Summary:
Attapulgite (palygorskite) clay, mined in the area of Grevena, Greece is a
natural material, environmentally friendly, readily available with proven
efficiency in environmental remediation of soils contaminated by heavy metals.
In this thesis, we present results of laboratory experiments on the adsorption
of Pb (II) and Cu (II) from standard aqueous solutions by comparing the
adsorption capacity of raw and thermally modified attapulgite.
Powdered attapulgite clay was supplied by Geohellas SA. The samples were
heat treated in muffle at temperatures of 130, 330, 500 and 800oC for 4 hours.
Mineralogical analysis by X-ray diffraction (XRD) before and after heat
treatment revealed that the crystal lattice of attapulgite gets destructed at
temperatures higher than 330oC. The heat treatment induces dehydration - loss
zeolitic H2O, at low temperatures (<150oC) and dehydroxylation at higher
temperatures (350-510oC), while the complete destruction of the clay occurs at
temperatures > 550oC. The comparative study of Pb and Cu adsorption efficiency
of heat treated and non-treated material, was performed for adsorbent dose from
2 to 15 g/L, initial metal concentration 50 mg/L, maximum shaking time 2h and
pH 4. The effect of ionic strength was studied by varying the concentration of
NaNO3 from 0 to 0.5 mol/L. All experiments were carried out in a chamber set at
22oC for Pb and 25oC for Cu.
At least 90% of Pb was retained by adsorbent dose of 5 g/L and 10 g/L
respectively for all tested materials, while an increase in the amount of dose
of adsorbent to 10 g/L for Pb and 15 g/L for Cu reached almost 100% retention
of all materials. An exception was observed in the instance of attapulgite
heated at 800oC, where 70% of Cu was retained by adsorbent dose of 10 g/L. The
optimum pH of standard aqueous solution was 4-6 for both metals and also for
all materials (thermally modified and raw). With the exception of attapulgite
clay heated at 800oC, maximum adsorption was achieved within the first 5 min of
reaction. A decrease in adsorption was observed as the concentration of NaNO3
was raised probably due to competition between Na+ and metal ions for
available adsorption sites.
Outer sphere complexation and ion exchange are probably the main adsorption
mechanisms at the studied pH. These data imply that significant changes in the
adsorption capacity of the used attapulgite clay occur after calcination in
temperatures > 550oC due to destruction of the crystal lattice of the material
and nanoporocity change.
Keywords:
Attapulgite, Heat treatment, Adsorption Pb, Cu
