Environmental-friendly montmorillonite-biochar composites: Facile production and tunable adsorption-release of ammonium and phosphate

The cleaner production of environmentally friendly materials for the controlled adsorption and desorption of ammonium and phosphate in wastewater treatment and soil fertility maintenance is a challenge. Here we show that a class of clay mineral-biochar composites which were produced via a facile cleaner pyrolysis process starting from two abundant natural materials (montmorillonite and bamboo powder). The montmorillonite-biochar composites (MBC) were characterized by X-ray powder diffraction, Fourier transform infrared spectroscopy, N2 adsorption/desorption isotherms, thermogravimetric analysis, and scanning electron microscopy. The adsorption and slow-release characteristics of MBC samples for NH4+ and PO43− in aqueous solution were evaluated. Montmorillonite (Mt) acted as a solid acidic catalyst and catalyzed the pyrolysis of bamboo powder to biochar and lower the pyrolysis temperatures. Both biochar and Mt contributed to the texture and structure, which provided varied surface and various adsorptive sites. The Langmuir maximum adsorption capacity of MBC samples for NH4+ and PO43− was 12.52 mg·g−1 and 105.28 mg·g−1. The adsorption of NH4+ onto the MBC sample was primarily controlled by surface adsorption and partially by a CEC process. The adsorption capacity of PO43− resulted from the electrostatic attraction or ionic bonding between PO43− and cations in the MBC samples such as Ca2+, Mg2+, Al3+ and Fe3+. The release of NH4+ and PO43− from the NH4+- and PO43−-laden MBC samples was merely 0.30–4.92% and 2.63–5.09% within 2–88 h. These findings implied that the MBC samples prepared from low-cost Mt and bamboo can remove NH4+ and PO43− from aqueous solution by adsorption and the resultant NH4+- and PO43−-laden MBC samples can be used as an effective slow-release fertilizer of N and P.