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Classification of naturally-occurring metalsaccording to toxicity & availability in the hydrologicenvironment. Wood (1974)Non-toxic Low Toxicity Mod-high Tox.Aluminium Gold CadmiumIron Tin ChromiumManganeseCopperLeadMercuryNickelZinc

Nieboer & RichardsonClass A (O 2 seeking) Borderline Class B (S or N 2seeking)Calcium Zinc CadmiumMagnesium Lead CopperManganese Iron MercuryPotassium Chromium SilverStrontiumSodiumCobaltNickelArsenicVanadiumNieboer & Richardson (1980); The replacement of the nondescriptive term heavymetals by a biologically and chemically significant classification of metal ions. Environ.Pollut. Ser. B 1:3-26.

Toxicity ratingB>Intermediate>A• Class B most effective at binding with S-H groups, N-containing groups• Displace Intermediate/Class A metals• Can form organometals• Intermediates displace other intermediates/Class A metals• Class A – displace other Class A metals• Metals do not metabolize, change by binding to othermolecules• Metals regarded as ‘essential’ or ‘non-essential’

SOURCES & SINKS OF METALS IN THE ENVIRONMENTErosionBiological & ChemicaltransformationsVolatilizationAttachment/releasein sedimentUptakeSettling/resuspensionStorage

Anthropogenic Enrichment Factor (AEF)Indicates extent to which human activity contributesto global metal cyclesMetal Anthropogenic Natural Total (T) AEFSource (A) source ([A/T] x 100)Cadmium 8 1 9 89%Lead 300 10 310 97%Manganese 40 300 340 12%Mercury 100 50 150 66%(All values 10 6 kg per year)

MERCURYLiquid at roomtemperatureEdu.Switches (car)ThermostatsLampsSwitches(lights)MedicalDentalDenseUsed in fungicides,manufacture of paper &plastics

No biological role, but abundant in environment and presentin all organismsExposureMedicalUsed as disinfectant in India in C12th, as diuretics,laxatives, and to treat syphilis. Still part of traditionalChinese medicineRespiratory (e.g. occupational)Gastrointestinal (e.g. Minamata disease)Dental – amalgam used in tooth fillings


Toxicity – Dependent on formElemental - Hg 0 : Vapour, highly toxicInorganic – Hg + , Hg 2+ :Not easily absorbed, but risk of kidney damage if ingestedOrganic – R-Hg, e.g. methyl mercury:More toxic than inorganic, risk of brain & CNS damage.Can cross blood-brain barrier and placentaAttraction for sulphur groups, thus can inactivate enzymesSodium/potassium-adenosine triphosphate (Na + /K + -ATPase) enzyme particularly sensitive to HgNa/K-ATPase essential for working of CNS

JAPANMinamata Bay Incident 1950sNew factory used mercuric chlorideas catalyst*Minamata BayNew illness apparent in fishermen &their families (memory loss, visualimpairment, cerebral palsy)Children & newborns most severelyaffectedHg discharged principally in inorganicform – bacteria in sedimentsproduced methyl mercury700 poisonings, > 70 deaths

Some animals concentrate Hg without obvious effectsE.g. Tuna, swordfish, sharksTop predatorsActive swimmersSeabirds:Majority of Hg in methylated formin feathersSeabirds & marine mammals:High levels of selenium, protectsagainst mercury toxicity

LEAD Galena (PbS) –main lead oreUses of LeadBatteriesLead mined for > 6000 yearsPigmentCablesAmmunition

No biological roleLEADMost dangerous via chronic exposure, can be stored inbones as lead phosphateExposureMedicalUsed by Romans to treat skin complaints, by Victorians(plus opium) to treat diarrhoeaRespiratory (e.g. leaded petrol)Gastrointestinal (e.g. Roman cooking implements &water pipes)Industrial – mining, paints

Start of C20thLarge-scale lead poisoning of children noticed,phenomenon of pica primarily responsible for PbingestionBIO-ACCUMULATION

Toxicity: Pb substitutesfor Ca(Microgramsper decilitre)Alters function ofenzymeProtein Kinase CInterfereswithNeurotransmittersSymptoms:Gut paralysisFluid retentionon brainAnaemia

Absorbed into bloodDeactivates enzymesthat make haemoglobinPrecursor ALA buildsupPbInhibitsParalyses gut,Fluid on brain,Affects reproductivesystemAnaemia

CADMIUMProduced as by-product from zincsmeltingBelieved to be nonessentialbiologicallyReadily accumulatedby marine organismsEssential for diatomsp. Thalassiosira

Poorly absorbed from gutCan be taken up by inhalation e.g. Severn Bridgeworkers 1966Present incigarettesmokeCauses kidneydamage

TINHeavily mined in Asia, Bolivia & BrazilOrganotin highly toxicUsed in antifouling paints, woodpreservatives and stabilizersPacific oyster

Metals – do not metabolizeChange speciation by binding to chaperonesPrinicipally excreted via gut

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