The Chemical behaviour of some toxic matter

Prof. Sachinath Mitra
Department of Geological Sciences
Jadavpur University
Calcutta - 700 032, INDIA

The 4.6 b.y. old Earth s surface evolved four spheres: lithosphere (rock), hydrosphere (water) and biosphere (organic) with a dynamic atmosphere (gas). These four spheres total to 0.4% mass of the Earth. Element cycling amongst these spheres involve various material and energy interactions.

The energy sources are solar radiation (uv-vis-IR), mechanical (kinetic and potential), thermal/nuclear (decay of 40K, U, Th, etc). and chemical.

We wouldn t have talked about the environment had there been no life. There would have been no life had there been no weathering of crustal rocks. Indeed, it is commonly believed that in the genesis of life the clay group of minerals particularly the species kaolinite with small particle size with large size molecules having residual charges on them, played an important role.

Clay minerals in Soil/Sediments

These clay minerals are the weathering products of rocks, and are effected by the chemical factors such as chemistry of water, pH and temperature, reactivity of species with CO2/H2O (called carbonation), hydrolysis, solubility and redox properties.

Some of the important weathering reactins affecting rocks are:

4 KalSi3O8(s) + 22 H2O ------>
Al4Si4 (OH)8 (s) + 8 H4SiO4 (aq) + 4 K+(aq) + 4 OH- (aq)

4 KAlSi3O8(s) + 18 H2O + 4 H+ ------> Al4Si4 (O)10 (OH)8(s) + 4 K+ (aq) + 8 H4SiO4 (aq)

4 KAlSi3O8(s) + 22 H2O + 4 CO2 ------> Al4Si4O10(OH)8(s) + 4 K+ (aq) + 8 H4SiO4 (aq) + 4HCO-3 (aq)

Thus K+ and silicic acid are released and sheet silicates are formed. Heavy elements in the rocks are also liberated during weathering.

In ordinary clay sediments we do see micas and hydrated alumina, formed by the reactions:

3KAISi3O8(S) + 12 H2O + 2H+ ------> KAI3Si3O10(OH)2(s) + 2 K+(Aq) + 6 H4SiO4 (aq) ------ (1) (mica)


KAI3 Si2 O10 (OH)2 (s) + 18H2O + 2H+ ------> 3AI2O3H2O + 2K+ (aq) + 6H4- SiO4 (aq) ------ (2) (gibbsite)

From the free energy formation data of various oxide species the free energy of the first reaction has been calculated to be 73KJ. The equilibrium constant, K, is measured as:

        K  =  ----------------

       log K  = --------  =  -12.8
                2.303 RT

Hence, -18.6 = 2 log ------ + 6 log [H4 SiO4]

The free energy for reaction (2) is calculated to be 106KJ and log K = -18.6.

Hence, -12.8 = 2 log ----- + 6 log [H4 SiO4]

A plot of log ([K+]/[H+]) against log [H4SiO4] sets the boundaries in the stability field diagram for K-feldspar, K-mica and gibbsite (Fergusson, 1982).

These soil (sediment) minerals, having large size molecular structures are capable of retaining or transfering a host of toxic elements, including the p-block element the chemical characteristies of which are discussed in the following.


The toxic and heavy elements of the p-block are : Gr II (Cd, Hg), IIIB (Ln, Tl), IVB (Pb), VB (As, Sb, Bi) and VIB (Se, Te), which correspond to the periodic groups 12, 13, 14, 15 and 16 of IUPAC.

IIB     IIIB    IVB             VB      VIB         Long      n
(12)    (13)    (14)           (15)     (16)        Period
Zn       Ga      Ge             As       Se         First     4 
Cd       Ln      Sn             Sb       Te         Second    5
Hg       Tl      Pb             Bi       Po         Third     6
 <----Metallic---->          <--non-metallic-->

P-block heavy metals

The 1st and 2nd long periods have filled d-shell and correspond to the n=4 n=5

electronic structure [Ar/Kr] (n-1) d10 ns2 px : [x = (]. The 3rd long period has the structure [Xe] 4f14 5d10 6s2 px, having filled f- and d-shells.

Elements of IIB, IIIB and IVB show metallic structure, whereas those of groups VB and VIB are non-metallic. A distortion in the metallic structure produces less dense meterial as happens with Hg.


Because of their large size the heavy elements show high coordination numbers, PbSO4 the coordination, no. of Pb is 12. The p-block elements in high oxidation states are predominantly covalent in character, whereas in low oxidation state they are predominantly ionic. The filled inner d- and f-shells cause the covalency, which makes the element polarizable. This is why these show affinity for polarizable ligands such as s2- and less affinity for more electronegative elements.

Such properties of the heavy p-block elements are illustrated with mercury and selenium. In the inorganic. Earth the toxic elements, which contributes to the threat of disturbing the balance in the organic living world, neither created nor destroyed by human activities. The hazards- environmental and health- arise from the redistribution of these elements by industrial, agricultural and mining operations, which enhance the bioavailability of these metals. The danger arises because of their ability for bioaccumulation through the food chain or bioconcentrate in human tissues. The anthropogenic emission of a toxic element disturbing the ecobalance is specified by its mobiliazation factor.

The mobilization factor (MF) of toxic metals is the ratio:

        anthropogenic emission rate
  MF =  ---------------------------
           natural emission rate

     = Pb > Sb > Cd > Se, As > Hg

The enrichment factor (EF) in the air is:

        [M]air       [Al]crust
  EF = --------      ---------
       [M]crust       [Al]air

EF > 1 implies enrichment in atmosphere relative to soil tract. In the North Atlantic and South Pole the metals with positive EF in Se > Pb > Sb > Cd.

The mobility of metal ions depends on pH and redox condition of the system. In this talk I like to touch upon lead, mercury and cadmium. Followed by and with emphasis on the chemical behaviour of the arsenic, as it has caused much concern in eastern part of India particularly Calcutta. The conclusion would be drawn outlining the salient features of the problem of waste disposal, particularly nuclear wastes. The mobility of metal ions depends on pH and redox conditions of the system.


Pb is contributed to the environment both by natural and anthropogenic sources. Till recent years leaded gasoline is used extensively. Now lead emissions have been dramatically reduced by removal of tetraethyl lead from gaolines. Now the principal sources of lead are smelters, battery plants and chemical plants.

Particulate lead as large as 2æ in diameter are transported by air. Smaller particulates may be transported as far as 1000 km from the source.

In water, solubility of Pb decreases with increase in salt content. In soil Pb migration is facilitated by water-soluble chelators and decreased soil pH.

Lead gets into human system by inhalation, but mostly through ingestion of food and drinking water. Lead-containing solders in water distribution systems and lead-containing paints also cause lead exposure.

High exposure results in high B.F. (cardiovascular effects), anemia (inhibition of heme synthesis), nephropathy (toxicity in kidney). Low level exposure causes neurobehavioural effects, such as hyperactivity, poor class room record and decrement in IQ. scores and vocabulary.


It occurs in elemental, inorganic or organic forms. Degassing of the Earth through volcanoes, fissures, fumeroles etc. is the major source. Anthropogenic sources are: mining, smelting and buring of fossil fuels- oil, gas and coal.

Metallic Hg readily oxidizes to Hg2+ in aquatic environment. Anaerobic bacteria methylates mercury, and methymercury is bioamplified and find ways to human food chain.

Metallic Hg is lipid soluble and readily crosses the membrane of the lung. It has high affiity for red blood cells. Methylmercury is readily absorbed (90-95%, Goyer, 1986) in the gastrointestinal tract, while inorganic salts of mercury is much less (7%) absorbed.

For all forms mercury, developing placenta acts as a sink following maternal exposures.

Early poisoing symptoms include tremors in fingers and eyelids, due to disturbance in fine motor functions, loss of memory, depression, halucination. Gingivitis and and severe salivation are two distinct manifestation of Hg toxicity.

The disaster of the fish eaters of Minamata Bay, methylation of the industrial discharge and subsequent bioamplification in fish (~11mg/kg Hg).

Consumption of wheat seeds treated with methylmercry fungicide enacted the Iraq episode, involving degeneration of neurons of the cerebral cortex, cerebral palsy, paresthesia, ataxia (eye muscle defect), dysarthia (speech defect) and deafness.


The mining and smelting of lead and zinc ores produce cadmium as a by-product. This is mainly used in electroplating or galvanizing, as pigments, as cathode in Ni-Cd batteries.

Zn and Cd are similar chemically and Cd goes very well for zinc binding sites in biologically active proteins.

Aerobic surface water carry a good concentration of soluble CdCl+ ion. At anaerobic water depths Cd content is rather low and microbial reduction often produces insoluble cadmium sulfides (Manahan, 1991).

Plants take up Cd more readily than Pb or other heavy metals. Rice and wheat thus concentrate Cd when the soil has it in high proportion. Shell fish, oyesters etc. constitute the other organisms concentrating it.

A smoker is prone to Cd poisoning because a single cigarette contains 1 to 2æg of Cd. Acute exposure to Cd can produce chemical pneumonitis and pulmonary edema.

In this context of arsenic poisoning it becomes pertinent to study the chemical behaviour of the Group 15 (IUPAC) elements viz. As, Sb and Bi. These show similar behaviour. As belongs to the first long period and follows the first transition metal series.

The covalent bond strength of these elements decreases in the order; As > Sb > Bi. For this reason BiH3 and organobismuth compounds are unstable.

Arsenic is released to the environment when arsenopyrite, FeAsS, commonly present as gold bearing ores reacts with water and oxygen. The redox process transforms S2- to SO42- and As (III) to AsO43- as :

4FeAsS + 13O2 + 6H2O ----->
4SO42- + 4AsO43- + 4Fe2+ + 12H+

The possible ion-electron half reactions are:

O2 + 4H+ + 4e -----> 2H2O, E0 = 1.23V

S2- + 4H2O - 8e -----> SO42- + 8H+, -E = -0.76V

AsO2- + 2H2O - 2e -----> AsO43- + 4H+, -E = 0.56V

From rocks and minerals, arsenic is released by the process of oxidation and hydrolysis. These arsenic products get into the water system and cause health hazards as seen in W.Bengal.

These elements manifest non-metallic character and form complexes, oxyanions etc. in water as M(H2O)xn+. Arsenic forms oxyanion eg. arsenates and arsenites, antimony forms oxyanions and oxycations, and bismuth forms oxycations and possible hydrated cations. Thus, these elements show increasing metallic and basic character from As ----> Sb ----> Bi. The basicity of oxides increases as: As < Sb < Bi, and M (V) < M(III). The oxides are not very soluble in water, but As (III) and Sb (III) oxides are soluble in basic solutions giving oxy/hydroxy species. Bi(III) oxide is soluble in acid only.

FeAsS when roated generates As4O6. Both As and Sb oxides are obtained from high pressure and temperature oxidations of M2O3 with O2. Pentavalent As (V) oxide is obtained by oxidation of trivalent oxide using nitric acid, followed by dehydration of the arsenic acid produced.

As (III) oxide in basic solution gives the specias As (OH)3, AsO2 (OH)2- and AsO3-. Arsenite ion is pyramidal with a lone pair occupying the fourth position. Arsenous acid is a weak acid:

H3AsO3 ======> H+ + H2AsO3-; K = 6 x 10-6

Many arsenites are not soluble in water, but alkali metal arsenites are. Arsenic (V) oxide gives in solution the arsenate ion AsO43- in which oxygen atoms lie at the tetrahedral corners around the central As ion.

Arsenates have low solubility in water. For this reason some arsenates of copper and chromium are used as wood preservatives. Who knowns if such wood preservative have not been used in the logs for piling the base of the multi-storied buildings (barrages and dams?) of Calcutta and around ?

The Eh-pH diagram of As is shown in the Figure.


The halides of these As, Sb and Bi hydrolyses in water. The halides are formed by reaction of the elements or the M (III) oxides with HX (where X are halogens).

Pentafluorides are known for all three elements. The relative stability of MCI5 compounds is Pb >> As << Sb. The process of chlorination goes as:

Mcl3 -----> MCl32+ -----> MCl4+ -----> MCl5
      -2e           +Cl-         +Cl-

The low stability of AsCl is attributed to the extra promotional energy of arsenic. The stronger attachment of the electrons to As (III) may be a result of stabilization of the orbitals of arsenic, due to the contraction effect of the preceeding transition metals.


The antimony sesquisulphides are not soluble in water, As4S3,, As4S4 and As4S6 (orpiment) occur in a number of modifications including realgar. For M2X3 compounds the trend in the band gap for doped materials for M : As > Sb >Bi, and for x: S> Se >Te giving n-and p- semiconductors.


The stability of organocompounds is the order As > Sb > Bi. because of their respective M-C bond dissociation energy

    M          M(CH3)3         MPh3
    As           238            280
    Sb           224            267
    Bi           140            200


For the analysis of As, Sb and Bi the methods used are : NAA (destructive), AAS, ET-AAS, Emission spectroscopy, Polarography etc. As in sediment can be directly measured by INAA for the concentration range of 18-474 ægg-1. The dust, soil, aerosol can be acid/chorate digested to form hydride and analysed by AAS; As in water and urine by polarography.

PCBs :

PCBs are used in capacitor and transformers because of their dielectric propertries and fire resistance. They are highly lipophilic and tend to accumulate in fat, skin, and other high-fat containing organs. Long term exposure may cause enzyme induction to hepatocellular necrosis. They affect cutaneous tissue and skin forming "Chlorgene".

Disposal :

According to U.S. Environmental Protection Agency (EPA) in U.S. alone the municipal and industrial solid waste is being generated at a rate of 400 million tons/yr. While liquid waste is about 10 trillion gal/yr. Of the solids 60 million tons may be hazardous.

The wastes are handled by incineration, bacterial digestion, reverse osmosis, shallow band burial. A large number of organic compounds are created as direct or indirect byproducts, of which about 10,000 compounds are toxic. Some of these are biodegradable but the host are stable compounds, persisting for decades or centuries and accumulated to dangerous levels.

From the waste disposal sites toxic contaminants such as As, Cd, Hg and Pb may come out and contaminate the ground water. Usually, when the pH of the water is close to 7 the toxic metals are nearly insoluble. But when the infiltrating water is slightly acidic the contaminants may increase the toxicity of the water. This may be neutralized by adding CaCO3 or some substances to precipitate/ adsorb the metal ions.

Much known such compounds are dichloro-diphenyl-trichloroethane (DDT) and the group of compounds know as polychlorobiphenols (PCBs). These are not oxidised nor are transformed by enzymatic reactions promoted by bacteria. Thus this problem is still persisting causing our anxiety.

Nuclear Waste :

The hazardous feature of radioactive waste lies in the radiation it generates. Health damage from radiation is cumulative and may lead to illness many years after initial exposure.

The radioactive isotopes in the intensely radioactive fuel rods or nuclear reactors that have been removed after many months of neutron bombardment of their contained uranium, may create havoc in radiation hazard if allowed to lie unshielded in the environment. Such 'high- level' waste needs deep burial several hundreds of meters underground. 'Low-level' waste with short-lived isotopes may be buried in shallow covered trenches.

Exploring Nature for purifying environment:

Trees :

    1. Fixing CO2 and generating O2

    2. Ayurvedic medicinal preparation produces least pollution of air, water and soil

    3. Nuclear waste treatment

I would conclude my talk with this single aspect of our trees. To show how some of our Indian trees come to help the nuclear waste treatment (Nature, 365, 1993, p.779).

A tree found growing in the forests of Andhra Pradesh (even shown to me by Tirupati geology techers the trees as being ordered by Japan) identified as Strychnos potatorum, is found to be capable of removing Cd, Hg and other toxic heavy metals from factory effluents, and manifests potential uses in the mining industry as it binds metals viz., Au, Ag, Co, Cu and Ni.

Powdered seed of this tree, had been used by tribals over the centuries for flocculation purposes, to transform muddy water into clean drinking water. The binding efficiency has been found to be high. One mole of bioflocculant can apparently absorb between 20 and 25 moles of metal. Dr. Durga Prasad of Girijan Cooperative Corporatio (GCC), collaborating with Prof. Clement Furlong, Dept. of Biochemistry, Washington University, states that certain proteins in the seed have the unique property of binding metals, Prof. Furlong and his collaborators, are trying to clone the genes coding for the proteins, so that they can be manufactured using the technique of biotechnology.

This herbal product has a very high affinity for uranium, and might therefore be used for removing traces of this metal from reactor effluent, a characteristic that has already attracted the attention of International Atomic Energy Agency (IAEA). Dr. K.Mahadev Rao of IAEA, has emphasised that the ability of this bioflocculant to selectively remove actinides makes the handling of the nuclear waste much easier.

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