Chemistry of Aquatic Systems: Local and Global Perspectives

Hydrogeochemical cycles couple atmosphere, land and water; here the effect of these cycles on the distribution of chemical species and the regulation of the composition of natural waters is discussed. Above all, the regulation of the delicate proton balance is emphasized, which in turn regulates the solubility of minerals, the availability of nutrients and the speciation of metal ions. In assessing the anthropogenic effect on the proton balance, it is important to distinguish between the H⁺, the H⁺ ion reservoir as given by the base neutralizing capacity (acidity) and the acid neutralizing capacity (alkalinity). Rigorous conceptual definitions of these neutralizing capacities are given. The effects of biological processes (production and mineralization of biomass, biological mediation of redox processes such as nitrification, denitrification, SO₄ ²⁻ reduction), and of weathering reactions on alkalinity are discussed. Chemical weathering (which consumes protons and produces alkalinity) is one of the major processes controlling the hydrogeochemical cycles of elements. The chemical erosion rate in the upper Rhine catchment area is exemplified. It is shown how concepts in the chemistry of the mineral-water interface can be used to predict surface reactivity and to account for the variables (pH, ligands) which influence the dissolution of minerals.

This paper reviews how biota, especially vegetation, may influence the physical and chemical pathways of acquisition of solutes by drainage water. The major processes involved are i) effects of canopies on atmospheric deposition, ii) effects of transpiration, iii) effects through soil formation and mineral weathering, and iv) effects through transformation and uptake of nutrients. After giving specific examples of each of these groups of processes, a number of specific ecosystems will be discussed in terms of their overall H⁺ budget.

The carbon cycle at the earth surface is a complex combination of organic and inorganic processes. If one excepts the calcium carbonate precipitation, the physical and chemical phenomena responsible for CO₂ exchange between the atmosphere, the continents and the ocean are very fast and can be adequately described by thermodynamic equilibrium models. After a brief presentation of the pre-industrial carbon cycle at the global scale, and of its anthropogenic perturbations, equilibrium concepts are used to evaluate some of the major inorganic carbon fluxes, as well as their influence on the composition of rainwater, river water and surface or deep seawater. A short discussion on the interactions of organic and inorganic carbon in the sediments is also given.

Clouds influence the chemistry of the troposphere in many ways. Cloud convective transport enhances vertical mixing, which is of particular importance for short-lived trace species. Lightning in thunderstorm clouds is a significant source of nitrogen oxides; the latter play a catalytic role in photochemical ozone formation. Further, clouds affect the budget of photochemically active short-wave radiation, they cause precipitation which removes soluble species, and they constitute a medium for heterogeneous chemistry. Some heterogeneous processes also occur on aerosol particles. Model calculations suggest that multiphase chemistry reduces the atmospheric oxidation efficiency and the abundance of tropospheric ozone, the latter by about 20–30%. Clouds also play am important role in the atmospheric sulfur cycle. Although liquid clouds occupy only about 5-6% of the volume of the troposphere, up to 80–90% of the sulfur dioxide transformation into sulfuric acid in the atmosphere may occur in the aqueous phase.

A simplified scheme is proposed for the carbon cycle in coastal waters, showing the main pathways and the two most important reservoirs, POC and DOC. The different steps are briefly discussed, including the sources (river inputs and primary production), the degradation steps in the water column and in the sediment, and how we can assess the capacity of the system to recycle or accumulate carbon. Several examples are presented on the different forms of carbon in estuaries and coastal zones, evidencing the specific character of rivers in different parts of the world, and the consequences in terms of carbon inputs and behaviour. The utilisation of molecular tracers is then presented as a tool to identify the origin of organic matter in coastal waters. Different examples are discussed in which one or several tracers, such as pigments, fatty acids, sterols are used to enlighten the nature and evolution of organic matter in natural environments. As a conclusion, it is shown how such regional studies, taking into account or showing the peculiarities of the studied systems, can be used to improve our knowledge on a global scale.

Microbial trasformations and interactions with metals are reviewed in relation to the biogeochemical cycle. Different strategies have been developed by microorganisms to cope with essential and toxic metals. The molecular mechanisms of metal resistance and metal transport are well known from the results of recombinant DNA analysis, sequencing and identification of the protein components involved in regulatory and membrane functions. Other mechanisms of metal interactions have only been studied at physiological level. The bioaccumulation of metals is discussed especially biological mineralization, in which bacteria are the nucleation sites for authigenic mineral formation in the enviroment. The environmental importance of the opposite process, namely mineral ore oxidation due to the biological leaching of metals from the ore reserve pool is also discussed. The paper touches on biological mining (biohydrometallurgy), now applied to metal leaching of sewage and industrial wastes. Moreover, it is presented the controversial question of the abiotic versus biological origin of metal methylation in the environment. Metal volatilization by metal alkylation and biohydrization important processes of metal mobilization in the biogeochemical cycle are also introduced.

Metal ions are complexed in natural waters by a variety of inorganic and organic ligands (carbonate, chloride, humic and fulvic acids, specific chelators etc.) and by particle surface groups. The free aquo metal ion concentration, that is a key parameter for the effects of metals, is regulated by the complex interactions between trace metal ions, ligands and major ions. Thermodynamic calculations of the equilibrium speciation give insights into the effects of various parameters. Ligand-exchange techniques appear to be useful as experimental tools for the indirect determination of free aquo metal ion concentrations.

Light-induced processes are of utmost importance in the aquatic environment, e.g., in the biogeochemical cycling of carbon, sulfur, and iron and in the transformation of organic and inorganic pollutants. Due to the ubiquitous nature of surfaces and interfaces in the aquatic environment, many environmentally important photochemical processes occur as heterogeneous processes, involving adsorbed species. In this chapter, the focus is on heterogeneous photoredox reactions. Iron oxide minerals play an especially important role as redox mediators. In Section 2, some basic principles of photochemical reactions are reviewed. In Section 3, three aspects of photochemical iron cycling are discussed: 1) formation of bioavailable iron; 2) mediation of the photochemical oxidation of reduced carbon and sulfur, and 3) mediation of hydrogen peroxide formation. In Section 4, some examples of both direct and indirect photochemical transformation processes of organic pollutants are given.

Recent evidence indicates that trace metal nutrients (most importantly iron, and to a lesser extent zinc and manganese) can profoundly influence the productivity, species diversity, and ecological interactions of marine phytoplankton communities. In this chapter, several aspects of marine trace metal/phytoplankton interactions are examined, including (1) the chemistry of bioactive metals in seawater, (2) the interaction of these metals with phytoplankton at the molecular, cellular, community, and ecosystem levels of biological organization, and (3) the mechanisms by which phytoplankton communities regulate the distributions, cycling, and chemical speciation of trace metals. In controlling the chemistry and cycling of metal nutrients, important feedback mechanisms are established, which can have global scale impacts on the physics, chemistry, and biology of the Earth.

Ocean colour data, derived from optical remote sensing of the water surface from space, have revealed for the first time the heterogeneity, from local to regional and global scales, in the concentration and distribution of various pigments present in water constituents, in particular those of phytoplankton. The assessment of the concentration of such water constituents allows the computation of plankton biomass indices, as well as visualisations of surface currents and of some deeper dynamic phenomena, such as upwelling; the determination of primary productivity, i.e. the rate at which photosynthesis proceeds; and the estimation of the rate at which the oceans sequester atmospheric carbon dioxide.

The same assessments of the plankton field used to investigate carbon cycling can also shed some light on other fluxes of importance in the ocean-climate system, i.e. those of nitrogen and sulphur and their potential role in atmospheric processes.

The mobility, biodisponibility and toxicity of trace elements is largely controlled by chemical reactions which take place at the particle/water interface. Regardless of their molecular structure, particles in aquatic systems present reactive functional groups of two kinds: cavities at the surface of permanently charged minerals (phyllosilicates and phyllomanganates) and surface hydroxy groups. The formulation of the laws of mass action for surface complexes formed with these two types of sites is described. The validity of use of constant partition coefficient, K_d, i.e., of the linear approximation, is discussed. Spectroscopic information is reviewed on the local structure of “amorphous” colloids and around ions sorbed (i) within the diffuse ion swarm, as (ii) inner-sphere or (iii) outer-sphere complexes. The importance of other potential sorption mechanisms, such as heterogeneous nucleation and solid diffusion, is explored.

Surface chemical reactions are of critical importance to separation processes in water technology. Adsorption reactions accumulate dilute solutes at phase interfaces, and play an important role in modifying interfacial forces and particle stability in solid-liquid separation. Dominant contributions to free energies of adsorption are: 1) chemical coordination bonding, 2) electrical interactions including coulombic forces between charged species, and 3) solvation energies that are particularly important in hydrophobic adsorption. Isotherms provide mathematical models of adsorption reactions at equilibrium, relating the chemical activity of surface species to the activity of solution species. Examples of adsorption reactions in water technology include uptake of ions by synthetic chelating or ion-exchange resins, adsorption of organic compounds on activated carbon, complexation of solute ions by organic polyelectrolytes or mineral surfaces. Adsorption reactions play a central role in modifying the particle-particle interactions controlling colloid stability and the kinetics of particle agglomeration and deposition. Surface processes in water processing provide necessary tools for the development of sustainable technologies constrained by water resource quality.

This work provides an introduction to organic reductants encountered in aquatic environments, and their role in the redox transformation of metallic elements. Sources and sinks for organic reductants are reviewed, and reaction pathways, stoichiometries, and thermodynamics are outlined. In order to illustrate differences in the reactivity of important metal oxidant species, reactions of chromium(VI) and Mn(III,IV), Fe(III), and Co(III) (hydr)oxides with a number of important reductants are reviewed. The multi-component and complex structural nature of natural organic matter presents a formidable challenge towards characterization. Detailed examination of reductant reactivity is needed to fully evaluate metal speciation and metal transport behavior in the environment.

Information is presented for assessing the potential of an organic chemical to undergo abiotic transformation in aquatic ecosystems. When predicting the environmental fate of an organic chemical, two primary questions must be addressed. First, what are the reaction kinetics for the transformation process of interest? Second, what are the reaction products resulting from the transformation process? In attempting to address these questions, the types of functional groups that are susceptible to abiotic transformations (i.e., hydrolysis and redox reactions) and detailed reaction mechanisms for their transformation will be presented. These elementary reaction mechanisms are used as a framework for discussing factors that affect reaction rates and product distributions.

Contaminants in aquatic systems are subject to a variety of transformation and dispersal processes. This paper attempts to summarize the progress made and try to examine how hydrogeology and geochemistry can further contribute to the understanding of the combined effects of all these processes on contaminant transport. Several concepts of modelling solute transport are discussed, depending on the choice of the elementary volume representative of the medium. In a simplest representation, this can have the dimensions of the entire water body, leading to the box model concept. Subdivision into very small boxes leads to a set of interacting continua. Emphasis is given to deterministic approaches for saturated porous media. The topics examined include: the relative effect of chemical equilibria and kinetics on reactive transport, with reference to simulation examples; the dynamics of suspended particles in natural aquifers, together with implications for contaminant transport; the factors influencing solute diffusion in soils taking into account adsorptive processes.

After recalling diffusion and transport equations in porous media and introducing the scale effect on hydrodynamic parameters, we compare deterministic and stochastic approaches applied to the transport of contaminants in natural media. Some examples show the interests and limitations of both methods in which many experimental measurements are necessary to obtain parameters with enough confidence. Then, we point out the methods which are important to estimate physico-chemical or kinetic parameters at the laboratory scale. Comparisons between field, laboratory and modelling underline the difficulties in including chemistry in modelling on a larger scale: a model is first of all a tool to improve our knowledge, the field scale is a source of questions and laboratory experiments are essential to investigate mechanisms which help in the understanding of field tests.

Computer models of hydrological and biogeochemical behavior of catchments have been designed to simulate changes in water and soil properties due to various anthropogenic inputs. The models consist of a set of hydrological mass balance equations, chemical mass balance equations, equation of electrical neutrality of solution, expression for the acid neutralizing capacity of solution, equilibrium equations of mass-action law, adsorption isotherms, kinetic rate laws of heterogeneous reactions and Arrhenius and van’t Hoff equations of temperature dependency of rate coefficients and equilibrium constants. The time progress is solved numerically using Euler’s method. Partial equilibria in the catchment are solved by iterative computation of a set of mass distribution equations for dissolved chemical components and a set of mass-action law equations. The major irreversible sources and sinks of chemical elements within the catchment are geochemical and microbiological reactions of rock weathering and organic matter transformations.

Scenarios with different anthropogenic inputs result in possible future chemical trends in the environment.

Soils are an important constituent of terrestrial surfaces. Since some parts of the solar radiation penetrates even dense plant canopies soils intercept much of the solar energy incident on these surfaces, thus taking an important role in climatological and biologic land processes which are driven by the absorption and reflection of solar radiation. Soils also influence the reflectance of composite land surfaces, in particular in regions of sparse vegetation. As such, they form an important element in the remote observation of terrestrial ecosystems. The nutrients contained in soil substrates and their water buffering capacity constitute important resources for the growth of agricultural crops and natural vegetation communities. Conversely, the degradation of soils has an enormous impact upon the regeneration and further development of the permanent vegetation cover. Identification and mapping of soil condition is thus of importance for assessing the present and future status of these ecosystems. Remote sensing provides possibilities for such mapping since spectral characteristics of the soil surface convey relevant information about the composition of the substrate. This paper summarizes important relationships between spectral reflectance features and the soil composition (i.e. organic matter, iron oxides and clay minerals), and it illustrates methods to use the remote detection of soil surface spectral variations for the assessment of environmental changes in semi-arid regions, such as the Mediterranean basin.