Analysis of K⁺ release kinetics of biomass ash with different pH

Biomass ash is a kind of plant straw residuum produced by combustion of straw at 800 ℃. Biomass ash contains lots of nutrients, including phosphorus (P), calcium (Ca), magnesium (Mg) and especially potassium (K). In biomass ash, K mainly occurs as K₂SO₄, KCl and K₂SO₃. In recent years, biomass ash has been used to produce potassium-based fertilizers. However, less work has been done on the principles and mechanisms of biomass ash K⁺ release. In the new production system and method proposed to produce potassium fertilizers from biomass ash, pH is critical for K⁺ release. This paper used four types of biomass ash (cornstalk, rice straw, sawn wood and chaff) with different pH of 4, 7 and 9 adjusted by ammonium acetate (NH₄Ac) to extract available K by the method of penetration exchange. This study discussed the cumulative quantity and rate of release of K⁺, and analyzed K⁺ release dynamics of four types of biomass ash using three kinetic models. The results indicated that K⁺ release attained equilibrium in 20 80 min with a total capacity of K⁺ release significantly different for different pH. Total K⁺ release capacity was 14.77 255.17 cmol·kg^-1 at pH of 4, 4.83 106.71 cmol·kg^-1 at pH of 7 and 12.11 224.33 cmol·kg^-1 at pH of 9. Total K⁺ release capacity under each pH condition showed that rice straw released the most K⁺, followed by cornstalk and sawn wood, while chaff released the least K⁺. Correlation analysis suggested that total K content of biomass ash was critical for the time and total capacity of K⁺ release. The correlation coefficient of released K⁺ amount and total K content was 0.991 0.997. A significant difference was noted in K⁺ release rate among biomass ash types. These implied that the greater content of K contain the more amount of K⁺ release. This indicated that equilibrium time and quantity of released K⁺ varied with pH and total K content of exchange solution. Under the three pH conditions, released K⁺ amounts (Q) of rice straw and cornstalk were in the order of Q₄ > Q₉ > Q₇, while those of sawn wood and chaff were Q₉ > Q₄ > Q₇. The order of total K⁺ release capacity of biomass ash was Q_{Rice straw} > Q_{Sawn wood} > Q_Chaff > Q_Cornstalks under the three pH conditions. Before equilibrium time, K⁺ release velocity (v) was logarithmically related with time (lnt) during each reaction stage. For the same biomass ash, the optimal kinetic mode of K⁺ release changed with pH condition. The best models at pH of 4, 7 and 9 were Elovich Equation (EE), Dual Constant Equation (DCE) and EE, respectively. Under the same pH condition, the model that best described K⁺ release process was the EE and DCE, while the Parabolic Diffusion (PD) was the most inappropriate model.