Calcium Effect on Arsenic (V) Adsorption onto Coal Fly Ash

Batch tests indicated that arsenic (As) leaching is significantly affected by the calcium concentration in fly ash in the alkaline pH range. Arsenic leaching from low calcium fly ash from eastern bituminous coal increased with increase of pH in the alkaline pH range (pH 7 – 12). Fly ash from eastern bituminous coal with a slightly higher calcium content exhibited an arsenic leaching peak at pH 9, followed by decreased arsenic leaching up to pH 12. For alkaline ashes derived from subbituminous coal with much higher calcium content, significantly less arsenic was leached in the alkaline pH range. To improve understanding of arsenic leaching behavior, batch experiments with washed ash were performed to evaluate partitioning of As(V) spiked into the system, along with different amounts of calcium addition. Results suggested that the addition of calcium significantly reduced the soluble arsenic ratio in the alkaline pH range. This phenomenon was explained by postulating the formation of two highly adsorbable neutral arsenic species, CaHAsO4 and Ca3(AsO4)2, in this pH range. An adsorption model was developed to quantify the calcium impact on arsenic