degradation and transport to shallow groundwater.     

Sorption of polar and nonpolar aromatic compounds to two humic acids with varied structural heterogeneity

The major objective of the present study was to evaluate the correlation between structural nature of humic acids (HAs) and sorption affinity of organic compounds with varied polarity. We compared the sorption behavior of three aromatic compounds-nonpolar phenanthrene (PHEN) and 1,2,4,5-tetrachlorobenzene (TeCB) and highly polar 2,4-dichlorophenol (DCP)-to a solid-phase coal humic acid (CHA) and a soil humic acid (SHA) suspended in aqueous solution. The structural nature of HAs was characterized using elemental analysis, ultraviolet absorbance, diffusive reflectance Fourier-transform infrared, and solid-state 13C nuclear magnetic resonance. The two tested HAs have very different structural properties: CHA consists primarily of poly(methylene)-rich aliphatics with high aromatic content and some COO/N-C=O but low polarity, while SHA consists of young materials of lignin, carbohydrates, and peptides with high polarity. In response to the structural heterogeneity of HAs, sorption of nonpolar and more hydrophobic solutes (PHEN, TeCB) to CHA is much greater than that to SHA because of the predominance of hydrophobic effects; however, disparities in sorption affinity between the two HAs become smaller for polar and less hydrophobic DCP because of the major role played by polar interactions. The influence of pH on the sorption of different solutes to the two HAs was also discussed. The results of the present work highlight the importance of structural heterogeneity of both solutes and HAs in the sorption process.     

Infrared investigation of the sequestration of toluene vapor on clay minerals

The Ca-exchanged clays were used to investigate the sorption of toluene under both high and low relative humidity conditions. According to the observed rates of sorption and desorption and the Fourier transform infrared (FTIR) spectra of sorbed toluene, the sorbate molecules on clays can be divided into three categories: labile (gaslike) phase, slow sorption (liquidlike) phase, and resistant-to-desorption phase with sorption/desorption time scales of less than 30 min, several hours, and several weeks, respectively. Aging changed the spectroscopic characteristics of the sorbed sorbates and contributed to the irreversibility of the sorption under both high and low relative humidity. Irreversibly sorbed portions of the sorbates and new characteristic peaks identified with FTIR spectrometry suggest the existence of chemical transformation of sorbed species under both high and low relative humidity conditions. It is suggested that the clay minerals, acting as catalysts, may contribute to abiotic natural attenuation of volatile organic aromatic hydrocarbons in soils.     

Sorption of p-nitrophenol on two Chinese soils as affected by copper

Heavy metals and organic contaminants often coexist in soils. However, very little information is available regarding the effect of metals on the sorption of organic contaminants onto soils and/or of organic contaminants on metal sorption. In the present study, the effect of Cu on the sorption of p-nitrophenol on two Chinese soils was investigated using a batch-equilibration method for three conditions: Copper and p-nitrophenol were sorbed simultaneously, either Cu orp-nitrophenol was sorbed previously, or the soil colloidal phase was removed in part previously. The results suggested that Cu suppressed the sorption ofp-nitrophenol on soils, whereas p-nitrophenol had little effect on Cu sorption because of the higher affinity of Cu for soils. Mechanisms of the Cu suppression effect were suggested by the results. First, large hydrated Cu occupy the siloxane surface of soils and prevent nonspecific interaction between p-nitrophenol and soils. Second, the soil colloidal phase is an effective adsorbent of p-nitrophenol; thus, more p-nitrophenol is retained in the aqueous phase. In addition, the aggregation of the colloidal particles may occur, which blocks soil pores, thereby decreasing the sorption of p-nitrophenol on the solid soil phase. Third, x-ray absorption spectroscopy revealed that Cu forms inner-sphere complexes with the carboxyl and hydroxyl functional moieties of the soil particles (clay minerals and organic matter). Fourier-transform infrared spectroscopy study indicated that these groups also react with p-nitrophenol through H-bond formation. These results suggested that Cu and p-nitrophenol have common sorption sites, at least in the soil organic matter domain, which is partially responsible for the observed overall Cu suppression effect.     

扩散法被动式挥发性有机化合物个体监测器的研制

研制了一种以单层活性炭颗粒作为吸收介质的被动式挥发性有机化合物个体监测器。空气中挥发性有机化合物依据分子扩散原理传质到采样器中，并吸附到活性炭上。采样后，取出活性炭吸附层，用二硫化碳洗脱，再用气相色谱法定性和定量。对监测器进行了实验室性能评价和现场验证。结果表明，采样器在风速２０～１５０ｃｍ／ｓ，相对温度１０％～８０％，温度－１０～３５℃范围内使用，采样速率分别为６０ｍｌ／ｍｉｎ（苯）、５４ｍｌ／ｍｉｎ（甲苯）、５４ｍｌ／ｍｉｎ（氯仿）和４９ｍｌ／ｍｉｎ（对－二甲苯），相对标准差小于５％。在现场与有泵活性炭管采样相比较，本个体监测器测定空气中挥发性有机化合物的总不确定度分别为１２％（苯）、１２％（甲苯）、２６％（对－二甲苯）和１６％（氯仿），可适用于室内空气污染和个体接触量的监测。     

Impact of coal structural heterogeneity on the nonideal sorption of organic contaminants

Carbonaceous geosorbents (black carbon, coal, and humin/kerogen) play a primary role in the nonideal sorption (isotherm nonlinearity, hysteresis, and multiphasic kinetics) of hydrophobic organic chemicals by soils and sediments. The present study investigated the impact of coal structural heterogeneity on sorption/desorption of two model monoaromatic compounds (1,3-dichlorobenzene and 1,3-dinitrobenzene). Due to the higher degree of aromaticity and condensation, anthracite showed stronger sorption affinity and nonlinearity and slower sorption kinetics than lignite. Removal of humic substances by alkali extraction and/or mineral fraction by acidification did not much affect organic carbon-normalized sorption coefficient to the coal, suggesting nearly complete accessibility of adsorption sites on the condensed organic carbon. However, the treatments greatly increased sorption kinetics and meanwhile alleviated hysteresis of 1,3-dinitrobenzene, as compared with the original lignite. These observations were attributed to the enhanced exposure of high-energy adsorption sites on the condensed organic carbon after exfoliating the surface coverage by humic substances and minerals. An empirical biphasic pseudo-second-order model consisting of a fast sorption phase and a slow sorption phase adequately quantified the overall sorption kinetics for the coal sorbents. The results indicated that the condensed organic carbon, in combination with other structural components, controls the nonideal sorption of unburned coal.     

Transport of Organic Compounds Through Porous Systems Containing Humic Acids

Soil pollution by the presence of different contaminants (e.g. heavy metal ions or pesticides) is one of the biggest problems worldwide. The positive affinity of natural humic acids towards these contaminants might contribute to the soil and ground water protection; therefore it is necessary to study the reactivity and barrier properties of humic acids. An original reactivity-mapping tool based on diffusion techniques designed to study the reactivity and barrier properties of polyelectrolytes was developed and tested on humic acids. The results of diffusion experiments demonstrate that the electrostatic interactions between humic acids functioning as a polyelectrolyte interpenetrated in a supporting hydrogel matrix (agarose) and cationic dye (methylene blue) as a model solute have a crucial impact on the rate of diffusion processes and on the barrier properties of hydrogels. The intensity of interactions was evaluated by fundamental diffusion parameters (effective diffusion coefficients and breakthrough time). The impact of modification of humic acids was also studied by means of diffusion experiments conducted on two types of standard humic acids (Leonardite 1S104H) and humic acids with selectively methylated carboxylic groups.     

The application of high-performance liquid chromatography humic acid columns in determination of Koc of polycyclic aromatic compounds

An improvement of a method for determination of the distribution coefficient for polycyclic aromatic compounds (PAC) between organic carbon and water (Koc) by high-performance liquid chromatography (HPLC) is presented in this paper. By use of silica-based HPLC columns with chemically immobilized humic acid (Aldrich, Zigma-Aldrich, Taufkirchen, Germany), the retention of PAC to humic acid can be measured, and the results can, when they are compared to those from a blank column, be used to a direct determination of Koc. These values agree quite well with literature Koc values for two- to four-ringed PAC. Our work has shown that it is possible to use this method to investigate how environmental parameters like electrolyte concentration, pH, and temperature affect the sorption of PAC to humic acid. Thermodynamic parameters can be estimated from variable temperature experiments. The advantage of the method is its cost and manpower effectiveness.     

Differential roles of humic acid and particulate organic matter in the equilibrium sorption of atrazine by soils

Recent studies have indicated that soil organic matter (SOM) may consist of physically and chemically different fractions, including particulate organic matter (POM), such as black carbon and unburned coal materials. The present study examined the differential roles of three different SOM fractions isolated from a peat and a topsoil in the equilibrium sorption of the herbicide atrazine (ATZ). The SOM fractions isolated from the two samples included humic acids (HAs), base-extracted humin (HM), and POM after demineralization of HM. A batch technique was employed to measure both the nonequilibrium ATZ sorption on the original and HA samples and the equilibrium ATZ sorption and desorption. The results showed that the phase-distribution relationships measured under nonequilibrium conditions were more linear and had lower sorption-capacity parameters compared with their respective isotherms measured under equilibrium conditions. The sorption isotherms were variously nonlinear, with POM exhibiting the greatest organic carbon-normalized sorption capacity. There existed apparent sorption-desorption hysteresis for each sorbent-sorbate system. It appeared that the extracted HAs could facilitate hydrolysis of ATZ when the reaction time extended to 4 d or longer. The equilibrium sorptive behavior of the HAs therefore was not examined. The present study indicated that both original samples showed lower organic carbon-normalized sorption distribution coefficients compared with their respective SOM fractions, suggesting that a fraction of sorption sites in soil aggregates were not accessed by ATZ.     

Tributyltin sorption to marine sedimentary black carbon and to amended activated carbon

Under marine conditions, tributyltin (TBT) is speciated mainly as an uncharged hydroxyl complex (TBTOH) that is expected to have a similar fate to hydrophobic organic contaminants. Earlier studies indicated that for the later compounds, sorption to black carbon (BC) can be more than two orders of magnitude stronger than sorption to organic carbon, notably at low and environmentally relevant concentrations. The aim of the present study was to investigate the sorption strength of spiked TBT to a sediment and its BC isolate. It was observed that carbon-normalized sorption coefficients were in the same range for the sediment total organic carbon (TOC) and for its BC (log K(TOC) 5.05 L/kg(TOC) and log K(BC) 5.09 L/kg(BC), respectively). This indicates that TBT does not sorb as strongly to BC as other hydrophobic organic contaminants. Activated carbon (AC), a strong man-made sorbent, has the potential to be used for in situ remediation of contaminated sediments and soils, in particular for polycyclic aromatic hydrocarbons and polychlorinated biphenyls. In the present study, both granular and powdered AC were found to strongly sorb TBT under marine conditions, with a log sorption coefficient of 6.8 L/kg(carbon). Tributyl- and dibutyltin concentrations in the pore water of a natively contaminated sediment were reduced by more than 70% on addition of 2% of powdered AC, whereas granular AC did not show a similar reduction. The results indicate that powdered AC might be a feasible remediation agent for sediments contaminated by organotins.     

Stage change in binding of pyrene to selected humic substances under different ionic strengths

Influence of ionic strength (I) on the pyrene binding constant (Koc) in six different humic substance (HS) solutions, three soil humic acids, one freshwater humic acid, one freshwater fulvic acid, and one freshwater natural organic matter was investigated by fluorescence quenching technique. Specific two-stage changes of Koc with increasing ionic strength were only observed in three soil humic acid solutions with varying to different extents. Two primary governing factors for the differences of these HS systems are binding potential and the abundance of acidic functional groups (carboxyl and phenolic) of HS. Experimental results demonstrated that within the ionic strength range studied in the present research, ionic strength had a major effect on molecular size of HS and brought about change of Koc. The salting-out effect did not. The change in molecular size was caused by the associations between cations and acidic groups of HS. Using pyrene as a probe, we also investigated the interaction of cation and dissolved HS and found relatively slow kinetics (at least 5 d), leading us to believe that the special cation-HS kinetics may influence the results of geochemical studies involving the measurement of ionic strength effect.     

The Influence of Black Carbon on the Sorption and Desorption of Two Model PAHs in Natural Soils

Black carbons (BC) which result from the incomplete combustion of farm waste [man-made (burned) BC] are highly absorbent. In Taiwan, the burning of farm waste known as slash and burn is common. The BCs from the burning may present an environmental challenge. Little is known about the effect of BCs on the transport of hydrophobic organic contaminants (HOC). This study investigates the sorption of anthracene and naphthalene to BCs in soil and efficiency of the surfactants Tween 80 and Triton X-100 in their removal. Both surfactants demonstrated 2–6 times increased solubility in the soils with the addiction of BC. Column experiments were performed to imitate the transportation of these contaminants in groundwater through soils before and after adding BC produced by burning farm waste in the lab. We found significantly increased sorption of anthracene in soil added with BCs produced in the lab, suggesting that fraction of organic carbon (f_oc) can contribute to sorption of such HOCs. Sorption of naphthalene was increased but not significantly. Comparing the concentrations of contaminants, we found the soil containing BC from burned farm waste absorbed HOC more efficiently than the organic BC (naturally-occurring) in the original soil. Therefore, sorption capacity and influence on the transport of HOC cannot be estimated simply by the f_oc of the soil because the two BCs differ greatly in their sorption ability. BC from farm waste absorbs more contaminants than naturally occurring BC in the soil.     

毛细管柱气相色谱法快速检测空气中14种挥发性有机物的含量

目的结合龙岗区实际情况,建立溶剂解吸一气相色谱同时快速测定工作场所空气中正己烷、丙酮、丁酮、乙酸乙酯、乙酸丁酯、苯、甲苯、对一二甲苯、间一二甲苯、邻一二甲苯、苯乙烯、三氯乙烯、三氯甲烷、环己酮等14种挥发性有机物含量的检测方法。方法用二硫化碳解吸样品,以毛细管柱分离,氢火焰离子化检测器（FID）检测。结果14种挥发性有机物在一定浓度范围内线性关系良好,相关系数r〉0．999,相对标准偏差1．2％～3．6％,最低检出质量浓度远低于职业化学中毒现场剂量水平,目标组分平均解吸效率在88．0％～103％。结论方法灵敏度高,重复性好,结果满意。关键词：挥发性有机物;工作场所,空气;气相色谱     

Biomedicine in the environment: sorption of the cyclotide kalata B2 to montmorillonite, goethite, and humic acid

Cyclotides are bioactive, stable mini-proteins produced in high amounts in Violaceae and Rubiaceae with promising pharmaceutical and agrochemical applications. Environmental issues must be addressed before large-scale plant cultivation of cyclotides for pharmaceutical or pesticidal purposes can commence. The objective of the present study was to investigate sorption of the cyclotide kalata B2 (kB2), because knowledge of cyclotide biogeochemistry will aid our understanding of environmental fate. The octanol-water partitioning coefficient was determined to be 2.8?±?0.6 (log?K(OW) =?0.4?±?0.1). Sorption of kB2 by montmorillonite, goethite, and humic acid was investigated at different concentrations and under varying acidity and reached equilibrium within minutes. The kB2 sorption at a solution concentration of 0.2?mg/L to montmorillonite was high (120?mg/g) compared to humic acid (0.60?mg/g) and goethite (0.03?mg/g). Kalata B2 intercalated the interlayer space of montmorillonite. The sorption isotherm for humic acid was linear up to a solution concentration of 0.8?mg/L and concave for montmorillonite and goethite up to an equilibrium solution concentration of 1.5?mg/L. Sorption to goethite was unaffected by pH, but sorption to montmorillonite and humic acid at pH near the isoelectric point (pI) was threefold the sorption when pH?>?the isoelectric point, suggesting that electrostatic interaction/repulsion between kB2 and sorbents play an important role. The strong sorption to montmorillonite reduces exposure to below toxic threshold values. In addition, the transport risk of soluble cyclotides is reduced, but particle-bound cyclotides may be transported to recipient aquatic sediments with the associated risk of adversely affecting sediment-dwelling organisms.     

The effect of aerosol on estimates of inhalation exposure to airborne styrene

Exposure to volatile organic contaminants usually is attributed to vapors alone; samples are collected on charcoal tubes or by passive dosimeters. This study demonstrated that aerosols, generated during the spraying of polyester resin solution, can contribute significantly to the exposure to volatile organic contaminants. Four spraying experiments were performed during which 64 samples were collected and analyzed to determine the styrene air concentrations. The results from the four spraying experiments showed that aerosols represented 30% +/- 3% of the total air concentration of styrene. The contribution of aerosols to inhalation exposure needs to be considered in other industrial situations where spray processes are used.