Production of biosorbents from waste olive cake

Chapter 1 - Introduction
The presence of heavy metals in the environment is an important issue because of their 
toxicity and threat to human life and environment itself. Heavy metal contamination exists 
in aqueous waste streams of many industries, such as metal plating facilities, mining 
operations, and tanneries. Chromium is one of the metals associated with these activities.
Chromium, one of the prior heavy metal pollutants, occurs in two stable oxidation states in 
aqueous solutions, Cr (VI) and Cr (III), and their properties are very different. Cr (VI) 
species, having mobile and strongly oxidant characters, are known as mutagen and 
potential carcinogen. In contrast, Cr (III), having a limited hydroxide solubility and toxicity 
is generally regarded as non-pollutant. Because of these dramatic differences in physical 
and chemical properties of two chromium types and benign character of Cr (III), 
detoxification and immobilization process of Cr (VI) is based on its reduction to Cr (III). 
Many different processes have been investigated for removing chromium from aqueous 
solutions. Treatment processes include chemical precipitation, membrane filtration, ion 
exchange, carbon adsorption, and co precipitation/adsorption. For Cr (VI) removal, the 
reduction-precipitation technique is abundantly practiced. In this technique, Cr (VI) is first 
reduced to Cr (III) and precipitation of insoluble chromium hydroxide is a final step in 
removal process. 
But these traditional techniques are incapable of reducing concentration to the levels 
required by law or are prohibitively expensive. In recent years the most promising 
alternative method for removal of these metal ions uses the sorption by waste materials, 
from industrial or agricultural operations, which are low cost and abundant. Biosorption is 
known as the removal of heavy metals by the passive binding to non-living biomass from 
an aqueous solution.
Recently, olive oil wastes have been tested as biosorbents of heavy metals with promising 
results. At present, this agricultural waste material obtained from the olive oil industry is 
used principally as a solid fuel and is available in abundance in the Mediterranean Region. 
The total production of olive oil reaches 1.75 million tons in the European Union, being the 
worldwide production around 2.3 million tons. Spain is leader in this field, with a 30% 
1
worldwide production, followed by Greece and Italy (MADRP). The olive-oil industry 
generates a big amount of wastes that are problematic due to its high content of organic 
carbon (cellulose, hemicellulose, lignin and polyphenols). Conversion of these wastes to 
useful adsorbents contributes not only for the treatment of contaminated waters and 
wastewaters but also to minimize the solid wastes.
In this context, aim of this study is to test different biosorbent materials prepared from 
waste olive cake, on the removal of Cr (VI) ions from aqueous solutions. The influence of 
several operating parameters, such as, particle size, different chemical treatment, contact 
time, pH, initial concentration and adsorbent dose was studied.
2
Chapter 2 - Chromium
Chromium was discovered in 1797 by the French chemist Louis Vaquelin. Its name comes 
from the Greek chroma, “color”, because of the many different colours found in its 
compounds. Chromium is the earth's 21
st
 most abundant element (about 122 ppm) and the 
sixth most abundant transition metal. The principal chromium ore is ferric chromite, 
FeCr
2
O
4
, found mainly in South Africa (with 96% of the world's reserves), Russia and the 
Philippines. Less common sources include crocoite, PbCrO
4
, and chrome ochre, Cr
2
O
3
. The 
gemstones emerald and ruby owe their colors to traces of chromium. (Mohan and Pittman, 
2006)
Chromium occurs in different oxidation states but only Cr (III) and Cr (VI) are stable 
enough to occur in the environment (Kotas and Stasicka, 2000; Saha and Orvig, 2010). 
Cr(III) predominates at pH < 3.0. At pH > 3.5, hydrolysis of aqueous Cr(III) produces 
mononuclear species CrOH
2+
, Cr(OH)
2
+
, Cr(OH)
4
-
, neutral species Cr(OH)
3
0
 and 
polynuclear species Cr
2
(OH)
2
 and Cr
3
(OH)
4
5+
. Trivalent chromium is insoluble and thus 
immobile under ambient conditions and is classified as a hard acid, forming relatively 
strong complexes with oxygen and donor ligands that precipitate as amprphus hydroxide. 
(Mohan and Pittman, 2006; Cheung and Gu, 2007)
Chromium(VI) is known to have more toxicity than Cr (III) because of its high solubility 
and mobility, as well as easy reduction. Chromate and dichromate are the most soluble, 
mobile and toxic forms of hexavalent chromium. Under aerobic conditions hexavalent 
chromium is rapidly reduced to trivalent chromium which can form insoluble trivalent 
hydroxides and oxides that cannot leach. (Mohan and Pittman, 2006; Cheung and Gu, 
2007; Saha and Orvig, 2010 )
Depending on the pH, hexavalent chromium exists primarily as salts of chromic acid 
(H
2
CrO
4
), hydrogen chromate ion (HCrO
4
-
) and chromate ion (CrO
4
2-
). H
2
CrO
4 
predominates at pH less than about 1.0, HCrO
4
-
 at pH between 1.0 and 6.0, and CrO
4
2-
 at 
pH above about 6.0. When the concentration of chromium exceeds approximately 1g/L, 
dichromate ion (Cr
2
O
7
2-
), a dimer of HCrO
4
-
, is formed. (Dionex, 1996; Mohan and 
Pittman, 2006)
Trivalent chromium is an essential trace element in mammalian metabolism. It is 
responsible for reducing blood glucose levels, and is used to control certain cases of 
diabetes, in addition to insulin. It has also been found to reduce blood cholesterol levels by 
3
diminishing the concentration of (bad) low density lipoproteins “LDLs” in the blood. 
Cr(III) is supplied in a variety of foods such as Brewer's yeast, liver, cheese, whole grain 
breads and cereals, and broccoli. Chromium is claimed to aid in muscle development: 
dietary supplements containing chromium picolinate (its most soluble form) are very 
popular with body builders. In contrast, Cr(VI) is hazardous by all exposure routes (Mohan 
and Pittman, 2006).
Acute exposure to Cr(VI) causes nausea, diarrhoea, liver and kidney damage, eardrum 
perforation, dermatitis, internal haemorrhage and respiratory problems. Inhalation may 
cause acute toxicity, irritation and ulceration of the nasal septum, asthma, bronchitis, 
pneumonias, inflammation of larynx and liver and increased incidence of bronchogenic 
carcinoma. Ingestion may affect kidney and liver functions. Skin contact can produce skin 
allergies, dermal necrosis and dermal corrosion and may result in systemic poisoning 
damage or even severe burns, and interference with healing of cuts or scrapes. 
Furthermore, Cr(VI) can accumulate in the placenta, impairing fetal development in 
mammals. Thus, the United States Environmental Protection Agency (USEPA) has laid 
down the maximum contaminant level for Cr(VI) in domestic water supplies to be 0.05 
mg/L. Almost every regulatory agency has listed Cr(VI) as a priority toxic chemical for 
control and the USEPA has identified it as one of the 17 chemicals posing the greatest 
threat to humans (Kotas and Stasicka, 2000; Mohan and Pittman 2006; Cheung and Gu, 
2007; Saha and Orvig, 2010)
The toxic properties of Cr(VI) arise from the possibility of free diffusion across cell 
membranes and strong oxidative potential. The action of this form itself as an oxidizing 
agent, as well as the formation of free radicals during the reduction of Cr(VI) to Cr(III) 
occurring inside the cell, originates the toxicological impact of Cr(VI). Cr(III) formed there 
in a significant concentration can cause further adverse effects because of its high 
capability to coordinate various organic compounds resulting in inhibition of some metallo-
enzyme systems. (Kotas and Stasicka, 2000; Saha and Orvig, 2010)
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2.1 Chromium chemistry
Chromium (Cr) can exist in several chemical forms displaying different oxidation numbers. 
The only two forms stable enough to occur in the environment are trivalent and hexavalent 
Cr. (Kotas and Stasicka, 2000; Saha and Orvig, 2010). Figure 2.1 shows the Frost diagram 
for Cr species in acidic solution.
The most stable oxidation state is Cr(III) (Figure 2.1) and converting it to lower or higher 
oxidation states would require considerable energy. Cr(VI) in acidic solution demonstrates 
a very high positive redox potential (Figure 2.1) which denotes that it is strongly oxidizing 
and unstable in the presence of electron donors. (Kotas and Stasicka, 2000)
Figure 2.1.: The Frost diagram for chromium (Cr) species in acidic solution. (Kotas and 
Stasicka, 2000)
pH and redox potential play a decisive role in the equilibrium between Cr(III) and Cr(VI) 
in aerated solutions (Figure 2.2).
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Figure 2.2.: Eh-pH diagram for chromium (Mohan and Pittman, 2006)
At low pH’s, HCrO
4
-
 reduction to Cr(III) is accompanied by the H
+
 consumption (Figure 
2.2, Kotas and Stasicka, 2000; Mohan and Pittman 2006):
HCrO
4
-
 + 7H
+
 + 3e
-
    Cr
3+
 + 4H
2
O (Eq. 2.1)
In more basic solution the reduction of CrO
4
2- 
to Cr (III) generates OH
-
 (Figure 2.2, Kotas 
and Stasicka, 2000; Mohan and Pittman 2006):
CrO
4
2-
 + 4H
2
O + 3e
-
    Cr(OH)
3
 + 5OH
-
(Eq. 2.2)
Cr(III) remains as hexa aqua ion in aqueous solution and its hydrolysis products. 
Cr(H
2
O)
6
3+
 is a moderately strong acid (pK ~4) and its deprotonated forms formulated 
shortly CrOH
2+
.aq, Cr(OH)
2
+
.aq and Cr(OH)
3
.aq are dominating successively within pH 4-
10. The last two forms are expected to be dominant forms of Cr(III) in the environment. At 
higher pH, Cr(OH)
3
.aq is transformed into the readily soluble tetrahydroxo complex 
Cr(OH)
4
-
. (Figure 2.2, Kotas and Stasicka, 2000; Mohan and Pittman 2006; Saha and 
Orvig, 2010):
Cr(H
2
O)
6
3+ 
+ H
2
O  Cr(OH)(H
2
O)
5
2+ 
+ H
3
O
+
(Eq. 2.3)
Cr(OH)(H
2
O)
5
2+ 
+ H
2
O  Cr(OH)
2
(H
2
O)
4
+ 
+ H
3
O
+
(Eq. 2.4)
Cr(OH)
2
(H
2
O)
4
+ 
+ H
2
O  Cr(OH)
3
.aq
 
+ H
3
O
+
(Eq. 2.5)
Cr(OH)
3
 (s)
 
+ 2H
2
O  Cr(OH)
4
- 
+ H
3
O
+
(Eq. 2.6)
6