Efficient nitrate adsorption by ball-milled biochar embedded polyvinyl alcohol/pectin/alginate double network...

Hydrogel-based adsorbents have attracted increasing attention for water treatment application due to their highly tunable and environmental-friendly properties, while its innate weak mechanical strength and excessive swelling remain challenging. In this work, physically crosslinked double network (DN) hydrogels (denoted as nBPPA) were successfully fabricated by using polyvinyl alcohol/pectin/sodium alginate (PVA/PE/SA) as the polymer matrix and ball-milled biochar (BBC) as functional additive through hydrogen bond-induced micro-crystallization and Fe²⁺-ionic coordination crosslinking. The effect of BC with different properties and contents, PVA contents and mass ratio of PE-to-SA on adsorption behaviors were explored for hydrogels proportion optimization. The samples were characterized by techniques including scanning electron microscopy, X-ray diffraction spectra, thermogravimetric analysis and differential scanning calorimetry, Brunauer-Emmett-Teller measurement, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy, etc. On these basis, the adsorption kinetics, isotherms and thermodynamics as well as adsorption mechanism were discussed in detail. The gel exhibited low, stable swelling (∼2.5) with salt & acid-alkali resistance and high compressive strength of 1.95 MPa at strain of 50 %. Under optimal conditions, the maximum adsorption capacity for NO₃^-/g was ∼44.0 mg NO₃^-/g with equilibrium time of 240 min. It also showed good environmental adaptability in regeneration and reuse experiment and real water test. These findings demonstrated that nBPPA could provide an efficient, environmental-adaptable and stable platform for nitrate removal from aqueous solutions.