Epsilon Archive for Student Projects

Abstract

Determining the cation exchange capacity (CEC) of biochar is needed to evaluate its potential to improve soil fertility and nutrient retention. However, standard soil CEC methods applied to biochar can give inflated values because soluble salts and carbonates dissolve during extraction. This study measured the CEC of five biochars: coffee husk (CH), coconut shell (CS), wood chips (WC), high-temperature Meva Energy biochar (ME), and seed waste (SW). Two methods were used: the ammonium acetate base cation summation method (CECsum) and the Na⁺ index cation displacement method (CECNa), applied at pH 4.8 and pH 7 on both acid-washed (AW) and non-acid-washed (NAW) samples. An ion exchange resin (IER) and sea sand were included as positive and negative controls.
Acid washing significantly reduced CECsum across all five biochars at both pH 4.8 and pH 7, with reductions ranging from approximately 30% in coconut shell to approximately 85% in wood chip biochar. The magnitude of the reduction scaled with the acid neutralising capacity of each biochar, confirming that soluble salts and carbonates dominate apparent CEC in non-acid-washed samples. In coffee husk and seed waste, K⁺ accounted for nearly all the removed fraction, with acid washing reducing K⁺ by 94% and 87% respectively. In wood chip and Meva Energy biochars, Ca²⁺ and Mg²⁺ were the dominant fractions removed. CECNa was 0 cmolc kg⁻¹ for WC, ME and SW in both NAW and AW samples at both pH values, confirmed by supplementary CaCl₂ extraction, indicating that these freshly produced biochars lack Na⁺-accessible surface exchange capacity. For coffee husk and coconut shell, elevated CECNa values measured at pH 7 are best interpreted as experimental artefacts arising from colloidal biochar particle migration during NaOAc saturation rather than as measurements of true exchange capacity. The IER reference confirmed that the underlying displacement procedure functioned as expected, with CECsum at pH 7 within 3% of the calculated reference value.
These findings show that non-acid-washed CEC values overestimate the true surface exchange capacity of biochars, and that the divergence between CECsum and CECNa exceeds an order of magnitude for the low-ash biochars. Acid washing followed by index cation measurement is recommended for reliable biochar CEC determination, with quantification of colloidal particle losses incorporated as a standard quality control step in future studies more especially when percolation tube procedure is employed.