•
Introduction
Trivalent
chromium salts are the most widely used tanning agent in the world
for the transformation of hides and skins into leather. Chromium tanned
leather has been used for almost a century for the manufacture of
consumer products such
as clothing, gloves, footwear, furniture, automobile upholstery, as
well as a variety of personal leather goods.
The
leather manufacturing process generates liquid and solid wastes containing
chromium (III). This manufacturing activity
and the resulting by-products prompt questions concerning their impact
upon the environment and mankind. This
article summarises the current state of knowledge on this subject.
•
The transformation of hide into leather
The
use of leather goes back to prehistoric times. Archaeological evidence
shows that the material was widely used in antiquity. The raw material
is mammalian skin, which today is derived principally from animals
which are butchered for the food industry, and to a lesser extent,
skins from reptiles, fish and birds(1).
The tannery operations consist of transforming the raw hide, a highly
putrescible material, into leather, a stable product which can be
conserved indefinitely and which has a significant commercial value.
These operations follow a sequence of organised chemical reactions
(using reactive products) and mechanical processes using specialised
machinery. Amongst these, tanning is the fundamental stage which confers
to leather its stability and essential characteristics.
•
Tanning
Tanning
is the most important step in the production of leather and it is
carried out in an aqueous environment with water in rotating drums(2).
Its objective is to process the skins which arrive from the abattoirs
in a dried, salted state, to a condition which will facilitate the
chemical operation of tanning. During this operation, collagen, the
principal protein of the skin, will fix the tanning agent to its reactive
sites, thus stopping the putrefaction phenomenon(3).
The products which are capable of being fixed to skin to achieve tanning
are many and varied.
They can be classified into three groups :
- vegetable type tannins (mimosa, chestnut, quebracho)
- mineral tannins (chromium, iron, zirconium, in salt form)
- other organic tannins (formaldehyde, synthetic tannins, fish oil
...)
The
most widely used today are chromium salts. The process which was invented
at the turn of the century uses chromium (III) hydroxysulfate salts
in powder form, on skins which have been previously prepared following
methods which were derived from work carried out in 1959(4).
Regarding chromium tanning, only chromium (III) sulphate possesses
tanning properties with respect to skin collagen. This tanning agent
is produced from a natural product found in the form of chromite.
To obtain good quality leather, it is necessary to use a quantity
of chromium salts representing 2 to 2.5 % (calculated as Cr2O3) of
the mass of skins to be tanned. This implies the use of 8 to 10 %
of commercial product containing on average 25 % of Cr2O3(5).
This procedure has thus replaced all the techniques based upon the
use of chromium (VI) which, in the past, necessitated the reduction
to a valency of (III) in the tannery, before tanning.After tanning,
the leather is stored for several days which allows the consolidation
of the chromium/collagen bonds. The resulting product can resist temperatures
as high as 120°C whilst collagen is denatured at 50°C(6).In
order to be transformed into a commercial product, the leather needs
to be split and shaved to an even thickness, then superficially re-tanned
with low quantities of tanning agents such as chromium, aluminium
salts or vegetable/synthetic tannins, dyed with colouring agents,
then fat liquored with natural or synthetic fats in order to render
the product flexible. After drying, it is embellished with a film
of more or less pigmented products on its outer surface in order to
attain the appearance and degree of protection required for its final
use. The last stages are called leather finishing(7).
•
The advantages and applications of chromium tanned leather
Chromium
tanning is preferred because the process is quick, simple, reproducible
and is very cost effective. It yields a
material with a high mechanical and thermal resistance and a pronounced
capacity for dyeing, so that a wide range of
colours, rich or pastel shades are possible. The collagen chromium
bond is actually the strongest known today amongst the various alternatives,
including vegetable and synthetic tannins. By virtue of their quality
and mechanical characteristics, chromium tanned leathers are well
suited to a wide range of applications : gloves, footwear, leather
goods, luggage and upholstery. Despite
extensive investigation in laboratories around the world, no fully
satisfactory alternative to chrome tanning has
been found or is likely to be found in the foreseeable future.
•
Health risks for tannery staff
For
many years the health risk associated with the use of chromium salts
in the tanning industry has been questioned. Unlike chromium (VI),
chromium (III) salts of the type used for tanning are less likely
to penetrate and sensitize the skin(8).
Further to the low risk of allergies due to chromium (III)(8),
it was feared that leather dust could have an adverse effect on the
nose, throat and lungs. Dust is produced during several operations,
notably by the buffing of the outer surface during finishing, which
produces particles of which 50 % have a diameter < 5 µm(9).
The International Agency for Research on
Cancer (IARC) evaluated studies(9,
10) dealing with the incidence of nasal cancer in tannery
workers and has not reported significant findings(11).
The results of analyses carried out on air from tanneries(9),
have revealed chromium (III) contents ranging from 5.5 to 8.0 µg/m3
which is far below the French or American-ACGIH Time Weighted Average
(TWA) values, which are 0.5 mg/m3(12,
13). From an epidemiological point of view, an American
study(14) related
to a period from 1940 to 1982 in two tanneries in Minnesota and Wisconsin
concerning 9,365 workers, did not show higher death rates from nasal
or lung cancer than that observed in the general population. An English
study(15) related
to a period from 1939 to 1982 concerning 833 tannery workers of which
573 were concerned with vegetable tanning and 260 with chromium tanning,
did not reveal a higher incidence of death from cancer (of the stomach,
large intestine, lung, rectum or prostate) compared to the general
population. IARC concluded in its monograph that the only study specific
to tanning failed to reveal a statistically significant risk(9).
•
Impacts on the environment
Chromium
salts which are not fixed to the collagen during the tanning process
are discharged as effluents to the
environment, notably water and the ground.
•
Water
- It has been estimated that, with traditional tanning methods, from
4 to 9.5 kg of chromium (calculated as Cr2O3) per ton of skins are
not chemically fixed during processing if it is not carried out with
a suitable high exhaustion tanning system(16).
Two thirds are rejected in the liquid effluent at the tanning stage(17).
According to a Canadian scientific bibliography, in an aqueous medium
of neutral pH, chromium (III) compounds form oxides, hydroxides and
highly insoluble phosphates, which bind themselves to solids in suspension.
This is why soluble chromium (III) compounds are rapidly eliminated
from surface water into sediment. This chromium complex is relatively
stable and slightly biologically available(18).
This aptitude for precipitation was confirmed in a study carried out
in Costa Rica on a river polluted by several tanneries(19).
Work was carried out by the CTC on activated sludge in an effluent
treatment plant. Using the respirometry method, the purpose was to
measure the toxicity of chromium. It was shown that whilst chromium
is maintained in an insoluble form (pH neutral or alkaline), its toxicity
on activated sludge cannot be detected(20).
World wide regulatory limits fixing the chromium (III) content of
water which can be discharged into surface waters, vary from 0.5 to
15 mg/l(21). Widely
practised efficient treatment of effluent combined with clean technologies
(which we will mention later and which are increasingly being adopted
by tanneries), enable tanners to comply with these strict regulations.
To measure the effect of chromium (III) compounds, studies were carried
out on living species in water. They showed that because of its low
solubility (experimental conditions pH 7), chromium (III) did not
prove to be toxic to bacteria, seaweed or fish. Only daphnia showed
a marked sensitivity to concentrations in the order of 6 to 9 mg/l(20).
•
Ground
- The manufacture of chromium tanned leather also generates solid
wastes or scrap resulting from mechanical operations of which the
chromium (III) content (calculated as Cr2O3) is, on average, between
2 and 5.5 %(22). In
Europe, these are not listed as hazardous waste(23)
and in the USA they are specifically exempt from Federal hazardous
waste regulations(24,25).
The US Environmental Protection Agency (EPA) has conducted an extensive
risk evaluation of chromium (III) in sludge used for agricultural
land application and could find no adverse effect for any pathway
of exposure(26). Therefore,
chromium limits for land applied sludge have been eliminated(27,28).
The Organisation for Economic Cooperation and Development (OECD) considers
that solid wastes produced by tanneries are to be included on the
«green» list, which signifies that they only need to conform
to commercial requirements with respect to their transboundary transport.
On the other hand, leather dust and sludge are included on the «amber»
list which signifies that the tanner and recipient of the goods are
required to inform their local or national environment administration
concerning the commercial transaction(29).
In the USA, one publicly-owned waste water treatment plant receives
95 % of its input from a tannery. Since 1977, it has been dumping
on land sludge containing 34 g of chromium (III)/kg. For the past
ten years, the town has been monitoring ground water immediately below
the landfill at down gradient wells. It reported that the chromium
level in ground water was below the detection limit of 0.01 mg/l(30).
When chrome containing sludge is mixed with earth, the appearance
of chromium (VI) could theoretically occur at redox potentials typically
found in well aerated soils(31).
However, in practice, the oxidation of chromium (III) to chromium
(VI) does not occur in earth even under experimental conditions combining
maximum aeration and high pH(32,
33, 34).
•
Ground life and micro-organisms - On micro organisms(35),
the inhibitory effects of chromium (III) (concentrations up to 1,000
ppm) were noticed on short term exposure but were no longer evident
after a period of six weeks. However, 10,000 ppm of chromium (III)
completely blocked nitrogen transformation. Earthworms could survive
even with a chromium (III) concentration in their stomachs of 100
ppm(36). With higher
concentrations, toxic effects were noticed : the individuals were
less numerous and their size decreased. However, after extensive risk
evaluation, the US EPA has established guidance cleanup for chromium
(III) at land disposal sites of 78,000 mg/kg(37).
This limit value is related to the most relevant pathway risk which,
concerning chromium (III) and according to the US EPA, is ingestion
of contaminated soil. Over this limit value, the site owner can be
asked to perform a site specific risk evaluation and even to clean
up the site. As a conclusion, care should be taken to avoid high accumulation
which could produce harmful effects.
•
Vegetables and animals
- Many studies have been carried out to determine the impact of chromium
(III) compounds on different crops : chicory, wheat, peas, tomatoes,
fennel, etc., particularly with respect to the spreading of sludge
onto agricultural ground(38,
39, 40, 41). These studies concluded that chromium (III)
compounds can be considered as non-toxic in concentrations up to 500
mg/kg(38). Once again
care should be taken to avoid an accumulation of chromium (III) compounds
in the ground. No toxic effect was detected on rats consuming water
containing 25 mg/l of chromium (III) over a 6-month period(39).
•
Impact on the consumer
The
consumer, the end-user of articles made from chromium tanned leather
has a right to ask whether the use of such leather constitutes a health
risk. As we have already indicated(8),
unlike chromium (VI), chromium (III) compounds are only weakly allergenic.
The question, however, is whether there is a risk of the appearance
of chromium (VI) in leather articles. A French study has demonstrated
that under specific conditions of humidity, ultraviolet-C light and
pH, there is a possibility of transforming chromium (III) compounds
into chromium (VI) compounds which could migrate from the leather(40).
The values found were in the order of «not detectable»
to 17 mg of chromium (VI) per kg of dried degreased leather. As a
precautionary measure, the German authorities require(41)
that chromium (VI) content which could be leached should be below
the limits of detection and reproducibility of the analytical methods
available to industry , that is to say of the order of 2 to 3 mg/kg
of leather(42). Furthermore,
based on a similar precautionary principle, the European standard
for safety gloves, EN 420(43)
specifies a maximum limit for leachable chromium (VI) compounds, as
being 2 mg/kg of leather. However research is still needed to better
understand the transformation mechanism of chromium (III) to (VI)
in finished leather as well as the potential toxic effects to man
at this very low range of values.
•
The development of clean technologies
The
aforementioned studies did show some harmful effects on species living
in the ground. Accumulation into the environment should therefore
be avoided. Motivated by this objective and in order to economise
chemicals and water, tanners have developed manufacturing methods
whose aim is to reduce chromium emissions into the environment. These
efforts are often based upon methods for the optimisation of the use
of chromium, through the exhaustion of tanning baths and the re-use
of residual water, developed by research institutes and manufacturers
of chromium salts(44, 45).
The chromium exhaustion ratio in tanning floats, traditionally 70-80
% can attain 97 % with the new processes(17).
Effluent treatment plants are increasingly being installed throughout
the world in order to purify liquid waste, often providing a common
treatment service in highly concentrated tannery zones. In parallel,
work is being carried out concerning the recovery of chromium (III)
compounds from tanned waste and tannery sludge(46,
47).
•
Conclusion
Chromium
(III) salts in the hydroxysulfate form, remain an irreplaceable tanning
agent due to their ease of use and the
quality that they confer to leather. Studies
known today conclude that chromium (III) tanned leather produces no
toxic effect on the consumer. On
the other hand, regarding fauna and flora, high accumulation in the
environment is to be avoided. This is why tanners are concentrating
their efforts on the application of clean technologies and processes
for the recycling of effluent and solid waste containing chromium
(III).
•
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