Phytoremediation of waters contaminated with medicines and personal care products. Methods for phytotechnological effective removal of known drugs such as salicylic acid, sulfadimethoxin, atenolol, caffeine, tetracycline, etc. from water.
The use of phytotechnology is increasingly used in various fields of human activity, including the protection and ecological reconstruction of water bodies. For example, in the USA, artificial wetlands are considered to be an integral part of the system of sustainable development and renewable nature management. In the U.S. Environmental Protection Agency, this topic is a long-term priority and is being developed by the National Risk Management Research Laboratory. Research and development in this area are also a priority in EU member states, especially for the sustainable treatment and purification of industrial, agricultural and municipal wastewater. In total, research in this field is carried out in over 80 countries, mainly in North America, Europe, South-East Asia and Oceania. Since 1991, about 4,000 papers have been published worldwide and the number is growing exponentially, doubling approximately every 2-4 years. The aim of this paper is to analyze and summarize current trends in the use of phytotechnology for water treatment and protection.
Phytotechnology is a fast-growing industry in many countries of the world.
In the last decade, the amount of research carried out in China has increased dramatically, as well as the number of bioengineering facilities created using phytotechnology. According to statistics, 425 such facilities were in operation in China in 2011, mainly for the treatment of agricultural irrigation runoff, artificial flooding of water bodies and reforestation. And every year 15-30 new biorefinery installations are built and put into operation.
The first bioengineering structures using higher aquatic vegetation (HABs) were developed in the 1960s, but these technologies have gained mass development in recent decades, both in developed and developing countries. In addition to differences in the species composition of the macrophytes used, the fundamental difference between artificial wetlands is the presence or absence of a free water surface. Structures without a free water surface can be divided into those with horizontal and vertical flow. In the latter, contaminated water can flow from top to bottom, from bottom to top and in a U-shape. There are a number of combined hybrid systems where different schemes are used in individual sections. This is due to the need for optimum conditions for aeration of the water and substrate, as well as comfortable conditions for the existence of BBB and associated micro-organisms, which ultimately should give maximum efficiency in water treatment. Facilities with subsurface water flow are generally more efficient than those with a free water surface.
To varying degrees, phytotechnology can treat hundreds of different pollutants and their combinations, which is particularly relevant for municipal, municipal, industrial and agricultural wastewater. The main advantages of phytotechnology for water treatment are the versatility of use for almost any pollutant, the natural nature of water treatment, the absence of any side effects, the long life and low operating costs of structures and installations, low resource and energy costs, small and simple infrastructure. Construction is carried out using local materials, does not require highly skilled personnel or special machinery and equipment. In the last decade, the use of WWTPs for water protection and purification has significantly expanded. In addition to traditional treatment and interception of biogenic and organic substances, heavy metals and radionuclides, rapidly developing technologies of water treatment for medical and personal hygiene products, as well as technologies based on improvement and management of water treatment by natural macrophyte associations, plant selection and genetic modification to obtain the desired characteristics, use of nanoparticles, biofumigation, combining water treatment with biofuel production, risk assessment of the risks of water pollution and its impact on the environment have appeared.
Facilities and plants for the recovery of nitrogen compounds and organic matter from wastewater using phytotechnology are developing rapidly worldwide, primarily due to low operating costs. The classical way of transforming and extracting nitrogen compounds includes biological processes (ammonification, nitrification, denitrification, biomass assimilation, etc.) and physical-chemical processes (adsorption). The new approaches are based on microbiological metabolism. The fundamental difference is that these processes do not require the presence of organic carbon as an electron donor.
Organic compounds in phytotechnology systems decompose under aerobic and anaerobic conditions. In the aerobic zone, oxygen is supplied from the atmosphere by convection-diffusion processes and through the roots of macrophytes. Anaerobic conditions are created in the closed pores of the fill soils.
The level of nutrient and organic matter extraction from the wastewater is highly dependent on the conditions created: pH, temperature, availability of available oxygen, presence of organic carbon, load on the structure, conditions and regime of pollutant input, time of water in the system, hydrological regime, removal of vegetation mass. The usual substrates are sand and gravel, but in some cases wood shavings, rice hulls, zeolite, mica, ash, coal slag, peat, dead seston, compost are used to improve the treatment conditions.
The second large group of pollutants, in the detoxification and removal of which phytotechnology is used, are heavy metals, metalloids (sometimes referred to as heavy metals) and radionuclides. Lead, cadmium, arsenic and mercury are the most dangerous toxicants, primarily because they are not biodegradable. Plants involved in the decontamination of heavy metals and radionuclides must meet certain requirements: 1) grow rapidly; 2) have a high tolerance to metals; 3) be resistant to diseases and pesticides; 4) have a well-developed root system and shoots and the ability to synthesise specific substances as a response to a toxic external environment; 5) be unattractive to animals to avoid the transfer of contaminants to higher trophic levels; 6) not be specific to certain elements so that there is the possibility to extract and detoxify other metals.
The main types of phytoremediation of heavy metals and radionuclides are:
Phytostabilisation/phytosequestration - conversion of chemical compounds to a less mobile and active form;
phytoaccumulation/phytoextraction - accumulation of hazardous contaminants in plants;
phytovolatilisation - evaporation of water and volatile chemical elements by plant leaves;
Rhizofiltration - roots absorb water and chemical elements necessary for plant life.
In addition, organic and inorganic pollutants containing heavy metals can be removed from water and bottom sediments through various biosorption mechanisms: adsorption, absorption, precipitation, surface complexation or ion exchange. Both living plants and dead phytomass are active biosorbents, as the main agents in this case are bacteria.
In recent years, phytotechnologies for the decontamination of water contaminated with arsenic, which is one of the most dangerous carcinogens, have advanced significantly. The increased interest in finding low-cost, environmentally friendly and sustainable methods of treating water from arsenic compounds on a large and small scale is due to the vast areas contaminated with this metalloid. For example, more than half of the groundwater in India and Bangladesh is contaminated with arsenic above the MAC and in some areas its concentrations are as high as 200 MAC for drinking water. The situation is similar in Latin America, Portugal and California (USA).
Good concentrators and hyperconcentrators of arsenic are the peppermint (Polygonum hydropiper), polyrhiza (Spirodela polyrhiza), Lemnagibba, Wolffia globosa (Wolffiagiobosa), Azolla caroliniana (Azolla caroliniana). The latter species belongs to the Salviniaceae family. In this regard, another species worth considering is Salcinia natans, which is the only species in this family native to Russia.
Salvinia floatifolia is a floating fern and is found in abundance in the southern regions of European Russia, Western Siberia and the Far East. This species is also a hyper-concentrator of heavy metals. According to our data obtained in the Lower Volga, Salvinia floating is able to concentrate the amount of copper almost 100 times more than common reed (Phragmites communis) and narrow-leaved cattail (Typha angustifolia).
Somewhat lower values for zinc, with salvinia concentrating this element almost 10 and 2 times more than reed and cattail respectively. It should be noted that salvinia is a freshwater plant, but the average content of zinc, nickel, cobalt, copper and chromium in this species is close to their content in marine plants. Given that salvinia is a free-floating species and can be easily removed from the water surface, its application in phytotechnology holds great promise for both artificial installations and the treatment of natural water bodies.
As has already been mentioned, a new trend has emerged in recent years - the purification and disinfection of pharmaceuticals and personal care products using phytotechnology. The presence of medicines in water has long been noted, but only in the last 15 years, when new analytical methods became available that allow the determination of extremely low concentrations (from nano to micrograms per litre), have they started to be considered as pollutants. New analytical methods make it possible to quantitatively analyse the presence of about 3000 bioactive chemical compounds in the environment in order to investigate their sources, behaviour, transformation as well as control, as the presence of pharmaceuticals in water, even in low concentrations, can have long-lasting side effects and pose risks to the ecological well-being of aquatic ecosystems and human health.
The main mechanisms of treatment of water polluted with pharmaceuticals in bioengineered systems with WWTPs are photolytic degradation, sorption, plant uptake and phytodegradation, microbial degradation. In principle, the design of phytotechnology-based extraction and decontamination facilities for pharmaceuticals differs little from that for treatment of other pollutants. They all have a substrate, a WWTP and a bacterial population.
There are units with a free water surface and sub-surface water movement in the horizontal or vertical directions, as well as hybrid units. Rooting and free-floating plants are used. Production (non-experimental) vetlands for these purposes are usually larger than 100 m2 and have water depths up to 30 cm. Depending on the design of the wetland, the water residence time in the system varies from 1 to 15 days. Such systems are already in operation in the USA, Canada, Denmark, Italy, Portugal, Spain, China and Singapore and make it possible to treat and purify wastewater from 115 pharmaceuticals.
The most commonly used substrates in these plants are gravel, less commonly lightweight expanded clay, expanded perlite and clean sand. Sandy, sandy loam or sandy loam based soil is used for the extraction of antibiotics. Sometimes stones and volcanic tuff are used as substrates.
More recently, biosorbents such as rice hulls, pine bark and granulated cork oak bark have been used. Common plants in such wetlands are Typha angustifolia and Typha latifolia, Phragmites communis and Scirpus validus. The latter are often grown using hydroponics methods.
The method is based on the fact that the roots of the plants do not get nutrients from the soil, but from water. It has been observed that reeds are more efficient than cattails. The density of the plants is usually 10-50 pcs/m2. Submerged and free-floating macrophytes such as Elodea canadensis, Ceratophyllum demersum, Salvinia molesta, Leinna minor are used less frequently.
According to the average efficiency of extraction from water by phytotechnology, medicinal products can be divided into easily-removable, moderately-removable, poorly-removable and almost non-removable. Drugs that are easy to remove (average removal efficiency over 70%) are acetaminophen, salicylic acid, sulfadiazine, sulfadimethoxin, sulfamethazine, sulfamethoxazole, sulfapyridine, trimethoprim, atenololol, metoprolol, furose-mide, caffeine and tetracycline. Drugs with an average removal efficiency of 50-70% include ibuprofen, naproxen, doxycycline and gemfibrozil. Weakly removable by phytotechnology (average removal efficiency 20-50%) include diclofenac, ketoprofen, amoxicillin, clarithromycin, triclosan, sotalol, 2-(2-methyl-4-chlorophenoxy) propionic acid and carbamazepine.
Finally, there are a number of drugs whose removal by phytotechnology is problematic (removal efficiency less than 20%). These are primarily ampicillin, erythromycin and lincomycin.
Among promising new methods for improving phytotechnology, the use of genetic engineering and the combination of water purification and biofuel production technologies should be mentioned. In this context, genetic engineering techniques are intended to help produce genetically modified microbes and BWPs with characteristics that allow micro-organisms and macrophytes themselves to be resistant to pollutants and to accumulate, absorb and disinfect wastewater more actively and sustainably. As far as the microbial population is concerned, the microorganisms that inhabit the substrate as a whole and, separately, the rhizosphere microflora - the aggregate of microorganisms contained in large numbers in a narrow zone around the roots - are of fundamental importance. So far, we are not aware of the existence of transgenic WWFs, although for land plants the first results have already been obtained on Thal's Rhizosphere (Arabidopsis thaliana) and common tobacco (Nicotiana tabacum).
Controlling overgrowth, recycling and disposal of contaminated plant matter are some of the main problems in using phytotechnology for water treatment, especially for fast-growing plants with considerable phytomass. An example of such a plant is water hyacinth (Eichhornia crassipes), a perennial free-floating plant that is a good concentrator and de-toxicator of many pollutants, but due to its rapid spread and invasion of new areas as an invasive species can create several environmental problems. Eichornia can be used to extract and decontaminate phosphates, sulphates, phenols, synthetic surfactants, heavy metals, radionuclides, formaldehyde, dimethylamine and even rocket fuel (dimethylhydrazine). However, it should be recalled that it is one of the ten worst weeds in the world. This particular invasive species, which has its origins in the Amazon basin, has now invaded the waterways of Europe, Africa, Asia and North America. In some countries, aquatic hyacinth has spread to such an extent that it has filled all bodies of water and become a national disaster. In Pova Guinea, for example, the proliferation of vegetation introduced into the Sepik River basin has caused lakes and streams to become overgrown, leading to the death of fish, starvation of the local population and forced migration. To combat the overgrowth of water bodies by water hyacinth, Japan has set up a facility to process the biomass of the plant into biofuel.
Thus, it is increasingly clear that phytotechnology holds considerable promise, exploiting the natural nature of water purification and having significant economic advantages. In this connection, more attention should be paid to the practical implementation of the results of scientific research on the use of phytotechnologies for water treatment, both in Russia and abroad, when developing energy- and resource-saving programmes at the state and regional levels.