Application of ferrates for Arsenic removal from groundwater
Ferrates are a general name for compounds of iron in a high oxidation state - Fe(IV), Fe(V) and Fe(VI), which show highly oxidizing effects and form non-toxic ferrous oxides and oxohydroxides that can act as coagulants. Laboratory tests have confirmed an interesting application potential for these compounds in the area of water treatment technologies. In 2014 an affordable technology for production of ferrates for industrial application was successfully developed and implemented. AECOM researched the possibility of application of ferrates to treat ground water with a naturally elevated content of arsenic (As, arising from regional geological conditions). During reaction of ferrate and As from groundwater, a very strong covalent binding occurs between Fe and As within three to five minutes. Subsequently it is necessary to finely separate the resulting micro-flakes containing hydroxides of Fe and As. Actual groundwater was tested from two sites (site MEZ, site KLU) each with an As content of approximately 100 µg/L, i.e. ten-fold greater than the Czech limit for drinking water (10 µg/L as stated in Regulation 252/2004 Sb). Based on successful laboratory tests, a technological process for separation of As using ferrates was designed, on which basis a mobile pilot plant with capacity of 100 L/hour for on-site testing was constructed. Configuration of operational pilot testing involved: a) addition of ferrate (5 mg/L, 10 mg/L, 15 mg/L, 20 mg/L), b) pH adjustment to pH 7 using diluted HCl, c) adding polymer flocculant, d) sand filtration, e) microfiltration through 10 µm and 1 µm, f) ultrafiltration through 0.45 µm. All pilot tests were performed in duplicate.
The quality of cleaned water achieved by the mobile testing device from both sites (KLU and MEZ) complied with the requirements for drinking water – the final As and Fe concentrations were both lower than the required limits of 10 µg/L As, and 200 µg/L Fe and a substantial disinfection effect was also observed. The minimal doses of ferrate for effective treatment to below the requirements were 10 mg/L for the MEZ site and 15 mg/L for the KLU site. During the washing of filters of the pilot test equipment no measurable quantity of sludge was detected. The main practical advantages of the tested technology in comparison with other technologies are therefore the minimal consumption of reagents, minimal production of waste sludge with As content and the associated disinfection effect.