Yixin Yan

Protein hydrolysate is critical for the recovery of sludge proteins. Ultrasonic-assisted enzymatic hydrolysis (UEH) is an efficient method for extracting and hydrolysing sludge protein. The process of protein extraction and hydrolysis during UEH and enzymatic hydrolysis (EH) was compared, and it was found that UEH facilitated the extraction of the protein and therefore did not significantly restrict the subsequent hydrolysis of protein as EH did. The extraction of protein conformed to the first-order kinetic equation, and the constants of UEH (k = 1.63 h^− 1) were twice those of the EH method. The formation of peptides, an important intermediate product of the proteolytic process, conformed to the tandem kinetic equation, was significantly affected by the dose of enzymes instead of ultrasonic assistance. Further studies on intracellular and extracellular protein extraction and hydrolysis showed that ultrasonic pretreatment effectively reduced the EPS barrier by approximately 50%. Coupled with the synergistic effect of ultrasound and enzyme, the release of intracellular protein was 1.9 times that of EH, which was beneficial to the yield of peptides by removing the restriction in tandem reactions.

In this research, a new composite adsorbent (SC@ZVI@CS-AA) was designed and synthesized, and its application for the removal of Cr(VI) in groundwater was investigated. The interaction between SC@ZVI@CS-AA and Cr(VI) conformed to a pseudo-second-order model, and the adsorption process was dominated by chemisorption. The effects of material ratios, pH, temperature, SC@ZVI@CS-AA dosage, and coexisting ions on the removal of Cr(VI) were investigated. The removal efficiency of Cr(VI) by SC@ZVI@CS-AA reached 95%, and the reaction was significantly inhibited when SO₄²⁻ was present. Thermodynamically, the adsorption of Cr(VI) proceeded spontaneously above 35 °C (ΔG^θ < 0). According to scanning electron microscopy, X-ray photoelectron spectroscopy, Fourier transform infrared spectrometry, and synchronous thermal analysis, the removal mechanism of Cr(VI) by SC@ZVI@CS-AA was attributed to electrostatic attraction and reduction. In addition, SC@ZVI@CS-AA had good cyclic adsorption performance. Overall, the SC@ZVI@CS-AA composite showed great potential in the remediation of Cr(VI)-contaminated groundwater.

The effects of Cr(VI) pollution in groundwater run wide and deep. Without effective remediation, Cr(VI) harms the environment and human health. With in situ treatment methods in which permeable reactive walls serve as the main body, chitosan-acrylic acid sols (CS-AA) were used to stabilize zero-valent iron (ZVI), and acid-modified sulfonated coal (SC) was used as the carrier to prepare SC@ZVI@CS-AA. The interaction between SC@ZVI@CS-AA and Cr(VI) conformed to a pseudo second-order kinetic model suggesting chemisorption. The Langmuir model provided a good fit to the data for adsorption of Cr(VI) by SC@ZVI@CS-AA. SC@ZVI@CS-AA was characterized using XRD, FTIR, SEM and TG − DSC analyses. The effects of contact time, initial Cr(VI) concentration, temperature, material ratios, pH, dosage of SC@ZVI@CS-AA and interfering ions on the adsorption of Cr(VI) were investigated. Under the optimal conditions, the efficiency for removal of Cr(VI) reached 98.5%. The adsorption of Cr(VI) was significantly inhibited by SO₄^2-. In addition, the capacity for adsorption of Cr(VI) by SC@ZVI@CS-AA remained above 70% after three use cycles. Overall, SC@ZVI@CS-AA proved to be a promising and environmentally friendly medium for removal of Cr(VI) from groundwater.

Excess sludge contains a high amount of protein, which can be recovered to prepare protein foaming agents and other products with high added value. Enzymatic hydrolysis (EH) is a promising technology for the recovery of protein from excess sludge, and ultrasound has been identified as a potential method to assist in sludge disintegration. Ultrasonic pretreatment was combined with alkaline protease hydrolysis to extract protein from excess sludge produced by A²/O treatment (S1) and an oxidation ditch treatment (S2), and the extraction effects and changes in sludge dewatering performance were studied. The effects of the six factors ultrasonic power density, ultrasonication time, enzyme dose, pH, hydrolysis temperature and hydrolysis time were analyzed. The results showed that the ultrasound-enhanced enzymatic method could effectively extract sludge protein. Although the extraction efficiencies for the different municipal sludges were different, their extraction conditions were relatively similar. Considering the protein extraction rate and sludge dewatering performance, the selected extraction conditions were as follows: ultrasonic power density, 1 W/mL; ultrasonication time, 20 min; enzyme dose, 3500 U/g; pH 11; hydrolysis temperature, 60 °C; and hydrolysis time, 3 h. Under these conditions, the protein extraction rate (R_p) of S1 and S2 reached 55.9% and 52.3%, respectively. Moreover, the improvement in sludge dewatering performance (D_w) of S1 and S2 was 49.5% and 52.4%, respectively. Comparison of the protein, polypeptide, and amino acid contents obtained from ultrasound-assisted enzymatic hydrolysis (UEH), EH, and ultrasonic hydrolysis (UH) further demonstrated the beneficial effect of ultrasound application on enzymatic hydrolysis.

The protein in excess sludge can be extracted effectively by the alkali-thermal method, and the extracted protein can be used as a foaming agent and in other industrial raw materials to realize its resource utilization. In this paper, the factors influencing sludge protein extraction by the alkali-thermal method were optimized based on the protein extraction rate and the polypeptide content, which determine the foaming performance of the extracted protein. The results showed that the optimal conditions were a pH of 12, a temperature of 120 °C, a reaction time of 4 h, and a sludge moisture content of 92%. Under these optimized conditions, the extraction rate of protein and the concentration of polypeptides were 88.3% and 6599 mg/L, respectively. Additionally, the foaming performance of the extracted protein solution was tested, and the foamability and foam stability were close to 450% and 88.8%, respectively. Therefore, the sludge protein extracted by the alkali-thermal method can meet the relevant standards of foam extinguishing agents and concrete foaming agents in China. In addition, the dewatering performance of the hydrolyzed sludge was improved by 93.1%, which provided favorable conditions for the subsequent separation of the protein solution.

In this paper, the hydrolysis conditions to extract protein from excess sludge by thermal pretreatment assisted enzymatic were studied. The optimum technological conditions were the sludge moisture content of 95%, thermal pretreatment temperature of 100 °C, heating time of 3h, initial pH of enzymolysis sludge of 11, enzyme dosage of 3000U/g dry sludge, enzymolysis temperature of 55°C and time of 3h. Up to 52.8% of the protein content of the sludge was extracted, and the dehydration performance of the hydrolyzed sludge was 87.46% higher than that of the original sludge. Therefore, thermal pretreatment-assisted enzymolysis can effectively extract protein from excess sludge and improve its dewatering performance.