Development of Green Subcritical Water Separation Technology
Theoretically speaking, water under temperature and pressure conditions below its critical point is called subcritical water. However, the term “subcritical water” normally refers to hot water up to 374 C and pressure up to 218 atm. As the temperature increases, the hydrogen bonding between water molecules weakens, making water less polar at elevated temperatures. In chemical and pharmaceutical industry organic solvents are extensively used for extraction, chromatography, chemical analysis, and remediation of toxic chemicals. It requires a large amount of money not only for purchasing the expensive organic solvents but also properly disposing of their wastes. Subcritical water technology has shown a great potential in eliminating or minimizing the use of organic solvents in the processes mentioned above as it solely uses subcritical water as the solvent. The objective of this research is to further develop the green subcritical water separation technology. Specific aims include organic analyte stability in subcritical water, subcritical water extraction and analysis of active pharmaceutical ingredients (APIs) from medicinal herbs, subcritical water chromatographic separation of APIs from cold drugs, development of thermally stable stationary phase materials, and destruction of toxic polychlorinated biphenyls (PCBs) under subcritical water conditions. The stability of vanillin and coumarin in subcritical water was studied at 100-250 C. Vanillin was stable up to 60 min at all temperatures tested. While coumarin was stable at temperatures up to 150 C, it started to degrade at 200 and 250 C after heating for 60 min. Vanillin and coumarin were also extracted from vanilla beans and whole Tonka beans using subcritical water at 100-200 C. The vanillin quantity extracted increased from 10.7 mg per gram of vanilla beans (10.7 mg/g) at 100 C to 19.9 mg/g at 200 C. Similarly, the coumarin quantity extracted increased from 23.9 mg per gram of whole Tonka beans (23.9 mg/g) at 100 C to 36.8 mg/g at 200 C. Due to effectiveness, wide availability, and low side effects and costs, the use of herbal medicine has increased all around the world. Danshen (salvia miltiorrhiza) is a Chinese medicinal herb used to treat patients suffering from stroke, angina, chronic liver disease or other diseases. Active pharmaceutical ingredients including protocatechualdehyde, ferulic acid and caffeic acid were extracted from Dan Shen using water at 75 to 150 C. Our results show that the extraction efficiency for all three APIs is improved with increasing temperature. To eliminate the use of organic solvents in high-performance liquid chromatography (HPLC), subcritical water chromatography (SBWC) methods were developed for aspirin and metformin. SBWC separation of metformin HCL and aspirin were achieved at 95 C and 125 C, respectively. The recovery for both pharmaceutical active ingredients obtained by SBWC is 99% in comparing with the stated content of each drug. The relative standard deviation is less than 1% for SBWC assays developed in this work. This level of accuracy and precision achieved by SBWC is the same as that resulted by the traditional HPLC analysis also carried out in this work. For subcritical water chromatography separations, thermally stable stationary phases are critical. Since zeolite is superior to the traditional silica in thermal stability, a method was developed to synthesize organic-bonded zeolite through the substitution of silanol groups with C18. In FTIR-ATR experiment stretching band of CH2 and CH3 clearly revealed the presence of carbon chain after the modification. HPLC column was packed with C18 modified zeolite for the evaluation of chromatographic separation. Caffeine peak was detected at lower concentration at 272 nm wavelength. It also showed some separation between vanillin and coumarin at 284 nm. However, the HPLC column was unable to perform separation above 0.4 mL/min due to the small pore size, and it is one of the limitations of zeolite modified stationary phase. Lastly, the destruction of PCB-118, PCB-156 and PCB-180 congeners under subcritical conditions has been investigated. The percent degradation of PCBs was mostly improved by increasing the heating time, concentration of hydrogen peroxide and sodium hydroxide in water. All three PCB congeners investigated were completely destroyed after heating in 0.2% sodium hydroxide at 350 °C for an hour.
Doctor, Ninad. (August 2019). Development of Green Subcritical Water Separation Technology (Doctoral Dissertation, East Carolina University). Retrieved from the Scholarship. (http://hdl.handle.net/10342/7605.)
Doctor, Ninad. Development of Green Subcritical Water Separation Technology. Doctoral Dissertation. East Carolina University, August 2019. The Scholarship. http://hdl.handle.net/10342/7605. November 27, 2021.
Doctor, Ninad, “Development of Green Subcritical Water Separation Technology” (Doctoral Dissertation., East Carolina University, August 2019).
Doctor, Ninad. Development of Green Subcritical Water Separation Technology [Doctoral Dissertation]. Greenville, NC: East Carolina University; August 2019.
East Carolina University