Adsorption isotherms of ammonium on coir

Coir (coconut mesocarp pith) is the fibrous material that constitutes the thick mesocarp (middle layer) of the coconut fruit (Cocos nucifera L.). It has ion‐exchange and gas adsorption properties that can be utilized to adsorb 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 + adsorption process and the pH buffering capacity of the coir were studied to quantify its effectiveness as an NH4 + adsorbent. The one‐ and two‐surface Langmuir, van Bemmelen‐Freundlich, Temkin, and first‐order models were fitted to the NH4 + sorption data at an equilibrium pH range of 4.6 to 5.1. Each model was found to describe the NH4 + sorption data with comparable success (r2≥0.88), with the Langmuir and Temkin being superior (r2≥0.99). Although the equilibrium pH did not change much after adding buffers of pH 4 to 7 to the coir, there was a consistent increase in the amounts of NH4 +‐N sorbed with increasing pH, as estimated using the two‐surface Langmuir model. The estimated amounts of NH4 +‐N sorbed were 516 mmol kg‐1 at pH 4.6, 521 mmol kg‐1 at pH 4.63, 571 mmol kg‐1 at pH 4.88, and 573 mmol kg‐1 at pH 5.1. The coir showed a good buffering ability at pH range 3.7 to 10.4. The coir buffer strength, defined as 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 to 10.4. The high CEC, surface area, and buffer strength of the coir indicated its high ability to retain cations and buffer against pH change.