Enhanced phosphorus recovery from biogas slurry by biochar of straw with Mg/La oxide

Phosphorus has been widely used as the essential component in the fertilizer and feed of agriculture, but it is a non-renewable resource. Biogas slurry rich in phosphorus can serve as a recyclable agricultural waste resource. Phosphorus recovery from the biogas slurry can be widely expected to realize nutrient management and utilization, particularly for the green and sustainable development of circular agriculture. Therefore, it is necessary to recover the phosphate from the biogas slurry for sustainable agriculture. Alternatively, adsorption is one of the main technologies in nutrient recovery. Different types of adsorbents have been developed to capture phosphate, including biochar, metal-based materials, minerals, functionalized silica, and various modified wastes. Among them, the layered double oxides (LDO) can often be coupled with the biochar-based materials, which are taken as an adsorbent and as a host substance, due to the multiple parent materials. The introduction of biochar materials can improve the recovery rate of phosphorus in a cost-saving way. More importantly, the biochar-based materials with unique pores and abundant surface functional groups can greatly contribute to the pore structure and surface functional group density of composites, where the ions can be highly concentrated in recycled materials. Herein, a new composite adsorbent of biochar-LDO was proposed to recover the nutrient elements from the biogas slurry using adsorption, in order to effectively recover the phosphorus for better resource utilization. The synthesis temperature of composite material was also evaluated to improve the adsorption performance of phosphorus. It was found that the layered bimetallic oxide (Mg/La0.1-LDO) synthesized by the quantitative control of magnesium and lanthanum presented rich interlayer ions and nanoparticles in the early stage, indicating an excellent performance in the phosphorus adsorption. However, the dispersion of nanomaterials in the solution and the high cost of raw materials relatively reduced the value of material recycling and application. Therefore, the straw of crops (rice and corn) was added to fix and strengthen the Mg/La0.1-LDO, where the biochar-magnesium/lanthanum bimetal oxide nanocomposite (BC-LDO) was prepared for the phosphorus recovery from biogas slurry using the co-pyrolysis at different temperatures (400℃, 500℃ and 600℃). The resulting BC-LDO was then characterized to determine the recovery capacity by the SEM-EDX/TEM/FTIR/XRD techniques. The results showed that the pyrolysis at a high temperature (600 ℃) was more conducive to the fixation of nanoparticles on the straw biochar (6YBC-LDO), which was rich in meso- and micro-porous materials. The particle size of the nanocomposite decreased significantly (between 12 and 20 nm), with the increase in pyrolysis temperature. These nanocomposites were also easier to combine with the phosphate to form LaPO4 precipitation and Mg3(PO4)2 inner-sphere complexation. According to the Langmuir equation, the maximum phosphate uptake of 6YBC-LDO was estimated to be 366.39 mg/g, which was significantly higher than that of 5YBC-LDO and 4YBC-LDO. The kinetic investigation showed that the 6YBC-LDO effectively adsorbed the high phosphorus (69 mg/g) within 30 min, and strongly enriched the straw biochar, indicating an excellent phosphate adsorbent. The 6YBC-Mg/La0.1-LDO presented a high selective adsorption capacity for the phosphate, which was reused many times after desorption. The 6YBC-LDO rapidly removed more than 90% phosphate in the biogas slurry, where the phosphate concentration was reduced to less than 8.25 mg/L within 1 h. Consequently, the 6YBC-LDO has an excellent application prospect in the nutrient recovery from the biogas slurry.