Metals, their Biological Functions and Harmful Impacts - Mining and the Environment: From Ore to Metal - page 476

FIGURE 11.1
Periodic Table of Known Elements

Gas
Metalloid
Metal
Transition Elements

1
H
3
Li
11
Na

2
He
10
Ne
18
Ar
36
Kr
54
Xe
86
Rn

4
Be
12
Mg

5
B
13
Al
31
Ga
49
In
81
Tl

6
C
14
Si
32
Ge
50
Sn
82
Pb

7
N
15
P
33
As
51
Sb
83
Bi

8
O
16
S
34
Se
52
Te
84
Po

9
F
17
Cl
35
Br
53
I
85
At

From the 109 known elements most
are metals, but over 80% of the Earth's
crust is made up of nonmetals. From
the economically important elements
only 4 elements (aluminium, iron,
potassium and magnesium) exceed
the 1% mark. In any given rock, all
remaining economically important
elements combined such as gold, silver,
copper, lead, zinc, tin, nickel, or platinum
typically account for less than half a
percent of its mass.

19
K
37
Rb
55
Cs
87
Fr

20
Ca
38
Sr
56
Ba
88
Ra

21
Sc
39
Y

22
Ti
40
Zr
72
Hf
104
Unq

23
V
41
Mb
73
Ta
105
Unp

24
Cr
42
Mo
74
W
106
Unh

25
Mn
43
Tc
75
Re
107
Uns

26
Fe
44
Ru
76
Os
108
Uno

27
Co
45
Rh
77
Ir
109
Une

28
Ni
46
Pd
78
Pt

29
Cu
47
Ag
79
Au

30
Zn
48
Cd
80
Hg

57-71
(La-Lu)

89-103
(Ac-Lr)

Lanthanide
Series
(La-Lu)
Actinide
Series
(Ac-Lr)

57
La
89
Ac

58
Ce
90
Th

59
Pr
91
Pa

60
Nd
92
U

61
Pm
93
Np

62
Sm
94
Pu

63
Eu
95
Am

64
Gd
96
Cm

65
Tb
97
Bk

66
Dy
98
Cf

67
Ho
99
Es

68
Er
100
Fm

69
Tm
101
Md

70
Yb
102
No

71
Lu
103
Lr

this does not mean that similar effects occur when being exposed to copper, nickel, zinc, or
other mined metals. There are also, of course, the indiscriminate claims that all ores con-
tain arsenic, cadmium, mercury, or even cyanide; these have become 'angst' words com-
monly used by anti-mining advocates to oppose mine developments.
This chapter aims to provide enough information on some mined metals and their
harmful impacts to facilitate an objective environmental assessment of a mine. What fol-
lows is a highly summarized overview of what is a highly complex subject. Undoubtedly
chemists and toxicologists may feel that the subject matter is dealt with in an overly sim-
plii ed manner. The text, however, provides sufi cient reference to the ample specialized
literature on this subject to assist the reader interested in further study.
What dei nes a metal? In some contexts, the dei nition of a metal is based on physi-
cal properties. Metals are characterized by high thermal and electrical conductivity, high
rel ectivity and metallic lustre, strength and ductility (Masters 1991). From a chemical per-
spective, it is more common to use a dei nition that says a metal is an element that gives up
one or more electrons to form a cation in an aqueous solution. With this latter dei nition,
there are about 80 elements that can be called metals.
Among metals, heavy metals are environmentally of most concern. 'Heavy metals' is an
inexact term used to describe some elements that are metals or metalloids (meaning ele-
ments that have both metal and nonmetal characteristics). Examples of heavy metals
include chromium, arsenic, cadmium, lead, mercury and manganese, common associates in
base metal sulphide deposits. By some dei nition, heavy metals are those metals that have
densities above 5 g/cm 3 comprising 38 elements in total (UNEP 1994). Others dei ne heavy
metals as those metals with an atomic number 20 and greater (Eby 2004). Another, more
strict dei nition, identii es those metals heavier than the rare Earth metals, which are at the
bottom of the periodic table. More commonly, the term is simply used to denote metals that
are toxic, since none of the heavy metals is an essential element in biological systems and
additionally, most of the better known heavy metals are toxic in fairly low concentrations.
To avoid bias, however, it is best to use the neutral term 'metal' in environmental studies.
Nature has distributed elements, including metals, in different concentrations but
rather equally within the Earth's crust. Only eight elements are present in the Earth's crust
in amounts exceeding 1% crustal abundance ( Figure 11.2 ) . These eight elements constitute
more than 98% of the Earth's crust by weight. Furthermore, the two most common ele-
ments in the crust, silicon and oxygen, constitute about two-thirds of the crust's weight.

What defi nes a metal?

'Heavy metals' is an inexact term.

None of the heavy metals is an
essential element in biological
systems.