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Advanced methods for the removal and monitoring of polar organic contaminants

Final Report Summary - POLARCLEAN (Advanced methods for the removal and monitoring of polar organic contaminants)

The aim of the PolarClean project was to study new efficient methods for removing emerging polar contaminants from wastewater and drinking water (Objective 1), and to use the principles governing the uptake of such contaminants to create pre-concentration methods for monitoring purposes (Objective 2). The safety of the most successful prototypes has been assessed in vitro and the purification process has been scaled up which brings the technology closer to practically solving real water purification problems (Objective 3).

The quality of drinking water and aquatic ecosystems is threatened by anthropogenic polar organic contaminants. The aim of the PolarClean project (http://www.brighton.ac.uk/set/research/projects/PolarClean.php) was to develop new methods for the removal and monitoring of emerging polar contaminants in waste and drinking water. These new methods have been tested against a range of polar emerging contaminants representing pesticides, hormones, pharmaceuticals and plasticisers with distinctive chemical structure and functionalities, and nanoparticles. The properties of the materials with higher removal capacity have been applied in the development of new analytical tools for trace contaminant detection.

Synthetic phenolic carbon materials with controlled pore structure and surface chemistry have been developed and identified as highly efficient water purification tools that, with some degree of selectivity, can remove polar organic contaminants by adsorption. The interconnected and abundant micro and mesoporosity together with the low charged surface have been identified as features that boost the removal capacity of these adsorbents compared to the granular activated carbon (GAC) currently used in tertiary treatment in water treatment plants. This part of the work, with special emphasis on the removal of a contaminant that is particularly challenging to treat, metaldehyde, has been carried out in collaboration with an SME (MAST Carbon International Ltd.). As an example of the enhanced performance of the synthetic carbon, the maximum adsorptive capacity achieved for metaldehyde (75 mg/ g carbon), atrazine (641 mg/ g carbon), malathion (682 mg/ g carbon), diclofenac (65 mg/g carbon) and estradiol (46 mg/g carbon) substantially outperformed the treatment obtained with standard granulated carbon used in the water industry when tested under the same conditions. Relevant studies for up scaling the technology to industrial scales have been carried out, such as adsorption isotherms; adsorption with presence of competing ions; through flow studies; kinetic of adsorption; in-vitro toxicity of the water filtrate; characterisation and treatment of the sub products in the production of the synthetic carbon and regeneration studies of the sorbents. The technology was selected by the Technology Approval group (http://www.isleutilities.com/tag.php) as one of the more innovative in Europe in 2012 among the 500 technologies annually reviewed (London, December 2012). Currently, the synthetic carbon materials are being tested at pilot scale in collaboration with the Water Industry (UK) and compared with other technologies for the removal of metaldehyde as well as for other organic contaminants and the results so far indicate that its performance is competitive and has potential to be used in water treatment plants. A patent was filed in November 2012 “Carbon materials and their applications GB 1221227.0” between the University of Brighton and MAST Carbon International Ltd.

The physico-chemical features found to be important in the synthetic carbon for the removal of polar emerging contaminants have also assessed in various natural materials. Hence, agricultural crop by-products with low commercial value but with advantageous natural coatings and porosity have been studied and assessed for their use in water clean-up. This is the first time to our knowledge that the purification of organic contaminants by these natural materials has been explored and could imply an advance in water treatment in regions with limited resources as well as to obtain value and use to agricultural “wastes”.

Carbon nanoparticles such as carbon nanotubes (single and multi-walled), with a range of oxidation degrees, have shown lower adsorption properties than the above mentioned micro- mesoporous synthetic carbon micro beads probably because of the limited pore size range. Single and multi-layered graphene oxides have also been evaluated for their potential to remove estradiol, atrazine and metaldehyde but the adsorptive capacity was lower than that found in the above mentioned synthetic carbon which has controlled and “tunable” micro- and mesoporosity. The high cost, limited adsorption and safety concerns of free nanopaticles limit their applicability for the purification of water effluents in treatment plants. Nevertheless single layer graphene oxide (SLGO) has been observed to play a role in the degradation of organic contaminants assisted by hydrogen peroxide. The fundamentals of this degradation mechanism have been studied for atrazine and metaldehyde; however, whereas atrazine removal was caused by both degradation and adsorption, metaldehyde did not adsorb onto graphene (pH 5-14) and this allowed detailed examination of catalytic degradation assisted by SLGO. The role of iron cations, pH and amount of the carbon nanomaterial on the degradation of metaldehyde have been investigated, and the optimal performance was found at 0.1mg SLGO/ml water, whereas pH and iron cations were not found to be critical parameters. The possibly catalytic reaction carried out by SLGO has been found to cause degradation at a similar rate to that from the Fenton’s reaction using iron sulphate and hydrogen peroxide and pH 5-8. The potential to incorporate SLGO in water treatment has been further evaluated by physically attaching it to a polymer matrix and carrying out the degradation of metaldehyde in a sequence of batch reactions.

Nanoparticles can also be considered a polar emerging contaminant since they are being incorporated in products of daily use (cloths, toothpaste, paints) and have application in nanomedicine or environmental remediation, and their fate and toxicity is not well understood in some cases. The Polarclean project has allowed studying the fate of some metal oxide nanoparticles currently used when in contact with epithelial cells and their effect on cell proliferation and metabolism. Besides, a new method based on their entrapment from water and dry surfaces within a self-assembly polymer from marine origin has been developed.

A new analytical methodology based on LC-MS (multiresidue determination of 15 emerging contaminants) and fast determination of a single contaminant with FTIR, have been developed to assess the water purification achieved with the studied water treatment methods. The structure and composition of the synthetic carbon beads that have shown to be so effective in water purification, as well as some agricultural waste products, have been used as solid phase extraction analytical pre-concentration tools. For this application, the carbon beads have been embedded in different shapes of macroporous polymer (acrylamide, polyvinyl alcohol and hydroxyethyl metacrylate), a matrix that holds the adsorptive phase and does not prevent the diffusion of the polar contaminants from the water to the core of the analytical tool.

The new water treatment methods developed have potential to substitute the GAC used today in water treatment plants and provide water with the lowest level of micropollutants possible. These carbons can be a solution to the background level of pollution in ground and surface water present for decades; with further research they could substitute the Fenton’s reaction in waste water treatment plants and avoid or minimise the addition of iron for catalytic degradation (SLGO) and be able to capture nanoparticles (self-assembly polymer); provide cleaner water in river basins and developing countries (via adsorption onto agricultural waste materials); and become a new generation of analytical tools. The results achieved in the PolarClean project have a clear socio-economic impact and provide a step forward in the state of the art in water research.