Phosphorus recovery from biogas fermentation liquid by Ca-Mg loaded biochar

Shortage in phosphorus （P） resources and P wastewater pollution is considered as a serious problem worldwide. The application of modified biochar for P recovery from wastewater and reuse of recovered P as agricultural fertilizer is a preferred process. This work aims to develop a calcium and magnesium loaded biochar （Ca-Mg/biochar） application for P recovery from biogas fermentation liquid. The physico-chemical characterization, adsorption efficiency, adsorption selectivity, and postsorption availability of Ca-Mg/biochar were investigated. The synthesized Ca-Mg/biochar was rich in organic functional groups and in CaO and MgO nanoparticles. With the increase in synthesis temperature, the yield decreased, C content increased, H content decreased, N content remained the same basically, and BET surface area increased. The P adsorption of Ca-Mg/biochar could be accelerated by nano-CaO and nano-MgO particles and reached equilibrium after 360min. The process was endothermic, spontaneous, and showed an increase in the disorder of the solid-liquid interface. Moreover, it could be fitted by the Freundlich model. The maximum P adsorption amounts were 294.22, 315.33, and 326.63 mg/g. The P adsorption selectivity of Ca-Mg/biochar could not be significantly influenced by the typical pH level of biogas fermentation liquid. The nano-CaO and nano-MgO particles of Ca-Mg/biochar could reduce the negative interaction effects of coexisting ions. The P releasing amounts of postsorption Ca-Mg/biochar were in the order of Ca-Mg/B600 〉 Ca-Mg/B4S0 〉 Ca-Mg/B300. Results revealed that postsorption Ca-Mg/biochar can continually release P and is more suitable for an acid environment.

Removing phosphorus from aqueous solutions by using iron- modified corn straw biochar

Iron-modified corn straw biochar was used as an adsorbent to remove phosphorus from agricultural runoff. When agricultural runoffs with a total phosphorus （TP） concentration of 1.86 mg.L-1 to 2.47 mg.L-1 were filtered at a hydraulic retention time of 2 h through a filtration column packed with the modified biochar, a TP removal efficiency of over 99% and an effluent TP concentration of less than 0.02mg.L-1 were achieved. The isotherms of the phosphorus adsorption by the modified biochar fitted the Freundlich equation better than the Langmuir equation. The mechanism of the phosphorus adsorbed by the modified biochar was analyzed by using various technologies, i.e. scanning electron microscopy （SEM）, X-ray diffraction （XRD） analysis, X-ray photoelectron spectroscopy （XPS）, and Fourier transform infrared spectroscopy （FTIR）. The results indicated that the surface of the modified biochar was covered by small iron granules, which were identified as Fe304. The results also showed that new iron oxides were formed on the surface of the modified biochar after the adsorption of phosphorus. Moreover, new bonds of Fe- O-P and P-C were found, which suggested that the new iron oxides tend to be Fe5（PO4）4（OH）3. Aside from removing phosphorus, adding the modified biochar into soil also improved soil productivity. When the modified biochar-to-soil rate was 5%, the stem, root, and bean of broad bean plants demonstrated increased growth rates of 91%, 64%, and 165%, respectively.