Water treatment using cryogel-based adsorbents - Targeting environmental pollutants at low concentrations
Abstract: The lack of safe, clean water is strongly linked to poverty, underdevelopment and poor health. Water is needed not only to sustain life, but to produce food and in industrial processes. Anthropogenic pollution and natural events can jeopardize access to safe water. Inorganic pollutants or toxic metal ions such as arsenic and cadmium are of special concern since these are persistent and accumulate in nature. Contaminated water used for direct consumption or for irrigation of crops can thus lead to direct and indirect exposure to humans to these metals. To improve access to safe water, treatment must be developed to remove even low concentrations of toxic metal ions. Adsorption is a suitable technique. In this work, adsorbents were developed for various kinds of water sources; dinking water, wastewater, industrial water and leachate from a biogas process, all of which contained metal ions, such as As or Cd, at low concentrations. The adsorbents developed were based on a polymer cryogels, produced via cryoconcentration. Cryogels have a highly interconnected and porous structure, making them suitable for water treatment. The cryogels evaluated in this thesis were all based on the polymer polyacrylamide (pAAm). In the first study, a pAAm cryogel was chemically modified with chelating functionalities suitable for the removal of heavy metals (i.e. Cu2+, Zn2+, Pb2+ and Ni2). A rational design was suggested, involving preparation of the polymer inside plastic carriers to increase the mechanical stability of the material. Another strategy using particles was explored by embedding either nanoparticles (NPs) or molecularly imprinted polymers (MIPs) in pAAm-cryogels. The ability of Al2O3 NPs or MIPs in cryogels (called Alu-cryo and MIP-cryo, respectively), to remove As(V) was investigated. Both Alu-cryo and MIP-cryo exhibited adsorption capacities similar to those when the respective particles were applied in suspension. MIP-cryo showed higher selectivity but lower adsorption capacity than Alu-cryo. To improve the removal of As(III) without any pretreatment, co-precipitated Fe-Al hydrous oxides were prepared and embedded in cryogels. As(III) could be removed from smelting wastewater in flow-through mode, reducing the concentration to European industrial emission standards. The removal of Cd2+ from leachate resulting from the anaerobic digestion of seaweed was studied using hydrothermally modified TiO2 embedded in cryogels. This cryogel was shown to be successful in removing Cd2+ when tested in flow-through mode, and high adsorption capacities were obtained. Toxicity and particle leakage were studied in NP-embedded cryogels. Although no evidence of particle leaking was found, toxicity resulted from the leakage of the monomer acrylamide from the cryogel. The findings of this work show that cryogel-based adsorbents have potential in water treatment, due to their broad range of applications and the flexibility of the material. New and improved methods of particle immobilization in cryogels should be investigated, and pAAm should be replaced by non-toxic alternatives. Making cryogel materials with high adsorption capacities, which can be regenerated, can reduce the cost of water treatment. It is also important that novel technology meets environmental safety requirements to ensure the supply of safe water.
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