Macroscopic Mg(OH)2-loaded lignin hydrogel for N/P recovery from simulated digestate: Formation, direct retrieval, and acidic-soil application of a struvite-containing product

Immobilizing reactive Mg phases within macroscopic hydrogel matrices may couple nutrient mineralization with direct retention of the resulting solid product, but such material-integrated systems remain insufficiently explored. Here, a macroscopic Mg(OH) 2 -loaded lignin hydrogel (Mg(OH) 2 @LH) was developed for nitrogen (N) and phosphorus (P) recovery from simulated digestate. The self-supporting hydrogel contained 115.60 ± 20.90 mg Mg·g −1 while retaining its porous block architecture. For powdered Mg(OH) 2 @LH passed through a 100-mesh sieve, Mg(OH) 2 @LH achieved apparent NH 4 + -N and total phosphorus uptake capacities of 73.99 and 30.99 mg·g −1 , respectively. NH 4 + -N removal varied modestly and non-monotonically with MgCl 2 impregnation concentration, whereas TP removal remained statistically unchanged across the tested preparation range. Nutrient recovery was sensitive to solution pH and N/P stoichiometry. XRD, FTIR, and SEM–EDS analyses supported the formation of a struvite-containing phase during nutrient recovery, while the non-stoichiometric N/P removal indicated contributions from processes other than struvite formation to the decrease in aqueous NH 4 + -N. Importantly, the recovered struvite-containing lignin hydrogel (MAP-LH) retained a centimeter-scale, self-supporting block form and could be directly retrieved from the liquid phase without additional fine-solid separation. Although intact blocks showed lower nutrient uptake than powdered material, this revealed a trade-off between mass transfer and macroscopic retrievability. Subsequent acidic-soil column experiments showed that MAP-LH resulted in a 53.3% lower cumulative TN leaching ratio than soluble N–P fertilizer and partially alleviated soil acidity. These results demonstrate a retrievable hydrogel architecture that integrates Mg-phase immobilization, nutrient recovery, macroscopic product retention, and post-recovery soil application. Download: Download high-res image (261KB) Download: Download full-size image