Explain the concept of toxicity and bioavailability
Bioavailability are metal species which can cross the cellular membrane. If metal species are bioavailable they may be toxic to the cell but if not bioavaiable they are not toxic
Bioavailability and toxicity are functions of chemical speciation
Important pathways by which metals enter water bodies (schematic)
- Direct runoff from source
- Atmosphere
- Indirect runoff: collected via networks (waste, WWTP)
Important processes controlling the fate of metals in aquatic systems (schematic)
- sedimentation/resuspension
- adsorption/desorption
- diffusion
- bioturbation
Metal speciation in waters
To secure the highest stability of their outer electrons, metal ions are bonded or coordinated to other species (water molecules or electron donor partners)
Metal ions in water seek to reach a state of maximum stability through chemical reactions, including acid base, redox and complexation reactions
Hydrated metal cation: metal ion bonded to water molecule then undergoes acid-base redox and complexation or chelation reactions
- acidic character of some hydrated metal ions may contribute to acidity
Definition of Nernst equation
The equilibrium of a redox reaction can be deduced from information about its constituent half-reactions and is related to the Nernst equation
The Nernst equation in this from is similar to that of pH and offers some advantage in calculating redox relationship
- Low pE is reducing (like low pH is acidic)
- High pE is oxidising (like high pH is basic)
pE-pH diagrams
- Show the region of stability and the boundary lines for various metal species in water and wastewater
Importance of chelating agents in naturals water, mechanism of action
Chelating agents are common potential water pollutants. These substances can occur in sewage effluent and industrial wastewater such as metal-plating wastewater. There are natural sources of chelating agents e.g. humic and fulvic substances
e.g. EDTA :
- prevents some metals from binding to and settling out with biomass sludge in biological treatment
- It chelates most of the copper, nickel and zinc in wastewater effluents
- It is responsible for the migration rates of radioactive elements in natural waters
- it is poorly biodegradable
Transport of metal in natural waters; importance of the partition coefficient
Distribution of metals in surface water:
MeT = MeD + MeP
Kd = MeP/MeD
Where MeD is the dissolved metal concentration in the stream or bed pore water, MeP is the particulate metal concentration in the stream or bed pore water and Kd is the distribution coefficient -> Kd modelling
- Considering that Kd is an equilibrium constant this approach requires the knowledge of one state variable only, MeT
- Derived Kd values may be obtained from large databases while MeT are obtained from field observations
- but Kd is not constant and depends on environmental conditions -> to overcome this difficulty, one can simulate the Kd variability using empirical functions
Higher Kd = higher affinity for particles e.g. lead is mainly absorbed on particles
Modeling metal speciation in the dissolved phase
Metal speciation in the dissolved phase = Chemical Equilibrium Modelling (CEM):
MeD = sum of bioavailable (labile) and non-labile metal species
Speciation measurements are necessary for the study of metal toxicity for aquatic organisms and for understanding trace metal transport in rivers and estuaries
Modeling adsorption reactions
Metal speciation in the particulate phase:
MeP = sum of the sorbed metal species -> Surface Speciation Modelling (SSM)
There are many types of adsorption reactions - The models differ in their structural representation of the solid-solution interface, that is, the location and hydration status of the adsorbed ions:
- isotherm or empirical models: numerical relationships used to curve-fit data
- surface complexation models exist in two types of structural configurations:
Outer-sphere surface complexes contain at least one water molecule between the adsorbing ion and the surface functional group - involve electrostatic bonding and are less stable than inner-sphere surface complexes
Inner-sphere surface complexes contain no water molecules between the adsorbing ion and the surface functional group, strong complexes involving either ionic or covalent bonding - diffuse layer model: A double layer is a structure that appears on the surface of a solid particle when it is placed into a liquid. The first layer, the surface charge (either positive or negative), comprises ions adsorbed directly onto the object due to a host of chemical interactions
The second layer is composed of ions attracted to the surface charge via the coulomb force. This second layer is loosely associated with the object, because it is made of free ions which move in the fluid under the influence of electric attraction - constant capacitance model: provides a molecular description of adsorption phenomena using an equilibrium approach. Unlike empirical models, the constant capacitance model defines surface species, chemical reactions, equilibrium constant expressions, surface activity coefficients and mass and charge balance
Explain the importance for pE in water
The limits of pE in water
- There are pH dependent limits to the pE values at which water is thermodynamically stable
- The condition under which oxygen from the oxidation of water has a pressure of 1 atm can be regarded as the oxidising limit of water, while a hydrogen pressure of 1 atm may be regarded as the reducing limit of water, these are boundary conditions that enable the stability of boundaries of water
Explain: chelation
- The species that binds with the metal ion is called a ligand
- The product in which the ligand is bound with the metal ion is a complex, complex ion or coordination compound
- A special case of complexation in which a ligand bonds in two or more places to a metal ion is called chelation
- The chelating agent has more than one atom that may be bonded to a central metal ion at one time to form a ring structure
- Stability of the metal chelates tends to increase with the number of sites available on the ligand
Case study: aluminium speciation in drinking water
Al in water has risen because acid rain has caused higher levels in freshwater and in high concentrations in treated water it gives rise to turbidity, reduces disinfection efficiency and may precipitate as Al(OH)3 during the course of distribution
Determination of Al speciation in drinking water is important because
- The Al form determines its mobility, bioavailability and toxicity
- Labile forms are the most toxic to aquatic organisms and can exist in a mononuclear form of Al3+ or can be complexed by dissolved inorganic constituents such as OH-, F- or SO42-
- The presence of acetate, oxalate, citrate and malate anions may significantly increase the Al assimilation due to their ability to complex and solubilise aluminum at neutral pH
- Possibility of association between Al and dementia and Alzheimers
