《环境化学》课程PPT教学课件讲稿（英文版）Unit 17 Toxic Metals

Toxic metals

Toxic Metals

TAN.OCUs MAN GOUR ( n (V(V)(V) D) E un(V (v (V)(vI ML) |5253 57273747m|7|7 NNER TRANSTION CROUPS OhMM GiIn Dy He Er Tm ni L TU||{。|盒a■||w||L Elements shown in bold outline are known to be essential for human metabolism About 1/ 3 of all known enzymes require the presence of a metal ion for them to exhibit biochemical activity

• Elements shown in bold outline are known to be essential for human metabolism. • About 1/3 of all known enzymes require the presence of a metal ion for them to exhibit biochemical activity

oxic metals Some metals are essential for the normal development and well-being of humans and other creatures They include Na, K ca, Mg, cu, cr, and zn Other metals are toxic even when present in an organism at very low concentrations. Examples: Antimony, Arsenic, Barium, Beryllium, Cadmium, Lead, Mercury, Nickel, Selenium, and thallium (Arsenic and selenium are not metals but are often listed with them) The toxic metals can cause brain damage, kidney and liver disorders and bone damage

Toxic metals • Some metals are essential for the normal development and well-being of humans and other creatures. – They include Na, K, Ca, Mg, Cu, Cr, and Zn. • Other metals are toxic even when present in an organism at very low concentrations. – Examples: Antimony, Arsenic, Barium, Beryllium, Cadmium, Lead, Mercury, Nickel, Selenium, and thallium – (Arsenic and selenium are not metals but are often listed with them). – The toxic metals can cause brain damage, kidney and liver disorders, and bone damage

Maximum permissible levels for metals in drinking water(USEPA standards Metal Max Level Metal Max Level (ppb) (ppb) Antimony(Sb) 6 Lead 50 Arsenic 50 Mercury Barium 2000 Nickel 100 Beryllium 4 Selenium 50 Cadmium 5 Thallium 2 Chromium 100

Maximum permissible levels for metals in drinking water (USEPA standards) Metal Max. Level (ppb) Metal Max. Level (ppb) Antimony (Sb) 6 Lead 50 Arsenic 50 Mercury 2 Barium 2000 Nickel 100 Beryllium 4 Selenium 50 Cadmium 5 Thallium 2 Chromium 100

Biogeochemical cycle of metal species All metals cycle naturally through the environment Sources: weathering of rocks Transportation: uptake and processing by plants and microorganisms. Plants living in soils rich in metal species have evolved protective mechanisms These plants can be used in remediation of soils contaminated with metals Human intervention have greatly perturbed the natural biological cycles of the metals Mining and metallurgy Coal burning Metals can not be broken down into simpler, less toxic forms. As a result, they persist in the environment and bioaccumulate through food chain

Biogeochemical cycle of metal species • All metals cycle naturally through the environment. • Sources: weathering of rocks • Transportation: uptake and processing by plants and microorganisms. – Plants living in soils rich in metal species have evolved protective mechanisms. – These plants can be used in remediation of soils contaminated with metals. • Human intervention have greatly perturbed the natural biological cycles of the metals. – Mining and metallurgy – Coal burning • Metals can not be broken down into simpler, less toxic forms. As a result, they persist in the environment and bioaccumulate through food chain

Hexavalent Chromium Cr(vi) is a carcinogen Sources Steel-making cutting grinding and welding Spills and residues of electroplating baths Chromate emissions from cooling towers(used to inhibit corrosion)

Hexavalent Chromium • Cr (VI) is a carcinogen • Sources – Steel-making, cutting, grinding, and welding – Spills and residues of electroplating baths – Chromate emissions from cooling towers (used to inhibit corrosion)

Chemical basis for the toxicity of Hg, Cd, and Pb All three are soft"Lewis acids, i. e with large polarizability They have strong affinity for soft Lewis bases, such as the sulfhydryl side chain of cysteine residue The heavy metals likely exert their toxic effects by tying up critical cysteine residues in proteins 12 Lewis acid is a species that can form a H-C-CH2SH covalent bond by accepting an electron pair from another species COOH Lewis base is a species that can form a cysteine covalent bond by donating an electron pair to another species

Chemical basis for the toxicity of Hg, Cd, and Pb • All three are “soft” Lewis acids, i.e., with large polarizability. • They have strong affinity for soft Lewis bases, such as the sulfhydryl side chain of cysteine residue. • The heavy metals likely exert their toxic effects by tying up critical cysteine residues in proteins. cysteine NH2 C COOH H CH2 SH Lewis acid is a species that can form a covalent bond by accepting an electron pair from another species. Lewis base is a species that can form a covalent bond by donating an electron pair to another species

cO cO CO cO I Oxidn CHCH,SH HSCH2 CH CHcH2S一SCH2CH Redn. I NH NH Cystein residues provide structurally important cross-linking sites through formation of disulfide bridges. Heavy metals interfere with formation of such bridges

Cystein residues provide structurally important cross-linking sites through formation of disulfide bridges. Heavy metals interfere with formation of such bridges

Mercury · When ingested,Ha0,hg2〃 and Hg2 are not toxIC. Harmful forms: methylmercury ion( CH3)Hg+ regardless of the route of exposure hg vapor when inhaled (CH3)Hg+ is produced from mercury ions by methanogens living in sediments (CH3)Hg+ is soluble in water and bioaccumulates in the aquatic ecosystem food chain (in protein-laden tissues) ( CH3)Hg* binds with protein and peptide sulfhydryl groups. It is dangerous to eat large quantities of very big ocean fish such as tuna and swordfish!

Mercury • When ingested, Hg0 , Hg2 2+, and Hg2+ are not toxic. • Harmful forms: methylmercury ion (CH3)Hg+ regardless of the route of exposure, Hg vapor when inhaled. • (CH3)Hg+ is produced from mercury ions by methanogens living in sediments. • (CH3)Hg+ is soluble in water and bioaccumulates in the aquatic ecosystem food chain (in protein-laden tissues). • (CH3)Hg+ binds with protein and peptide sulfhydryl groups. • It is dangerous to eat large quantities of very big ocean fish such as tuna and swordfish!

Mercury sources related to human activities Coal-burning(Hg Hgt, Hg bound to particulate Chlor-alkali plants Hg used as cathode for collecting metallic na Use in electrical devices and equipment, such as batteries, switches lamps Use in extracting gold or silver from ores Dental amalgam fillings Use of complexes of phenylmercury as paint preservatives. Organomercurials used as fungicides for seed treatment

Mercury sources related to human activities • Coal-burning (Hg0 , Hg2+, Hg bound to particulate) • Chlor-alkali plants Hg used as cathode for collecting metallic Na • Use in electrical devices and equipment, such as batteries, switches, lamps • Use in extracting gold or silver from ores. • Dental amalgam fillings • Use of complexes of phenylmercury as paint preservatives. • Organomercurials used as fungicides for seed treatment