University of Nottingham1
The process of periodically removing sediment accumulated in canal/river water and reed bed wastewater treatment plants to landfill as hazardous waste is becoming an unsustainable method due to spiraling costs and increasingly stringent environmental regulations. In fact, The Canal and River Trust Company who manage the canal network in England and Wales, intend to spend around Â£8 million each year for the next ten years on dredging and disposing of accumulated sediment to landfill and incineration. Consequently, more economical and environmentally sustainable processes should be investigated. One potential approach would be to classify contaminants in sediment as either hazardous or non-hazardous waste and to consider the potential of recycling the sediment for use as a construction material. Additionally, valuable metals (e.g. lead and aluminum) in sediment could be extracted and recovered, which will significantly reduce the concentration of pollutants in sediment. In order to identify the metals, total microwave digestion of sediment using hydrofluoric acid is an accurate method of preparing the sample solution for analysis by Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES). However, this method is expensive due to the sophisticated equipment and the high level of expertise required. A cheaper and simpler way to prepare the sample for ICP-AES analysis is to use the Aqua Regia partial digestion method. The focus of this research was to compare the extraction effectiveness between Aqua Regia and microwave total digestion on target metals analysis using ICP-AES, specifically Iron (Fe), Copper (Cu), Zinc (Zn), Lead (Pb), Nickel (Ni), Vanadium (V) and Chrome (Cr) - all of which are listed in the guidelines for landfill directive. It also gives an indication of the performance of Aqua Regia to be used for the extraction and recovery of heavy metals from sediment as a valuable resource.
To undertake the investigation, three certified reference materials (CRMs) containing different soil/sediment samples, SO1, SO3 and SO4, were to undergo digestion by using both microwave digestion and Aqua Regia digestion. In the microwave total digestion, each CRM sample was digested and heated in a closed vessel by microwaving in three stages. In each stage a different acid reagent and a different temperature were used. The first stage involved heating the sample with concentrated HNO3 for 15mins until reaching 190oC, which was then kept constant for 45 minutes. In the second stage, the sample solution used previously was allowed to cool to room temperature before adding HCl and HF. The sample was then heated for 20 minutes and held for a further 20 minutes at a constant temperature of 190OC. The third stage required the sample to be heated for 20 minutes until a temperature of 170oC was achieved before it was held for 10 minutes at 170OC while a solution of 4%w/v H3BO3 was added. To carry out the Aqua Regia partial digestion, an Aqua Regia solution comprising of a 3:1 volumetric ratio of concentrated hydrochloric and concentrated nitric acid was initially prepared. The first stage involved adding the Aqua Regia solution into the CRM sample and leaving to stand overnight. The sample was then heated on a hot plate to 120oC. Once the sample had reached this temperature it was taken off the heat and allowed to cool naturally overnight. In the second stage, using the same procedure as the first, Aqua Regia solution was added to the sample, which was allowed to stand overnight before being reheated to 120oC and then to cool naturally during the night. In the third stage, adding 10%v/v HNO3 to the sample before heating it up to 120OC and holding this temperature for 30 minutes. Each digested sample was then analysed using the ICP-AES to identify the concentration of the target metals. Two sets of multi-element standards containing all target elements at different concentration levels were prepared using a single element certified standard solution in 10% v/v HNO3 matrix, which was the same as the certified sample solution matrix. The ICP-AES standard solutions were used for calibration at the suitable wavelength for each metal of interest. The precision of the two methods was validated by comparing measured target metals concentration in digested solutions against certified values from three CRMs, as recovery (%). In order to ensure results are valid, blank reagents using the same matrix as the sample solution were digested using both digestion methods before analysis using ICP-AES.
The findings indicate that Aqua Regia digestion is a promising method for preparing samples prior to analysis using an ICP-AES. Microwave total digestion, as expected, was a thorough extraction method with a recovery of around 100% for all target metals in all three CRMs samples, which was a close match against the certified values. More interestingly, Aqua Regia digestion was more successful at extracting specific metals, namely Fe, Cu, and Zn with recovery around 85% - 100% in all three CRMs. For other target metals including V, Ni and Cr, however, recovery by Aqua Regia varied significantly between three different CRMs, with 80-90% recovery in SO1 but less than 50% in SO4. The recovery of Pb was considerably low in all three CRMs, about 45-55%. This could potentially be due to the mineralogical properties of the sediment/soil.
The findings show that more precise measurements can be obtained from microwave total digestion; however in situations where a high level of accuracy is not required, Aqua Regia can offer a simpler, cheaper and safer way of determining the concentration of the target metals in sediment. However, further work needs to be conducted on wider certified sediment/soil types, to find out the relationship between mineralogical properties of sediment/soil and extraction performance of Aqua Regia. An appropriate correction factor could then be determined to give a good indication of total elemental composition in sediment/soil to classify its toxicity and identify valuable resources. The performance of Aqua Regia partial digestion for other highly valuable elements like platinum group should undergo further investigation to establish any potential application in industry for extraction and recovery. 1. Rachel, G. (2013). Multi - Element Analysis of Canal Sediment to inform on Best management Practices.Â SETAC AU Pollution in Urban Environment Presentation, 7-25