Changes in solubility and chemical speciation of potassium, calcium, and silicon in wood ash by acid treatment

• Published in: Soil science and plant nutrition (Tokyo) Vol. 69; no. 4; pp. 231 - 239
• Main Authors: Takeuchi, Satoshi, Yamashita, Kosei, Hashimoto, Yohey
• Format: Journal Article
• Language: English
• Published: Kyoto Taylor & Francis 04.07.2023; Taylor & Francis Ltd
• Subjects: acid treatment; Calcium carbonate; Chemical speciation; Degree of polymerization; Dilution; fertilizer components; Fertilizers; Fly ash; Gypsum; Hardwoods; Hydrochloric acid; Phosphoric acid; Polymerization; Potassium; Power plants; Silica; Silica gel; Silicon; Sodium hydroxide; Solubility; Speciation; Spectroscopy; Sulfuric acid; Water chemistry; wood ash; X ray absorption; X-ray diffraction; XAFS; XRD
• ISSN: 0038-0768; 1747-0765
• DOI: 10.1080/00380768.2023.2230228

This study investigated the relationship between the solubility and the chemical speciation of plant nutritional elements (K, Ca, and Si) in wood ash treated with a dilute solution of sulfuric acid or phosphoric acid. The diluted acid solutions were added to fly ash (FA) collected from a power plant, and the untreated and acid-treated FAs were analyzed for their contents of K, Ca, and Si. The crystalline phases of the untreated and acid-treated FAs were identified using X-ray diffraction (XRD), and the Si species were determined using X-ray absorption near-edge structure (XANES) spectroscopy. Sulfuric acid and phosphoric acid treatments did not affect the solubility of K in FA. Water soluble and citrate soluble K in FA were decreased by the addition of these acids. The water soluble Ca content in FA treated with phosphoric acid did not show a significant change compared to that in the untreated FA, but it increased up to 65 times in FA treated with sulfuric acid. It is suggested that CaCO 3 , which was the main Ca species in the untreated FA, was transformed into syngenite and gypsum by the sulfuric acid treatment. Hydrochloric acid soluble Si in the acid-treated FA decreased compared to that in the untreated FA. In contrast, the NaOH-soluble Si fraction, a method for selectively extracting silica gel, increased especially in the phosphoric acid-treated FA. The results of XANES analysis showed that the FA samples treated with 10-30% sulfuric acid or 10-30% phosphoric acid had a lower degree of Si polymerization than the untreated FA, which corresponded generally to the increased Si solubility in FAs. However, FA treated with 30% sulfuric acid had lower soluble Si despite a lower degree of polymerization than FA, which was attributed to a lower pH. These results suggest that the change in solubility of Si in the acid-treated FA was influenced by the degree of polymerization and the decrease in pH. From the viewpoint of a fertilizer use of FA, sulfuric acid-treated ash, which consists of both water soluble Ca and citrate soluble Ca, is suitable for Ca fertilizer, and phosphoric acid-treated ash, which has high NaOH-soluble Si, is suitable for Si fertilizer