Tea saponins: effective natural surfactants beneficial for soil remediation, from preparation to application

Tea saponins, found in Camellia plants, are natural non-ionic surfactants that offer obvious beneficial effects in soil remediation. Most tea saponins are extracted from the Camellia oleifera seed meal, with the leaves and flowers of Camellia sinensis as potential sources. Water extraction and ultrasound-assisted water extraction combined with acetone precipitation are recommended for the industrial extraction and purification of tea saponins, considering multiple factors. The detailed physical, chemical and biochemical properties of tea saponins need to be clarified, especially whether tea saponins with slightly different structures from distinct sources have different soil remediation properties. Applied in leaching remediation, phytoremediation and microbial remediation, tea saponins desorb heavy metals from contaminated soil as well as enhancing their bioavailability. Tea saponins improve the accumulation of pollutants by hyperaccumulators as well as the degradation of organic pollutants by microorganisms. Currently the mechanisms of tea saponins are not clear, although they are proven to be effective natural surfactants for the remediation of contaminated soils. This review enriches our understanding of tea saponins from various aspects and encourages further studies of industrial extraction and purification, and the field remediation mechanisms of tea saponins, making better use of Camellia plants and contributing to environmental protection.

Soil, one of humanity’s most valuable resources, is severely polluted by heavy metals and organic pollutants all over the world. In China, for instance, the “Bulletin on the Survey of Soil Pollution in China” issued in 2014 (ref. 1) shows that the total soil over-standard rate in China is 16.1%, of which inorganic pollutants account for 82.8%, followed by organic pollutants. Cadmium (Cd), nickel (Ni), copper (Cu), arsenic (As), mercury (Hg) and lead (Pb) are major inorganic pollutants, and benzene hexachloride (BHC), dichlorodiphenyltrichloroethane (DDT) and polycyclic aromatic hydrocarbons (PAHs) are major organic pollutants. For arable land, the over-standard rate is 19.4% and the main pollutants are Cd, Ni, Cu, As, Hg, Pb, DDT and PAHs. Given the current severe state of soil pollution, it is not only of great urgency but also of significant importance to amend contaminated soil both for environmental protection and agricultural production, which is worth researching.

According to the principle of remediation, soil remediation is simply classified as physical remediation, chemical remediation or bioremediation. Soil remediation technologies have been developed over the past decades. However, complex methods, the long time required for remediation and the expensive investment, along with low remediation efficiencies, still hinder the large-scale application of soil remediation techniques, such as excavation/disposal, soil washing, soil vapour extraction (SVE), stabilization/immobilization, chemical reduction/oxidation, and electrokinetic and thermal desorption.2–4 With its advantages of environmental friendliness, low cost, low energy consumption and no secondary pollution,5–9 bioremediation is an effective method for soil remediation, although its efficiency is affected by the bioavailability of pollutants. This is mostly low due to the low mobility of the forms of pollutants existing in soils, thus reducing the efficiency of bioremediation for contaminated soils.

The use of surfactants like sodium dodecyl sulfate (SDS)10 or chelating agents,11–13 such as natural low molecular weight organic acids (NLMWOAs), ethylene diamine tetraacetic acid (EDTA), ethylene diamine disuccinate (EDDS) and humic substances (HS), can increase the bioavailability of pollutants in contaminated soil. EDTA may form a chemically and microbially stable complex with heavy metals in soil, in turn impacting soil quality and causing groundwater contamination;14 at the same time, there is a lack of detailed research on the persistence of EDTA–metal complexes in soils.15 As a result, surfactants, especially natural surfactants, may be a more suitable choice for increasing the bioavailability of soil contaminants.

Tea saponins, found in the roots, stems, leaves, flowers and seeds of Camellia plants,16–23 are natural non-ionic surfactants. It has been verified24–29 that tea saponins contribute to improving the bioavailability of hyperaccumulators and microorganisms on Cd, Pb, PAHs and polychlorinated biphenyls (PCBs) in contaminated soils. The helpful influences of tea saponins in the leaching remediation of soils have also been investigated and confirmed.30–33

Although the mechanism of tea saponins improving the bioavailability of pollutants in contaminated soils is not yet fully understood, and the application of tea saponins has been investigated with only a few heavy metals and organic pollutants, the results of existing studies suggest that tea saponins deserve in-depth and extensive researches, not just on their sources, structures and properties, but also on their preparation, effects and mechanisms. This will be greatly beneficial to fully utilize the resources of Camellia plants and increase the bioremediation efficiency of contaminated soils.

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