Adsorption isotherms of ammonium on coir

Coir (coconut mesocarp pith) is the fibrous material that constitutes the t hick mesocarp (middle layer) of the coconut fruit (Cocos nucifera L.). It h as ion-exchange and gas adsorption properties that can be utilized to adsor b the important plant nutrient N in its NH4+ and/or NH3 form, protecting it from losses such as during composting of N-rich wastes. The NH4+ adsorptio n process and the pH buffering capacity of the coir were studied to quantif y its effectiveness as an NH4+ adsorbent. The one- and two-surface Langmuir , van Bemmelen-Freundlich, Temkin, and first-order models were fitted to th e NH4+ sorption data at an equilibrium pH range of 4.6 to 5.1. Each model w as found to describe the NH4+ sorption data with comparable success (r(2)gr eater than or equal to 0.88), with the Langmuir and Temkin being superior ( r(2)greater than or equal to 0.99). Although the equilibrium pH did not cha nge much after adding buffers of pH 4 to 7 to the coir, there was a consist ent increase in the amounts of NH4+-N sorbed with increasing pH, as estimat ed using the two-surface Langmuir model. The estimated amounts of NH4+-N so rbed were 516 mmol kg(-1) at pH 4.6, 521 mmol kg(-1) at pH 4.63, 571 mmol k g(-1) at pH 4.88, and 573 mmol kg(-1) at pH 5.1. The coir showed a good buf fering ability at pH range 3.7 to 10.4. The coir buffer strength, defined a s the amount of OH- required to increase pH by one unit, was 11.3 cmol kg(- 1) for the pH range 3.7 to 8.4, and 22.1 cmol kg(-1) for the pH range 8.4 t o 10.4. The high CEC, surface area, and buffer strength of the coir indicat ed its high ability to retain cations and buffer against pH change.