As a consequence of mining, heavy metal ions can be exposed to the environment hence contaminate ground water and surface water amongst others. The natural polymer chitosan was proved to be an excellent adsorber material for the effective removal of iron and sulfate ions in batch as well as in column experiments. The adsorption behavior of iron ions, as well as sulfate ions was investigated by utilizing chitosan flakes as a natural adsorbent. The removal was studied using adsorbance measurements, SEM and SEM-EDX. The adsorption capacity of chitosan was determined at different times. The received adsorption capacities for iron ions were very promising with a maximum adsorption capacity of 85 mg/g and a rate of separation of 100%. The maximum adsorption capacity obtained for sulfate ions was 188.8 mg/g and a rate of 80%.
Since approximately 100 years, brown coal is demounted in different region of Germany. Because the decommissioning and flooding of many open pit mining the groundwater increase, iron salts and iron hydroxide are washed out from tipping areas as well as from naturally occurring pyrite layers. This leads to a considerable pollution of the surface water (i.e. iron hydroxide deposition) and causes problems with the drinking water abstraction due to the high iron concentration accordingly [
In waste water treatment, traditional coagulation and flocculation processes are suitable. Inorganic flocculants (e.g. aluminum hydroxide, or iron chloride) as well as synthetic water soluble polymers (i.e. polyelectrolytes such as polyacrylamide for example), or natural polymer (e.g. starch, or chitosan) are used [
Furthermore, adsorbents such as activated carbon or ion exchange resins are used for the treatment of waters and waste water. Synthetic adsorbents exhibit high adsorption capacities having the disadvantage of high production costs [
For this reason, the quest for effective and cost-efficient adsorbents for heavy metal ions has become a major topic in research during the last years. Industrial waste products mainly arising from agriculture are potentially interestingly adsorber materials due to their high abundance. These kinds of adsorber materials are many called biopolymers [
Chitin and its deacetylated derivate chitosan are two kinds of biopolymers (i.e. polysaccharides) which are obtained from crustacean shells and exhibit the capability of adsorbing a multiplicity of heavy metal ions. The strong affinity for heavy metal ions is attributed to the high impact of nitrogen in the polymer matrix in the form of primary amino groups. The capabilities of chitosan are sufficiently established in literature [
The adsorption properties of chitosan as adsorber material were investigated in batch and column experiments with respect to the binding mechanism of the heavy metal ions. Up to our knowledge, this is the first publication focusing on the adsorption potential of cation and anions simultaneously.
The adsorption properties of chitosan with respect to the anion sulfate and the cation iron were analyzed. The adsorption process was investigated in dependence of time and concentration of the two ions. The two contrary charged ions were examined as iron sulfate and for comparison as sulfuric acid without the influence of the iron ion in solution. The results can be outlined as:
・ Adsorption isotherm of iron sulfate and sulfate on chitosan, respectively.
・ SEM-EDX measurements of pure chitosan in comparison with chitosan after the adsorption process.
SEM-EDX measurements shall contribute the understanding of the adsorption mechanism of both contrary charged ions on the surface.
Chitosan in form of powder or flakes with a deacetylation degree of 90% (product name Ch90/200/A1) from the company BioLogHeppe GmbH were used for the experiments. The chitosan powder exhibited a D50 value of 200 µm obtained by laser diffraction measurements in deionised water. Chitosan flakes possessed a broad particles size distribution within the mm range. All investigations were performed in batch and column experiments. The iron(II) concentrations of the iron sulphate solutions varied between 0.04 mg/L and 1300 mg/L. Sulfuric acid was used with a concentration of 0.9 g/L and 0.5 g/L with a resulting pH value 2.02 and 2.25 respectively. 30 mL of the diluted sulphuric acid solutions were added to 0.1 g of chitosan for the batch experiments at r.t. The concentration of the iron sulphate solutions were 1 g/L for the batch experiments. The adsorption time (tads) of the experiments was 24 hours hence within the adsorption equilibrium. Additionally has the adsorption time been varied by constant salt concentration to determine the adsorption equilibrium.
The assignment of the heavy metal and anion concentration in solution was carried out with a DR6000 spectrophotometer from the company HACH Lange GmbH, Germany. The DR6000 microprocessor controlled spectrophotometer with a reference beam and a wave length region of 190 to 900 nm. The instrument is appropriate for routine analysis and distinct applications (programmable). Particular associated cuvette tests for each ion are provided from the company. The instrument recognizes the specific cuvette test with a bar code on each standardized cuvette.
Chitosan exhibited excellent results as a flocculation agent in previous examinations already [
In Scheme 1, the proposed adsorption mechanism of the iron sulfate ions on the chitosan surface is displayed. For a high content of atmospheric oxygen in water, iron(II) gets easily oxidized in solution to iron(III) oxide and iron(III) oxide-hydroxide. This is manly the case for the batch experiments were the chitosan flakes were stirred with the solution in an open baker. Hence, iron(II) and iron(III) oxide is concurrently adsorbed on the chitosan surface. The content of atmospheric oxygen seems to be lower for the column experiments leading to a higher adsorption of iron sulfate on the chitosan surface. However, a quantitative statement about the two possible iron species on the chitosan surface will be an important consideration in our nearby investigations.
The adsorption efficiency of iron(II) ions on chitosan powder and flakes is displaced in
Scheme 1. Schematic diagram of the adsorption mechanism of iron sulfate on chitosan.
The adsorption equilibrium for the adsorption of iron and sulfate ions has been reached after 24 hours.
In general, sulfate ions were less adsorbed than iron ions. After 24 hours, 99% (ceq/c0 = 0.01) of the iron ions were adsorbed in comparison with an adsorption efficiency of only 60% (ceq/c0 = 0.4) of the sulfate ions. Previous adsorption investigations of copper sulfate on chitosan showed an adsorption ratio of 1:1 for the two contrary charged ions [
where c0 is the initial concentration of the metal ion (Fe(II) ions) in solution, ce is the metal ion concentration at equilibrium, V is the volume of the metal salt solution, and m the mass of the adsorbent.
The maximum adsorption capacity of iron ions on chitosan flakes is 85 mg/g. In comparison, chitosan powder exhibits a higher adsorption capacity due to larger surface area.
Sulfate ions should undergo a strong interaction with the chitosan surface based on the protonated amine functionality. Therefore, the adsorption of diluted sulfuric acid on chitosan powder has been investigated (s.
Sample | pH-value | Max. adsorption | |||
---|---|---|---|---|---|
Before adsorption | After adsorption | Before adsorption | After adsorption | ||
0.9 g/L (≙ 0.092%) | 2.02 | 2.77 | 853 mg/L (≙mA/mC** 256 mg/g) | 240 mg/L (≙mA/mC** 189 mg/g) | 73% |
0.5 g/L (≙ 0.051%) | 2.25 | 5.40 | 511 mg/L (≙mA/mC** 153 mg/g) | 100 mg/L (≙mA/mC** 125 mg/g) | 79% |
*at the adsorption maximum; **mA/mC**: ratio mass anion/mass chitosan.
Furthermore, the adsorption analysis was supported by SEM images of untreated chitosan powder and flakes as well as after the adsorption process to get a better understanding of the adsorption mechanism.
On the surface of the chitosan flakes can be crystal like structures observed arising from the adsorption of the iron sulfate salt. The initial concentration of iron sulfate varied from 0.05 g/L, 1.0 g/L, 3.0 g/L, and 5.0 g/L (from left to right hand side in
The SEM image of chitosan after the adsorption process with the corresponding elemental distribution of Fe and S from EDX measurements can be seen in
However, for iron sulfate an equal elemental distribution can only be observed for sulfur (see
Additionally to the batch studies, adsorption investigations in the column were performed. Chitosan flakes was packed into a column and treated with an iron sulfate solution. Iron sulfate solution was transported through column at a speed of 80 U/min by a peristaltic pump. The solution leaving the column was collected and analyzed regarding the changes in sulfate and iron concentration.
Similar to the batch experiments, a coloration of the chitosan in the column can be observed (see
brown color is expected to arise due to the oxidation of the iron(II) ions to iron(III) oxide. On the upper part (in the direction of flow) of the column can the characteristic color of iron sulfate be observed. The separation two types of colors might be due to a gradient of atmospheric oxygen in the column. After an adsorption time of 3 hours, the column is completely colored. The color change as it can be seen in
The ratio between the iron concentration in solution before and after the column experiment in dependence of the adsorption time is represented in
Within the first hour of the column experiment, 100% of iron ions and 40% of sulfate ions were adsorbed. After 3 hours, minor amounts were adsorbed and a ratio for ceq/c0 of 0.7 for iron (i.e. 30% of iron ions were adsorbed) and 0.8 for sulfate ions (i.e. 20% of sulfate ions were adsorbed) can be observed.
As the adsorption can manly be observed as a crystallization process on the surface of the chitosan material we want to improve the chitosan towards lower material costs in the successive investigations.
Chitosan as an adsorber material has the ability to adsorb the positively charged, as well as the negatively charged ion of a heavy metal salt. This study indicates the adsorption of sulfate, as well as of iron ions. In batch experiments, the removal of iron ions occurs almost completely and sulfate could be removed with an efficiency of 60%. At initial concentrations of 1.0 g/L iron sulfate, an adsorption time of 24 hours was necessary. On the basis of the coloration of the original white adsorber material towards dark brown, an oxidation of the iron(II) ions to iron(III) oxide was proposed. The oxidation occurs due to the atmospheric oxygen in the water. Hence, the adsorption mechanism is competitive of iron(II) sulfate and iron(III) oxide. Additionally, the adsorption of iron sulfate on chitosan was investigated in a column experiment. Similar to the batch experiments, iron could be removed by nearly100% efficiency and 40% for sulfate ions. After 3 hours, a constant removal rate of 30% iron ions and 20% sulfate ions was achieved. Biopolymers like chitosan can be applied as non-hazardous substances.
The company BioLogHeppe GmbH is acknowledged for the supply of the chitosan powder and flakes applied in this investigation.
Simona Schwarz,Christine Steinbach,Dana Schwarz,Mandy Mende,Regine Boldt, (2016) Chitosan—The Application of a Natural Polymer against Iron Hydroxide Deposition. American Journal of Analytical Chemistry,07,623-632. doi: 10.4236/ajac.2016.78058