Over 2 million tons of citric acid residue (CAR) are generated from citric acid production in China each year. Thus, it is of great significance to find a suitable method for recycling the waste products of citric acid fermentation. Herein, activated carbon (AC) samples were prepared from CAR with phosphoric acid activation. Effects of impregnation ratio, activation time, and activation temperature on pore structure and surface functional group formation were evaluated. AC prepared by 1.5 h of pyrolysis in nitrogen at 550°C from CAR and phosphoric acid at an impregnation ratio of 2.0 (CAR-AC-2.0) had the largest specific surface area (786 m 2 /g) and total pore volume (0.71 mL/g). The surface morphologies and chemical properties of AC samples were characterized by scanning electron microscopy, thermogravimetric analysis, elemental analysis, and Fourier transform infrared spectroscopy (FTIR), and their adsorption capacities for methylene blue (MB) and iodine were tested. There were rare functional groups on the surfaces of CAR-AC-2.0, and the adsorption amounts of CAR-AC-2.0 for iodine and MB reached up to 729 and 121 mg/g, respectively. CAR-AC-2.0 was an ideal adsorbent for removing organic matters from solutions and wastewater. Adsorption efficiency of CAR-AC-2.0 for chemical oxygen demand (COD) from sugar-containing wastewater was 291.6 mg/g, and the removal efficiency was 80.9% when the dosage was 15 g/L. This study provided a new strategy for removing COD from fermentation wastewater with high efficiency and low cost.
Chemical modeling calculations and batch tests were carried out to investigate the effect of solution chemistry on formation of phosphate precipitation in synthetic flushed dairy manure wastewater for phosphorus recovery. Saturation index (SI) of different calcium phosphate precipitates in solution with a PO 4 3 -concentration range of 0.032-9.8 mM (1-300 mg P/L), Ca/P molar ratio of 1-20, pH value of 5.0-12.0, and the CO 3 2 -concentration of 0-100 mM was calculated separately using the geochemical aquatic modeling program, PHREEQC. Results show that the SI of calcium phosphate is the logarithmic function of Ca 2 + and PO 4 3 -concentration, increasing with the increase of Ca 2 + and PO 4 3 -concentrations. SI of calcium phosphate is the polynomial function of the solution pH value. SIs of hydroxyapatite (HAP) and tricalcium phosphate (TCP) increase with the growth of the pH value, while the SIs of octacalcium phosphate (OCP) and dicalcium phosphate dehydrate (DCPD) reach the maximum value at pH 9.0-9.5 and 7.0-7.5, respectively. The SI of calcium phosphate decreases with the growth of the CO 3 2 -concentration following a linear function pattern. Meanwhile, the SI of calcium phosphate decreases with growth of ionic strength following a logarithmic function pattern. In the case study of phosphate removal from synthetic flushed dairy manure wastewater, the PO 4 3 -removal trend under a different pH value and Ca/P molar ratios was closer to the predictions of thermodynamic modeling. CO 3 2 -can affect the PO 4 3 -removal efficiency and turn on the marked inhibiting effect on HAP growth, but does not obviously affect the structure of the precipitate.
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