Plants interact with and influence water quality through intricate biological and chemical processes. These capacities show how plants contribute to cleaner water, both in natural settings and in human-engineered systems. This article explores the science behind plants’ involvement in water purification, highlighting their capabilities and limitations.
How Plants Clean Water
Plants clean water through biological processes known as phytoremediation. Rhizofiltration involves plant roots absorbing, concentrating, and precipitating toxic metals and excess nutrients from contaminated water. Roots offer a large surface area, taking up heavy metals like lead, cadmium, copper, and zinc. Plants can also accumulate low-level radioactive contaminants.
Phytoextraction involves plants taking up pollutants from water and accumulating them in harvestable tissues, such as shoots and leaves. Harvested plants are then disposed of, removing pollutants from the water system. Certain species, called hyperaccumulators, concentrate metals at levels exceeding those in the surrounding water without experiencing toxicity.
Plants also engage in phytodegradation, breaking down organic contaminants into less harmful substances within plant tissues through metabolic pathways. Phytovolatilization allows plants to absorb pollutants and release them into the atmosphere as a modified gaseous form through transpiration. This process is effective for volatile organic compounds and some inorganic contaminants like mercury and selenium.
The rhizosphere, the zone of soil or water influenced by plant roots and their associated microorganisms. Plant roots release exudates that foster diverse bacteria and fungi. These microorganisms play a substantial role in breaking down complex pollutants into simpler, less toxic compounds, enhancing water cleaning capacity.
Natural and Applied Plant Filtration
Natural wetlands purify water. These ecosystems act as large-scale natural filters, removing sediments, excess nutrients like nitrogen and phosphorus, and various pollutants. Dense wetland vegetation slows water flow, allowing suspended particles to settle, while plant roots and microbes absorb and transform contaminants.
Constructed wetlands are human-engineered systems designed to treat wastewater, building upon natural processes. They utilize plants, soil, and microbes to purify municipal, industrial, and agricultural runoff. These wetlands offer a cost-effective and environmentally sound approach to water treatment, mimicking natural wetlands in a controlled environment.
Rain gardens and bioswales are practical applications for managing urban stormwater runoff. Rain gardens are shallow depressions that collect and filter rainwater. Bioswales are vegetated channels designed to convey and filter stormwater. These systems reduce stormwater pollution and improve water quality by allowing pollutants to settle and be absorbed by plants and soil microbes. Plants in these systems increase the uptake of nutrients like nitrogen and phosphorus, reducing their entry into waterways.
Small-scale systems leverage plants for water quality improvement. In aquaponics, plants absorb nutrients from fish waste, cleaning the water for aquatic organisms. This symbiotic relationship reduces the need for frequent water changes and maintains a healthier environment.
What Plants Cannot Filter
Plants have limitations in water purification. Filtration processes are often slow, requiring extended periods for contaminant removal compared to conventional methods. Phytoremediation effectiveness depends on pollutant concentration; high levels can overwhelm or be toxic to plants.
Plants are not a universal solution for all water contaminants. They do not remove viruses or many bacteria (pathogens). Certain toxic chemicals or persistent organic pollutants may not be degraded or absorbed. While plants filter suspended solids and some dissolved substances, very fine sub-micron particles might pass through.
For phytoextraction, disposing of harvested contaminated plant biomass presents a challenge. This biomass, enriched with pollutants, requires careful handling to prevent re-release into the environment. Specialized disposal methods are necessary to ensure extracted pollutants are safely contained.