Issue 40/2015 - Subcritical water extraction has great prospects for green environmental protection

The hydrogen bonding, ion hydration, ion association, cluster structure, etc.
of the fluid microstructure in the subcritical state have changed, giving subcritical water special application properties
in many aspects.
In today's increasingly concerned about environmental protection and energy saving, subcritical water extraction technology is receiving more and more attention due to its non-toxic, economical, mild reaction conditions and simple waste liquid treatment, which highlights many advantages and broad application prospects compared with commonly used extraction methods
.

First, the advantages of subcritical water extraction are prominent

Water is the most abundant substance
on the earth's surface.
When the pressure and temperature are close to the critical point (Tc=374 °C, Pc=22.
1MPa), the characteristics of liquid phase water and vapor phase water gradually approach until the critical point
is reached or exceeded.
In the temperature range from 100 °C to a critical temperature of 374 °C, water can be kept liquid by applying pressure, and such water is called subcritical water
.
The reason why subcritical water has attracted the attention of researchers is mainly due to its special physicochemical properties and the applications
derived from these properties.

Subcritical water extraction is one of
the promising application directions.
By changing the temperature and pressure, the polarity of water changes in a large range, so that water can have good solubility
for medium polar or even non-polar components in a wide range.
The subcritical state can not only reduce the dielectric constant of water, so that water can dissolve organic solutes, but also enhance the diffusion effect, improve the kinetic properties, and reduce the surface tension and viscosity
.
Subcritical water extraction is regarded as a green extraction technology
because it uses inexpensive, pollution-free water as an extractant.
Compared with common extraction methods, such as solvent method (SOX), steam distillation (SE), supercritical fluid extraction method (SFE), ultrasonic extraction (USE), accelerated solvent extraction (ASE), etc.
, in addition to environmental protection, it also has the advantages of short extraction time and good selectivity, through the control of extraction conditions can extract substances of different polarities, good reproducibility, extraction device and operation are simple, extraction conditions are not harsh, detection limit is low, high sensitivity and other advantages
.
See Table 1
for details.

The effect of subcritical water extraction is mainly affected by the following factors:
First, temperature
.
By adjusting the temperature, the water gradually changes from strong polarity to non-polarity, and the solute can be extracted from high to low polarity
.
In the subcritical water extraction process, the extraction rate generally increases with the increase of temperature, in order to maximize the extraction efficiency, the extraction should theoretically be carried out
at a higher temperature.
But this is not the case, too high a temperature will lead to the degradation of some components, which will reduce the extraction rate
.
Therefore, the extraction temperature should be selected
according to different raw materials and extracts.

The second is pressure
.
The role of extraction pressure in the subcritical water extraction process is relatively small, and due to the low compressibility of water in a certain temperature range, pressure has little
effect on the dielectric constant.
Therefore, only the appropriate pressure (generally in 1~6MPa) to keep the subcritical water in a pressure state
.

The third is the extraction time
.
Generally within
1h.
In addition to this, the extraction rate
can be improved by controlling the pH value.
In some cases, when the extraction capacity of subcritical water is insufficient, the extraction efficiency can be improved by adding organic solvents, and the extraction temperature can be reduced and the extraction time
can be shortened.

Project/methodology	SWE	SFE	ASE	SOX	USE	SE
Organic solvent dosage (ml)	not	not	15-45	Hundreds	Hundreds	not
Sample volume	1-30	1-30	1-30	10-30	30	1-30
Extraction time	5-15min	1h	10-15min	24h	5-10min	A few hours
Temperature(°C)	30-300	35-90	30-200	Higher boiling point than organic solvents	Above normal temperature	100
pressure	1-6MPa	7. 7-42. 5 MPa	1-2 MPa	Atmospheric	Atmospheric	Atmospheric
efficiency	high	high	Higher	low	Slightly higher	low
Sample morphology	Solid, semi-solid	Solid, semi-solid	Solid, semi-solid	Solid, semi-solid	Solid, semi-solid	Solid, semi-solid
Type of analyte	Medium polar and non-polar organics	Non-polar components with a molecular weight of less than 500	Non-volatile	Non-polar compounds with high boiling points	Non-polar compounds with high boiling points	Non-polar components below 100°C
Instrumentation	Simple	complex	Simpler static extraction	Simple	Simple	Simple
Online co-use	OK	OK	OK	OK	OK	OK
Influencing factors	temperature	Temperature, pressure, time, flow rate	Temperature, pressure, solvent volume	Time, temperature, solvent, pre-heating or pre-concentration	Time, solvent,	temperature

Project/methodology SWE SFE ASE SOX USE SE Organic solvent dosage (ml) not not 15-45 Hundreds Hundreds not Sample volume 1-30 1-30 1-30 10-30 30 1-30 Extraction time 5-15min 1h 10-15min 24h 5-10min A few hours Temperature(°C) 30-300 35-90 30-200 Higher boiling point than organic solvents Above normal temperature 100 pressure 1-6MPa 7.
7-42.
5 MPa 1-2 MPa Atmospheric Atmospheric Atmospheric efficiency high high Higher low Slightly higher low Sample morphology Solid, semi-solid Solid, semi-solid Solid, semi-solid Solid, semi-solid Solid, semi-solid Solid, semi-solid Type of analyte Medium polar and non-polar organics Non-polar components with a molecular weight of less than 500 Non-volatile Non-polar compounds with high boiling points Non-polar compounds with high boiling points Non-polar components below 100°C Instrumentation Simple complex Simpler static extraction Simple Simple Simple Online co-use OK OK OK OK OK OK Influencing factors temperature Temperature, pressure, time, flow rate Temperature, pressure, solvent volume Time, temperature, solvent, pre-heating or pre-concentration Time, solvent, temperature

 

Second, the application prospect is broad

A large number of experiments have shown that subcritical water extraction has great application potential
in extracting active ingredients from plants.
Plant phenols are an important part of natural products, including plant tannins, flavonoids, lignin and some simple phenols
.
At present, a large number of literature at home and abroad have reported the extraction, separation and utilization of
plant phenols.
Buderat et al.
used subcritical water to extract polyphenolic compounds in bitter melon, and the results showed that under the conditions of temperature 150~200 °C, pressure 10MPa, and flow rate of 2mL/min, the mass fraction of total polyphenols in the extract was 52.
63mg/g, which was much higher than that extracted by methanol6.
OOmg/g and 6.
68mg/g
extracted by boiling water reflux.

In terms of essential oil extraction, the application prospects of subcritical water extraction technology are also promising
.
Essential oils contain a variety of active ingredients and are widely used
in pharmaceutical, food, tobacco, cosmetics and other industries.
Traditional essential oil extraction methods, such as steam distillation and organic solvent extraction, will cause problems such as the destruction of essential oil components due to extraction solvent residue and excessive extraction time, which seriously affects the quality of
essential oils.
When relevant personnel studied the application potential of subcritical water extraction technology in plant essential oil extraction, by examining the subcritical water extraction technology in rosemary, cloves, oregano leaves, fennel, laurel, peppermint, calamus, sand kernels, dried peppercorns, onions and other plant extract essential oils, it was found that subcritical water extraction essential oils had high sulfide content, and saved time, energy, efficiency and high quality
.

In addition to the application of plant extracts, subcritical extraction technology also has a good application prospect
in food safety detection.
Food safety testing has the following characteristics: there are many types of samples, including grain, vegetables, fruits, meat, eggs, milk, aquatic products and their processed foods, and the matrix of these samples is different and complex; There are many types and quantities of ingredients to be detected, including organic synthetic pesticides, veterinary drug residues and pollutants, toxic and harmful trace elements, pathogenic bacteria, etc.
; Large sample volumes, wide sample distribution, low analyte content, and the need for fast, easy testing
.
At present, there have been reports
at home and abroad that subcritical water extraction technology has been applied to pesticide residues in grain, fruits, vegetables and meat products.
The results show that the subcritical water extraction method has the characteristics of
rapid, convenient, accurate and no organic solvent residue.

Since the application research of subcritical water extraction technology in the field of plant extraction is still in its infancy, most of them belong to laboratory research, so there are still problems
in engineering equipment, process flow and operation technology in industrial production.
However, as a green extraction technology, subcritical water extraction will receive more attention and application due to its significant advantages, creating more value
for society.