AbstractsChemistry

In terms of chemistry, phosphate (PV) and arsenate (AsV) are similar, but their biochemical roles are completely different. Phosphorus (P) is an important nutrient for all forms of life but when entering to watercourses it increases the eutrophication risk. Arsenic (As) is a metalloid, the inorganic species of which are toxic, with AsIII classified as a first group carcinogen. This work was undertaken to investigate the potential of Light Expanded Clay Aggregates (LECAs) to act as sorbents to remove P and As species from the aqueous phase. Detailed studies were undertaken to determine if the efficiency of LECAs can be improved by coating them with aluminum (Al) or iron (Fe) oxide. The ability of various LECA materials to remove P from water was compared at different pH values, and the reversibility and kinetics of the reactions were investigated. The results revealed that the oxide-coated LECAs were superior to LECAs as P sorbents. Furthermore, Al-LECAs was superior to Fe-LECAs, which is redox-sensitive and therefore may lose its sorption properties in temporarily prevailing anoxic conditions, e.g., in constructed wetlands (CWs) used for wastewater purification. Studies of the reaction kinetics, impact of the degree of P saturation (DPS) and P desorption revealed that the oxide-coated materials retained P more strongly than LECAs. The experiments with As were designed to simulate the conditions in groundwater where As exists as AsV or AsIII, depending on the redox potential. The optimal pH for the sorption of both As species, the effect of oxidizing agents on the sorption of AsIII and the effect of competing phosphate anions on As sorption were investigated in systematic studies, which revealed that the oxide coatings decisively increased the sorption of both As species. For AsV at a pH of 4, Al-LECAs appeared to be a better sorbent than Fe-LECAs. However, Fe-LECAs were superior to Al-LECAs at a pH of 6. The sorption of AsIII increased with elevated pH. At a pH of 9, sorption was greater onto Al-LECAs than Fe-LECAs. However, the opposite trend was obtained at a pH of 10. The oxidation of AsIII to AsV enhanced sorption by LECAs. Moreover, the oxidation-reduction reaction created new sorption sites for AsV on the manganese dioxide (MnO2) surface. The competition with P for sorption sites relatively diminished the retention of As onto LECAs compared to the oxide-coated LECAs. In the Al-LECAs, the P-induced decrease in the AsV sorption was larger than that in the Fe-LECAs. The competition between P and AsIII lowered the sorption by oxide-coated LECAs most markedly in acidic media. Thus, LECAs seem to be the best option for removing AsIII and AsV from groundwater with high P. The sorption properties of LECA materials were assessed by means of sorption isotherms describing the sorption as a function of the sorbate concentration in the solution after equilibration The actual P sorption capacity was assessed as the P retained at a P concentration of 20 μg L−1 in the equilibrium solution (X20), which is close to the upper…