Heavy Metals

Heavy metals group of metallic elements with relatively high densities, atomic weights, or atomic numbers that are toxic or poisonous at low concentrations * Characteristics * Naturally occurring elements * Persistent in the environment * Non-biodegradable * Can accumulate in living organisms * Sources of Exposure * Occupational (mining, manufacturing) * Environmental (contaminated soil, water, air) * Dietary (food, beverages) * Household (lead-based paint, plumbing)

General Chemical Properties of Heavy Metals

Metallic Bonding: Exhibit metallic bonding, resulting in good electrical and thermal conductivity
Variable Oxidation States: Can exist in multiple oxidation states, influencing their chemical behavior and toxicity
Complex Formation: Tendency to form complexes with various ligands, such as proteins, enzymes, and DNA
Reactivity: React with biological molecules, disrupting their structure and function

General Physical Properties of Heavy Metals

Appearance: Typically have a metallic luster
Density: Relatively high densities compared to other elements
Melting and Boiling Points: Varies depending on the specific metal
Solubility: Solubility of metal compounds varies widely depending on the specific compound and the solvent
Conductivity: Good electrical and thermal conductivity

Specific Heavy Metals of Toxicological Significance

Lead (\(Pb\))

Definition: A naturally occurring heavy metal that is toxic even at low concentrations
Sources of Exposure
- Lead-based paint
- Contaminated soil and dust
- Drinking water from lead pipes
- Occupational exposure (mining, manufacturing)
Chemical Properties
- Atomic Number: 82
- Atomic Weight: 207.2 g/mol
- Oxidation States: +2 (most common), +4
- Forms complexes with various ligands, including proteins, enzymes, and DNA
Physical Properties
- Appearance: Soft, gray-blue metal
- Density: 11.34 g/cm³
- Melting Point: 327.5°C
- Boiling Point: 1749°C
- Solubility: Insoluble in water, but lead compounds (e.g., lead acetate, lead nitrate) are soluble
Toxicokinetics
- Absorption:
  - Inhalation: Rapidly absorbed into the bloodstream
  - Ingestion: Absorption varies depending on the form of lead and the presence of food in the stomach
  - Dermal: Minimal absorption through intact skin
- Distribution:
  - Distributed throughout the body
  - Accumulates in bone (90%), blood (1-2%), and soft tissues (5-8%)
  - Crosses the blood-brain barrier and the placenta
- Metabolism:
  - Not metabolized
- Excretion:
  - Primarily excreted in the urine and feces
  - Excretion is slow, leading to accumulation in the body
Mechanism of Toxicity
- Interferes with various enzymatic processes
- Inhibits heme synthesis, leading to anemia
- Disrupts calcium homeostasis
- Causes oxidative stress
- Affects the nervous system, kidneys, and reproductive system
Signs and Symptoms of Lead Poisoning
- Neurological:
  - Adults: Headache, irritability, memory loss, peripheral neuropathy
  - Children: Developmental delays, learning disabilities, seizures, coma
- Gastrointestinal: Abdominal pain, constipation, nausea, vomiting
- Hematological: Anemia (microcytic, hypochromic)
- Renal: Kidney damage
Analytical Methods
- Atomic Absorption Spectrometry (AAS)
- Inductively Coupled Plasma Mass Spectrometry (ICP-MS)
- Anodic Stripping Voltammetry (ASV)

Mercury (\(Hg\))

Definition: A naturally occurring heavy metal that exists in various forms, each with different toxicological properties
Forms of Mercury
- Elemental Mercury (Hg⁰): Liquid at room temperature
- Inorganic Mercury (Hg⁺, Hg²⁺): Mercury salts
- Organic Mercury: Methylmercury, ethylmercury
Sources of Exposure
- Occupational (mining, manufacturing)
- Dental amalgams
- Contaminated fish (methylmercury)
- Industrial waste
Chemical Properties
- Atomic Number: 80
- Atomic Weight: 200.59 g/mol
- Oxidation States: 0, +1, +2
- Forms complexes with sulfur, selenium, and other elements
Physical Properties
- Appearance: Silver-white liquid metal
- Density: 13.534 g/cm³ (very dense)
- Melting Point: -38.83°C
- Boiling Point: 356.7°C
- Solubility: Insoluble in water, but some mercury compounds are soluble
Toxicokinetics
- Elemental Mercury:
  - Inhalation: Rapidly absorbed into the bloodstream
  - Ingestion: Poorly absorbed
- Inorganic Mercury:
  - Ingestion: Poorly absorbed
- Organic Mercury (Methylmercury):
  - Rapidly and completely absorbed from the gastrointestinal tract
  - Crosses the blood-brain barrier and the placenta
- Distribution:
  - Elemental Mercury: Accumulates in the brain and kidneys
  - Inorganic Mercury: Accumulates in the kidneys
  - Organic Mercury: Distributed throughout the body, including the brain and fetus
- Metabolism:
  - Elemental Mercury: Oxidized to inorganic mercury
  - Organic Mercury: Demethylated to inorganic mercury
- Excretion:
  - Primarily excreted in the urine and feces
  - Excretion is slow, leading to accumulation in the body
Mechanism of Toxicity
- Binds to sulfhydryl groups in proteins and enzymes, disrupting their function
- Causes oxidative stress
- Affects the nervous system, kidneys, and immune system
Signs and Symptoms of Mercury Poisoning
- Neurological: Tremor, memory loss, irritability, numbness, and tingling
- Renal: Kidney damage, proteinuria
- Gastrointestinal: Nausea, vomiting, abdominal pain
- Dermatological: Skin rash
- Minamata Disease (methylmercury poisoning): Neurological syndrome with ataxia, sensory disturbances, and cognitive impairment
Analytical Methods
- Cold Vapor Atomic Absorption Spectrometry (CVAAS)
- Inductively Coupled Plasma Mass Spectrometry (ICP-MS)

Arsenic (\(As\))

Definition: A naturally occurring metalloid that is toxic in both inorganic and organic forms
Sources of Exposure
- Contaminated drinking water
- Food (seafood, rice)
- Pesticides
- Industrial processes (smelting, wood preservation)
Chemical Properties
- Atomic Number: 33
- Atomic Weight: 74.92 g/mol
- Oxidation States: -3, 0, +3, +5
- Forms compounds with oxygen, sulfur, and other elements
Physical Properties
- Appearance: Gray, metallic solid
- Density: 5.727 g/cm³
- Melting Point: 817°C (at 28 atm)
- Boiling Point: Sublimes at 615°C
- Solubility: Insoluble in water, but some arsenic compounds are soluble
Toxicokinetics
- Absorption:
  - Inorganic Arsenic: Well-absorbed from the gastrointestinal tract and lungs
  - Organic Arsenic: Less well-absorbed
- Distribution:
  - Distributed throughout the body
  - Accumulates in the liver, kidneys, spleen, and skin
- Metabolism:
  - Inorganic arsenic is methylated to monomethylarsonic acid (MMA) and dimethylarsinic acid (DMA)
  - Methylation is a detoxification pathway, but some intermediates may be more toxic
- Excretion:
  - Primarily excreted in the urine as inorganic arsenic and methylated metabolites
  - Excretion half-life varies depending on the form of arsenic
Mechanism of Toxicity
- Inhibits enzyme function
- Causes oxidative stress
- Disrupts DNA repair
Signs and Symptoms of Arsenic Poisoning
- Acute:
  - Gastrointestinal: Nausea, vomiting, abdominal pain, diarrhea
  - Cardiovascular: Hypotension, arrhythmias
  - Neurological: Encephalopathy, seizures
- Chronic:
  - Dermatological: Hyperpigmentation, palmar-plantar hyperkeratosis (Mees’ lines on nails)
  - Neurological: Peripheral neuropathy
  - Cardiovascular: Increased risk of cardiovascular disease
  - Cancer: Increased risk of skin, lung, and bladder cancer
Analytical Methods
- Hydride Generation Atomic Absorption Spectrometry (HGAAS)
- Inductively Coupled Plasma Mass Spectrometry (ICP-MS)

Analytical Significance

Selection of Analytical Method: Physical and chemical properties guide the selection of appropriate analytical methods
Sample Preparation: Solubility and stability determine the optimal sample preparation techniques
Interpretation of Results: Understanding metabolism and excretion helps interpret analytical results and assess the severity of poisoning

Clinical Significance

Accurate Diagnosis: Prompt and accurate identification of the ingested heavy metal is crucial for appropriate treatment
Assessment of Severity: Measurement of heavy metal concentrations helps assess the severity of poisoning and guide clinical management
Monitoring of Treatment: Serial measurements of heavy metal concentrations are used to monitor the effectiveness of treatment and guide further interventions

Key Terms

Heavy Metal: A metallic element with a relatively high density that is toxic at low concentrations
Lead: A naturally occurring heavy metal
Mercury: A heavy metal that exists in various forms, each with different toxicological properties
Arsenic: A metalloid that is toxic in both inorganic and organic forms
Toxicokinetics: The study of the movement of toxic substances within the body
Metabolism: The process by which the body chemically alters a substance
Excretion: The process by which the body eliminates a substance
Chelation Therapy: The use of chelating agents to bind to heavy metals and facilitate their excretion
Atomic Absorption Spectrometry (AAS): An analytical technique used to measure the concentration of elements in a sample
Inductively Coupled Plasma Mass Spectrometry (ICP-MS): An analytical technique used to measure the concentration of elements in a sample
Hydride Generation Atomic Absorption Spectrometry (HGAAS): An analytical technique used to measure the concentration of arsenic in a sample
Cold Vapor Atomic Absorption Spectrometry (CVAAS): An analytical technique used to measure the concentration of mercury in a sample
Chelating Agent: A molecule that binds to metal ions
Metalloid: An element with properties intermediate between those of a metal and a nonmetal

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