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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

Jianbin Deng,Xiaohan Zhang,Qijun Zhang,Pu Wang,Deyun Li,Tian Hu,Kai Yao,Xiaoyuan Chen,Zhihao Wu,Yulong Zhang

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