Adsorption effectiveness of ammonium nitrogen by iron-modified rice husk biochars

Objectives The research studied the adsorption characteristics of rice husk biochar and three iron-modified rice husk biochar to ammonium nitrogen. The aim was to provide a basis for using them as carbon additives to manufacture new fertilizers.

Methods Biochars (RBC, and FDRBC, FWRBC, FWBC) were produced from rice husk without and with FeCl₃ through pyrolysis under an oxygen-limited condition at 500℃. The physico-chemical properties of biochar were detected by a specific surface area analyzer (BET), scanning electron microscopy (SEM), X-ray diffractometer (XRD), and Fourier transforms infrared spectroscopy (FT-IR). A chamber adsorption test was carried out using the rice husk biochar and three iron-modified rice husk biochars. Langmuir and Freundlich’s equations were used to fit the isotherm adsorption data of rice husk charcoal and three kinds of iron-modified rice husk charcoal. The quasi-first-order kinetic model and quasi-second-order kinetic model were used to fit the adsorption data, respectively.

Results 1) The iron-modified rice husk charcoal’s specific surface area was reduced by 2.4%–63.7%, the average pore diameter increased by 2.8%–319.2%, and the pH reduced to about 5. 2) The adsorption ability of NH₄⁺-N by FWRBC and FWBC was the best under the experimental condition with a pH of 6, while FDRBC and RBC had the best adsorption effect on NH₄⁺-N when the pH was 7. Moreover, the Langmuir equation could better simulate the process of isothermal adsorption of NH₄⁺-N by the 4 kinds of biochar, indicating that the adsorption of NH₄⁺-N by the biochars was mainly due to monomolecular adsorption. 3) The maximum adsorption capacity of RBC, FDRBC, FWRBC and FWBC for ammonium nitrogen were 2.22, 8.82, 4.67, and 3.67 mg/g, respectively. 4) The pseudo-second-order kinetics model better defined the kinetics of the 4 kinds of biochar adsorption of ammonium nitrogen.

Conclusions The adsorption of rice husk charcoal and the three iron-modified rice husk charcoals on ammonium nitrogen is mainly monomolecular layer adsorption, a subset of chemical adsorption. Iron-modification process enhances aperture and pH of rice husk biochar to a different extent. FDRBC shows the best adsorption capacity of ammonium nitrogen, so it is most prospective in increasing fertilizer’s retention ability as an additive.