Comparison and mechanism of ammonia nitrogen and phosphorus adsorption properties of six mineral-based fillers

In order to efficiently remove ammonia nitrogen (${\mathrm{NH}}_4^+ $-N) and phosphates (${\mathrm{PO}}_4^{3-} $-P) from contaminated water and screen mineral-based fillers with excellent adsorption performance used in ecological restoration projects, three natural fillers (zeolite, vermiculite, volcanic rock) and three artificial fillers (ceramsite, biological filter, phosphorus removal filter) were selected to carry out experiments on the adsorption kinetics and isotherm of ammonia nitrogen and phosphorus. The adsorption properties and mechanisms of different mineral-based fillers were compared and analyzed by characterization methods such as X-ray diffraction and scanning electron microscopy. The results showed that the adsorption kinetics of ${\mathrm{NH}}_4^+ $-N and ${\mathrm{PO}}_4^{3-} $-P of the six mineral-based fillers complied with the pseudo-second-order kinetic equation, and the chemisorption process mainly controlled the adsorption rates. Both Langmuir and Freundlich equations could well describe the isothermal adsorption curves, and the theoretical adsorption capacity of ${\mathrm{NH}}_4^+ $-N was ranked as zeolite (5.9416 mg/g) > vermiculite (3.6953 mg/g) > biological filter (3.2500 mg/g) > phosphorus removal filter (3.1389 mg/g) > volcanic rock (1.0000 mg/g) > ceramsite (0.8571 mg/g). The adsorption capacity of ${\mathrm{PO}}_4^{3-} $-P was as follows: phosphorus removal filter (4.2424 mg/g) > biological filter (2.7917 mg/g) > vermiculite (1.6250 mg/g) > ceramsite (1.2105 mg/g) > volcanic rock (1.1579 mg/g) > zeolite (0.5625 mg/g). The adsorption properties of different mineral-based fillers for ${\mathrm{NH}}_4^+ $-N and ${\mathrm{PO}}_4^{3-} $-P were related to their specific surface area, micropore structure, mineral composition and metal element content, etc. Among them, zeolite had the largest specific surface area and cation exchange capacity, thus exhibiting the strongest adsorption capacity for ${\mathrm{NH}}_4^+ $-N. The biological filter and phosphorus removal filter contained tobermolite, calcium, iron and other components with strong phosphorus binding ability, which removed ${\mathrm{PO}}_4^{3-} $-P from contaminated water obviously. On the whole, zeolite and vermiculite could be used for the treatment of ammonia nitrogen contaminated water, and biological filter and phosphorus removal filter could be chosen to remove phosphorus. When treating contaminated water containing both ammonia nitrogen and phosphorus, a variety of mineral-based fillers could be combined. Compared with other mineral fillers, the self-developed biological and phosphorus removal filters had obvious advantages in the simultaneous removal of nitrogen and phosphorus, and could be used as the optimal substrate fillers for ecological restoration projects of contaminated water.