Environmental Cycling and Pollution
Introduction
The cycling of substances by the environment is driven through energy fluxes under the Earth and at its surface. The radioactive decay of elements within the mantle and core drives tectonic processes which cause crust formation, hydrothermal processes and volcanic activity in aqueous solutions deep under the crust. Absorption of solar energy drives the physical circulation of ocean and winds currents. It also fuels the physicochemical hydrological cycle, that entails the evaporation of water from lakes and oceans, and consequent rainfall providing rivers that flow into the sea. Solar energy has additionally some direct chemical consequences, during photosynthesis through green plants, and atmospheric photochemistry, that relies on reactive species produced through absorption of UV radiation. Human activity devotes to these cycles by the burning of fossil fuels and the extraction and make use of elements within technology.
The presence of liquid water and the existence of life are two characteristics that make the chemistry of the Earth's surface exclusively complex between the known planets. Biological processes cycle a few elements (particularly C, N, O and S) via different oxidation states, and photosynthesis has provided us both buried fossil fuels and a strongly oxidizing atmosphere. Hydrological cycling entrains several other substances, via the chemical breakdown of rocks and through evaporation from the oceans.
Elements react to these driving forces in ways which rely on their chemical characteristics. Volatile molecules created through nonmetallic elements enter the atmosphere from volcanic emissions, like 'waste products' of life and industry and from human energy use. A few volatile compounds are quickly oxidized through photochemical processes, and a few are rapidly washed out through dissolving in rainfall. Elements (particularly metallic ones) that do not create volatile compounds within normal circumstances are confined to the solid and liquid parts of the environment. Soluble ions (example Na+, Cl-) are separated from rocks in weathering processes and end up in sea water. Another element (example Al, Ti) that creates extremely insoluble oxides or silicates are through comparison highly immobile.
A few pollutants from human activity are natural substances (example CO2) created in excessive amounts which unbalance the natural cycles. Other than these are synthetic (example organochlorine compounds) and are harmful either since they are toxic to life, or since they interfere with natural chemical processes (example in the ozone layer).