pH control for enhanced reductive bioremediation of chlorinated solvent source zones

Enhanced reductive dehalogenation is an attractive treatment technology for in situ remediation of chlorinated solvent DNAPL source areas. Reductive dehalogenation is an acid-forming process with hydrochloric acid and also organic acids from fermentation of the electron donors typically building up in the source zone during remediation. This can lead to groundwater acidification thereby inhibiting the activity of dehalogenating microorganisms. Where the soils' natural buffering capacity is likely to be exceeded, the addition of an external source of alkalinity is needed to ensure sustained dehalogenation. To assist in the design of bioremediation systems, an abiotic geochemical model was developed to provide insight into the processes influencing the groundwater acidity as dehalogenation proceeds, and to predict the amount of bicarbonate required to maintain the pH at a suitable level for dehalogenating bacteria (i.e., > 6.5). The model accounts for the amount of chlorinated solvent degraded, site water chemistry, electron donor, alternative terminal electron-accepting processes, gas release and soil mineralogy. While calcite and iron oxides were shown to be the key minerals influencing the soil's buffering capacity, for the extensive dehalogenation likely to occur in a DNAPL source zone, significant bicarbonate addition may be necessary even in soils that are naturally well buffered. Results indicated that the bicarbonate requirement strongly depends on the electron donor used and availability of competing electron acceptors (e.g., sulfate, iron (III)). Based on understanding gained from this model, a simplified model was developed for calculating a preliminary design estimate of the bicarbonate addition required to control the pH for user-specified operating conditions.     

地下水流速及介质非均质性对重非水相流体运移的影响

重非水相流体(DNAPL)在地下介质中的运移分布受多种因素影响,包括DNAPL的物化性质、泄漏速率、地下水流速和介质非均质性等。本文采用地质统计方法生成渗透率随机场刻画非均质性,运用T2VOC模拟不同流速情形下DNAPL在均质/非均质介质中的运移分布,以评估地下水流速和非均质性对DNAPL运移的影响。研究结果表明,地下水流速的增大显著促进了DNAPL的水平和垂向运移。相同流速情形下,非均质性的增强使得DNAPL污染羽分布形态及运移路径的空间变异性增强,出现明显的蓄积和绕流。当DNAPL运移的优势通道方向与流速方向相反时,水流对DNAPL水平运移的促进作用将减弱,反之,其促进作用增强。随着流速与非均质性的同时增强,水平方向上污染羽的扩散范围增大,另外水平和垂直方向上质心位置(一阶矩)及展布范围(二阶矩),污染池起始位置及其长度的空间变异离散程度增大。     

Microbial activity and distribution during enhanced contaminant dissolution from a NAPL source zone

Laboratory experiments were conducted to assess microbial reductive dechlorination in one-dimensional sand columns containing a 10 cm long source zone of uniformly distributed residual tetrachloroethene (PCE) nonaqueous phase liquid (NAPL), a 10 cm long transition zone directly down-gradient of the source zone containing some nonuniformly distributed NAPL ganglia, and a 40 cm long plume region down-gradient of the transition zone. The activity and distribution of Sulfurospirillum multivorans, a PCE-to-1,2-cis-dichloroethene (cis-DCE) dechlorinating bacterium, was evaluated in columns containing either a mixed-NAPL (0.25 mol/mol PCE in hexadecane) or pure PCE-NAPL. Significant dechlorination of PCE to cis-DCE was observed in the mixed-NAPL column, resulting in 53% PCE-NAPL mass recovery in the effluent with PCE-NAPL dissolution enhanced by up to 13.6-fold (maximum) and 4.6-fold (cumulative) relative to abiotic dissolution. Quantitative real-time PCR targeting pceA, the PCE reductive dehalogenase gene of S. multivorans, revealed that S. multivorans cells were present in the NAPL source zone, and increased in numbers (i.e., grew) throughout the source and transition zones. In contrast, minimal reductive dechlorination and microbial growth were observed in the column containing pure PCE-NAPL, where aqueous-phase PCE concentrations reached saturation. These results demonstrate that microbial growth within NAPL source zones is possible, provided that contaminant concentrations remain below levels toxic to the dechlorinating organisms, and that microbial growth can result in significant bioenhanced NAPL dissolution.     

Novel applications of scrap tire for organic sorption/separations

 This paper presents the results of a study of the use of scrap tire material as a sorbent medium for sorption of volatile organic chemicals from organic mixtures and water. The tire material was exposed to pure solvents to establish the sorption characteristics for classes of organics. The overall rate and selectivity of sorption of a solvent in a polymeric material is determined by its diffusivity and solubility in the polymeric material. Literature studies show that polymer-solvent interactions could arise due to electron acceptor-donor charge transfer complexes occurring between the functional groups in rubber and solvent functional groups. This was postulated to be a major contributor toward selective sorption of solvents in tire. The technical feasibility of separating organic solvents from both organic and aqueous mixtures by exploiting the diffusional and solubility related characteristics of various solvents in tire, which is a crosslinked rubber, has been established. Three types of separations were studied: (1) a mixture of two alkanes of significantly different sizes, represented by n-octane, a light alkane, from 2,6,10,14-tetramethylpentadecane, a branched C₁₉ heavy alkane; (2) a mixture of two organic solvents used in extraction and differing in polarity, represented by chloroform and methanol, that form a minimum boiling azeotrope; and (3) removal of an organic form water, represented by TCE contaminated water. Organic molecules having similar polarity could be separated based on substantial size differences, whereas polar/nonpolar organics separation could be achieved based on solubility differences of the organics in the tire phase. The feasibility of using tire material for the separation of trichloroethylene (TCE) from contaminated water was also established. The equilibrium partitioning of the TCE was linearly proportional to its concentration in the aqueous phase in the range of 25 to 600 mg/l. Desorption studies were performed to establish the ease of regeneration of the sorbent material, restoring its sorption capacity and recovering the solvent. The studies indicated that more than 97% of sorbed solvent (TCE) can be recovered at 25  °C under a vacuum of 20 Torr for under 10 minutes. Received: 4 December 1998 / Accepted: 10 February 1999     

Multi-response optimization of process parameters for remediation of tetrachloroethylene pools by surfactants: Application of Taguchi design of experiment and Artificial Neural Network

Within the scope of the study, the effectiveness of the experimental conditions was tested by performing a multi-response Taguchi experimental design for the optimization of the minimum cost remediation performance with Tween 80, Methyl beta cyclodextrine (MCD) and Sodium dodecyl sulfate (SDS) from tetrachloroethylene (PCE) contaminated porous media. Tween 80, MCD and SDS were extensively used in cosmetic industry as emulsifier. Both time of remediation and cost of remediation were studied as two separate response variables in three replicate experiments conducted according to the Taguchi L9 orthogonal experimental design. In the multi-response Taguchi analysis, the sensitivity analysis was performed by systematically changing the weights determined for two separate response variables in the calculation of total loss of quality (TNQLj). Optimum experimental conditions were determined with the help of the calculated multi-response signal/noise (S/N) ratios (MRSN). The results show that the type of Flushing Agent is the most important factor in optimizing the remediation time and remediation cost for the removal of dense non-aqueous phase liquid (DNAPL) PCE mass. Flushing rate is considered to be the least contributing factor. Furthermore, the results of the analysis of variance (ANOVA) showed that all parameters used in the system had a significant effect on the experimental results and the Taguchi method could explain 97.15% of the Remediation Time and 92.03% of the Remediation Cost. Afterwards, the data obtained from the experiments performed according to the experimental design were modelled using Artificial Neural Network (ANN) to estimate the remediation performance and remediation cost without performing new experiments.     

DNAPL在饱和多孔介质中的迁移规律及数值模拟

基于多孔介质两相流模型,编制了相关的有限元计算软件,对已有文献中关于重非水相流体(Dense Non-aque-ous Phase Liquids,DNAPL)在饱和多孔介质中迁移的离心模型试验进行了模拟,探讨了DNAPL的重度与粘滞度等参数对其在地下水位线下迁移规律的影响,并对DNAPL在均质以及非均质地层的迁移进行了模拟,计算结果对DNAPL在饱和多孔介质中的迁移过程、影响范围的描述以及评价有一定的参考意义。     

Design tool for estimation of buffer requirement for enhanced reductive dechlorination of chlorinated solvents in groundwater

To assist in the design of enhanced reductive dechlorination systems for in situ remediation of chlorinated solvent source zones, the software BUCHLORAC (BUffering of deCHLORination ACidity) was developed to predict the amount of buffer required to maintain the groundwater pH in a DNAPL treatment zone within the optimal range for dechlorinating bacteria. Reductive dechlorination is an acid-forming process with hydrochloric acid and organic acids typically building up in the treatment zone. Remediation of source zones is associated with such extensive localized dechlorination that it may be common for the soils' natural buffering capacity to be exceeded. As groundwater acidification may inhibit the activity of dechlorinating microorganisms and thus slow or stall the remediation process, sufficient alkalinity must be present to maintain a near-neutral pH. BUCHLORAC is a Windows Graphical Interface based on an abiotic geochemical model that is implemented through the program PHREEQC. BUCHLORAC allows users to estimate the buffer requirements for their specific operating and design conditions including, for example, site water chemistry, mineralogy, amount of chlorinated solvent to be degraded, design inhibition pH, and type of organic substrate and buffering additive.     

Experiences of Perozone and C-Sparge at Two Former Dry Cleaner Sites in the Netherlands

C-Sparge^TM, better known as ozone sparging (microbubble ozone), is used for treatment of the plume zone area with VOCs, mainly PCE, at a former dry cleaner site in Utrecht. The City of Utrecht has had good results with C-Sparge in combination with pump and treat. Prior to the application of C-Sparge, pump and treat was used for removal of mass in the plume until tailing of the groundwater concentrations occurred. Together the pilot test and full-scale system lowered PCE groundwater concentrations from ppm-level (max. 15 ppm) to low ppb-level (400 ppb) in about 125 days. Currently, the remediation is in the tail end, and the site remediation will be closed. Perozone® (peroxide-coated ozone) is used on a former dry cleaner site in Terneuzen. After demolition of the dry cleaner facility and excavation of the hot spot in soil, MIP-CPTs were done to investigate for possible DNAPL (Dense non-aqueous phase liquids) below the former dry cleaner facility. After installing 60 Laminar Spargepoints® over approximately 100,000 m³ soil volume, the system was started in October, 2005. The initial results indicate a quick removal of aqueous PCE concentrations. The experience indicates that treatment of the DNAPL with Perozone is possible. Mobilization effects were found in the source zone also. From extended monitoring appears a >99% reduction of this initial increased concentration. Currently, a total of about 85% mass has been removed within the 100,000 m³ of soil volume, and the remediation has reached the tail end.