Non-reactive hydraulic assessments during a freeze-thaw laboratory based simulation of a permeable reactive barrier

The impact of freeze-thaw cycling on a ZVI and inert medium was assessed using duplicated Darcy boxes subjected to 42 freeze-thaw cycles. Measuring bed heights and non-reactive tracer tests allowed the assessment of bed hydraulics. Reaction kinetics were also assessed using a step increases in contaminant (copper and zinc) concentration. All measurements were conducted before, midway and at the end of the freeze-thaw cycling. Two custom built Perspex Darcy boxes of bed dimensions: length 362 mm, width 60 mm and height 194 mm were filled with a mixture of 5 wt% Peerless iron (Peerless Metal Powders and Abrasive, cast iron aggregate 8-50 US sieve) and 95 wt% glass ballotini ground glass (Potters Industries Inc. 25-40 US sieve). This ratio of media was selected to ensure that most aqueous contaminant measurements were above the analytical limit of quantification (LOQ) for feed solutions at a realistic maximum Antarctic metal contaminant concentration at a realistic field water flow rate. All solutions were pumped into and out of the Darcy boxes using peristaltic pumps and acid washed Masterflex FDA vitron tubing.Dry media was weighed in 1 kg batches and homogenised by shaking and turning end over end in a ziplock bag for 1 minute. To ensure that the media was always saturated, known amounts of Milli-Q water followed by the homogenised media were added to each box in approximately 1 cm layers. 20 mm of space was left at the top of the boxes to allow for frost heave and other particle rearrangement processes.The process of each solution flow assessment took approximately 2.5 days. For the entire duration the flow rate of the upstream pump was set at 18.1 mL min-1. The height in the feed weir was maintained as closely as possible to 30 mm below the top of the box by fine adjustment of the downstream pump. During this time the electrical conductivity (EC) of the effluent was logged at 5 second intervals. Initially, Milli-Q water was passed through the box until the EC reduced to a constant value. After approximately 10 hours of water flow a conductivity-based pulse tracer test was conducted on the box. This was performed by changing the feed solution to 0.05 M sodium bromine for 20 minutes. Between 95% and 103% of the tracer was recovered in all tests as measured by an e curve method described by Levenspiel (1999). Residence times were determined using the exit age distribution method. The remaining assessment consisted of increasing step concentrations of copper and zinc solutions. This reactive tracer data is presented in Statham et al. (unpublished manuscript).After the sampling, metal clamps were tightened along the length at the base and top of the boxes to increase structural integrity when exposed to freeze-thaw cycling. The Perspex sides and bases of both Darcy boxes were covered with insulated panels of 25 mm of extruded polystyrene and the boxes were placed in a Sanyo MIR-153 laboratory incubator. The incubator was programmed to cycle through 4 days at -12 degrees C followed by 3 days at 10 degrees C. These temperatures were based on the lower limit of operation of the machine and a realistic field condition.Levenspiel, O. (1999) Chemical Reaction Engineering. 3rd Edition. John Wiley and Sons, New York.

本研究采用两台平行设置的达西箱（Darcy box）开展42次冻融循环试验，评估冻融循环对零价铁（Zero Valent Iron, ZVI）与惰性介质的影响。通过测量床层高度与非反应示踪试验，实现对床层水力学特性的表征；同时通过逐步提升污染物（铜与锌）浓度的方式，评估体系的反应动力学特征。所有测量均在冻融循环开始前、循环中期及循环结束后完成。
本研究采用两台定制的有机玻璃（Perspex）达西箱，其床层尺寸为长362 mm、宽60 mm、高194 mm；箱内装填质量分数为5%的Peerless铁粉（Peerless Metal Powders and Abrasive公司生产的铸铁集料，粒径对应8-50 US标准筛孔）与95%的玻璃微珠磨料（Potters Industries Inc.公司生产的25-40 US标准筛孔玻璃微珠）的混合介质。选取该介质配比的目的在于：在模拟南极金属污染物实际浓度与现场水流速率的条件下，确保大部分进水溶液中的污染物浓度均高于定量分析限（Limit of Quantification, LOQ）。所有溶液均通过蠕动泵泵入或抽出达西箱，管路采用酸洗后的Masterflex FDA认证维通橡胶管（Masterflex FDA vitron tubing）。
干燥介质以1 kg为批次称重，并置于自封袋中震荡翻转1分钟以实现均质化。为确保介质始终处于饱和状态，先向箱内分层加入约1 cm厚度的密理博超纯水（Milli-Q water），随后加入均质后的介质；箱顶预留20 mm空间，用于容纳冻胀及其他颗粒重排过程。
单次溶液流动特性评估流程耗时约2.5天。整个试验过程中，上游泵的流速设定为18.1 mL·min⁻¹。通过微调下游泵的转速，将进料堰的高度尽可能维持在箱顶下方30 mm处。试验期间，以5秒为间隔记录出水的电导率（Electrical Conductivity, EC）。初始阶段，先向箱内通入密理博超纯水，直至电导率降至恒定值。经过约10小时的水流后，对箱体开展基于电导率的脉冲示踪试验：将进料溶液切换为0.05 M溴化钠溶液，持续20分钟。所有试验中，通过Levenspiel（1999）提出的曲线法测得的示踪剂回收率介于95%~103%之间。采用出口年龄分布法确定体系的停留时间。其余评估流程为逐步提升铜与锌溶液的浓度，相关反应示踪数据已发表于Statham等人的未刊手稿中。
试验采样结束后，沿箱体底部与顶部的全长拧紧金属夹具，以提升箱体在冻融循环过程中的结构稳定性。两台达西箱的有机玻璃侧板与底板均覆盖25 mm厚的挤塑聚苯乙烯保温板，随后将箱体放置于三洋Sanyo MIR-153型实验室恒温箱内。恒温箱的程序设定为：先在-12℃条件下运行4天，随后切换至10℃条件下运行3天。该温度设定参考了设备的最低运行温度与实际现场环境条件。
Levenspiel, O. (1999) 《化学反应工程（Chemical Reaction Engineering）》第3版，John Wiley and Sons出版社，纽约。