The research content of environmental pollution chemistry includes the source, diffusion, distribution, circulation, form, reaction and destination of pollutants in the environment. The purpose of this study is to provide a basis for environmental quality evaluation, analysis, monitoring and control.
Environmental pollution chemistry is a new discipline, and its scope has not been clearly defined. Generally, it can be divided into air pollution chemistry, water pollution chemistry, soil pollution chemistry, ecological pollution chemistry and other parts, which study pollution chemistry problems in different fields such as atmosphere, water body, soil and ecosystem respectively.
Environmental substances have always been an important research object of natural science. After the problem of environmental pollution appeared, people began to study pollutants.
At first, most of the research work focused on investigating the sources and emissions of pollutants, focusing on exploring treatment and control technologies. Since the 1960s, people have gradually discovered the impact of the environment on pollutants, the impact of pollutants on the ecosystem, the generation of secondary pollutants, the migration and transformation of pollutants, etc. Will have an overall impact on environmental protection. These problems urge the study of environmental pollution to face the natural environment, so as to grasp the law of migration and transformation of pollutants in the environment more deeply and promote the formation and development of environmental pollution chemistry.
The main research objects of environmental pollution chemistry are pollutants discharged to the environment by human beings in production and consumption activities, such as sulfur oxides, nitrogen oxides, smoke and dust, volatile hydrocarbons, oxygen-consuming organic compounds, nitrogen and phosphorus nutrients, heavy metals, pesticides, polycyclic aromatic hydrocarbons, halogenated hydrocarbons, polychlorinated biphenyls, radioactive substances and so on.
There are many pollution-free natural substances in the natural environment, such as inorganic salts, metal oxides, clay minerals, humus and so on. , as well as various physical factors (such as light and radiation), meteorology, hydrology, geology and geographical conditions. And there may be polluted natural substances, which is the environmental background of pollutants. This environmental background either directly interacts with pollutants or indirectly affects pollutants.
Therefore, the research object of pollution chemistry should actually be a comprehensive system composed of pollutants and their environmental background. The natural environment is an open system, and there are many factors that affect the energy flow and material flow, and they often change, so the research object of pollution chemistry is very complicated.
Pollutants will migrate and transform in the environment. Migration includes source, diffusion, distribution and circulation, while transformation includes form, reaction and destination. On the surface, migration seems to be only a physical movement that changes the spatial position, but in fact it is intertwined with the transformation of pollutants, interdependent and mutually promoting, and contains complex chemical content. At the same time, the role of organisms in the migration and transformation of pollutants is also closely related to the chemical reaction process.
For example, sulfur dioxide, an atmospheric pollutant, can be oxidized into sulfur trioxide when it diffuses and migrates in the atmosphere, and sulfate particles will be formed when it meets ammonia or metal oxides. With the precipitation falling to the ground, it is washed into the water body by runoff and becomes sediment. Sulfate, as a hydrogen acceptor, can be reduced to hydrogen sulfide by sulfate-reducing bacteria under the condition of underwater hypoxia, and then enter the atmosphere.
Although this is only one of the circulating ways of sulfur in the environment, each step often involves physical, chemical or biochemical reactions. The oxidation of sulfur dioxide in the atmosphere involves complex photochemical reactions, forming various excited States and carrying out free radical reactions, and there are multiphase interfacial adsorption and catalytic processes.
The circulation of pollutants in the environment is often summarized as the circulation of various elements. Such as carbon, oxygen, nitrogen, sulfur, phosphorus and various metals, all form a cycle system with changeable forms and complicated chemical reaction process, which is usually called biogeochemical cycle.
In the study of pollution chemistry, a considerable part of work focuses on the form and distribution of pollutants. The existing forms of pollutants include valence state, bound state, structural state, bound state and so on. Different forms of pollutants have different chemical behaviors in the environment and show different pollution effects.
For example, hexavalent chromium has strong toxicity and trivalent chromium has weak toxicity; The toxicity of organic mercury, such as methyl mercury, far exceeds that of inorganic mercury; There are seven isomers of 666, among which γ-type has the strongest insecticidal activity; The carcinogenic activity of polycyclic aromatic hydrocarbons is related to its chemical structure. The binding state of trace pollutants with different carriers often determines their migration in the environment and so on.
The distribution of pollutants not only refers to the concentration distribution in environmental space, but also refers to the distribution of pollutants between different forms and different phases. Because only according to the total amount of pollutants, it is impossible to accurately grasp the nature of environmental pollution.
Taking mercury as an example, mercury pollutants in the atmosphere mainly come from the combustion of mercury-containing fuels, the smelting of mercury-containing minerals and the emission from factories, as well as the volatilization of mercury from soil or water. They exist in the form of metallic mercury and mercury chloride vapor, monomethyl mercury, dimethyl mercury and particulate mercury.
Water-soluble mercury in water not only has different valence states, but also can form complexes with a variety of inorganic and organic ligands, and under certain conditions, it will generate mercury sulfide and other precipitates. Mercury can also combine with clay minerals, humus, metal hydrated oxides and so on to form granular mercury. Inorganic mercury can be converted into methylmercury under the action of microorganisms or physical chemistry. Aquatic organisms can also accumulate mercury in the body, and various biopolymers often combine with mercury by sulfhydryl groups. The total amount of mercury in the environment is distributed in various forms according to a certain proportion. Explaining the distribution of mercury in the environment often involves dozens or even dozens of different forms.
In order to master the transformation mechanism of pollutants in the environment, it is necessary to understand the chemical reaction process. The influencing factors in the natural environment are complex and changeable, so it is difficult to reveal the essence and panorama of the reaction only by general chemical laws. In the development of pollution chemistry, many new topics have been put forward and explored one after another. For example, photochemical reaction in the atmosphere, formation of secondary pollutants and acid rain, chemical balance and imbalance of solutions in water, interfacial chemical reaction between soil and sediments, degradation and biological oxidation of organic pollutants in the environment, enzymatic chemical reaction of toxic pollutants in organisms, etc. Pollutants migrate in homogeneous or heterogeneous environmental systems and undergo various physical and chemical processes, such as diffusion, evaporation, condensation, adsorption, ion exchange, coagulation, flocculation, deposition and biological concentration. They all play an important role in the spatial position or phase change of pollutants.
In an open environmental system, most pollutants and background substances are not in equilibrium, but at most in steady state, so it is impossible to accurately describe their reaction process only by chemical thermodynamics.
Chemical kinetics is an important basis of pollution chemistry. Chemical kinetics or catalysis are involved in the transformation of nitrogen oxides in the atmosphere into nitrate aerosols, chemical oxidation and photooxidation of pesticides and organic chemicals, methylation of heavy metals in reduced water, destabilization and flocculation of aerosols and hydrosols.
The reaction of environmental pollutants is often carried out in flowing media such as air and water, which is inevitably affected by the fluid state. In recent years, a new discipline-environmental chemical kinetics has emerged, which specializes in the migration and transformation of pollutants in environmental fluids, indicating that pollution chemistry is becoming more and more theoretical and modeled.
There are three main research methods of pollution chemistry: direct measurement, theoretical calculation and simulation experiment. Each method can not fully reflect the real situation of the environmental system, so it is always complementary and comprehensive. The development trend of pollution chemistry can be summarized as trace, micro and particle in in-depth analysis and model, model and simulation in comprehensive inference.
In the early stage of environmental pollution research, the main concern was the pollutants with high content. With the deepening understanding of the impact of pollution, people's attention has gradually turned to micro and trace pollutants, such as chemical carcinogens, heavy metals, pesticides, eutrophic substances and so on.
Microscopic study of pollutants is to identify, analyze and observe pollutants at atomic and molecular levels, and explore their morphological structure, reaction mechanism, transformation process and intermediate products. The study of pollution effect is to quantitatively judge the toxicity or carcinogenicity of pollutants from the molecular structure at the molecular biological level. In microscopic research, infrared spectroscopy, X-ray diffraction, gas chromatography-mass spectrometry, electron microscopy, electron spectroscopy, laser detection and other means are widely used.
The research shows that micro and trace pollutants mostly combine with fine particles (particles) in the environment, migrate with particles as carriers and transform on the surface of particles. Environmental particles in the atmosphere, such as floating dust and metal powder, and environmental particles in water, such as clay minerals, metal hydrated oxides, humus, algae and bacteria, constitute various dispersion systems. Studying the interaction between these particles and trace pollutants has become an important aspect of pollution chemistry.
In recent years, the macroscopic comprehensive study of environmental pollution began with the establishment of a certain model, which concisely expressed the chemical mechanism of internal action with physical images or block diagrams. The model reflects the qualitative relationship and can be used to judge the direction and trend of environmental pollution. If further expressed by mathematical quantitative relations, it will become a model.
In addition to the general parameters such as pollutant concentration and environmental conditions, the pollution chemical model is often determined by experiments such as distribution coefficient, equilibrium constant, electrochemical energy, free energy of formation and kinetic constant. The research of atmospheric chemical model and water quality chemical model has made great progress and become an important direction of pollution chemistry research.
In order to confirm the proposed model and mode, in addition to direct observation, a large number of simulation research methods are adopted, mainly laboratory simulation and computer simulation, and sometimes field simulation experiments are carried out. At present, a variety of equipment, instruments and sensing elements have been developed for simulation experiments, using isotope tracing, fluorescence display, laser testing and other technologies.
In addition, complete sets of facilities such as photochemical smog box, environmental wind tunnel, water quality simulation system and comprehensive micro-cosmic ecosystem simulation experimental field have all come out. Computer simulation also puts forward a fixed simulation program for comprehensively solving various physical and chemical reactions by asymptotic method. The simulation research of environmental pollution system has become a very active field.