Removal of heavy metals on a carbon sorbent prepared from flax shive
Carbon is prepared from flax shive by treatment with sulphuric acid. The optimised
conditions of preparation were based on metal sorption (Cd2+, Hg2+), carbon yield,
and acid and energy consumption. The carbon product retains its fibrous woody
texture and XRD indicates an amorphous structure which has a very low surface area
determined by nitrogen adsorption. Elemental analysis of the sorbent shows a very
low amount of sulphur (-1%), while FTIR shows the presence of carboxyl, carbonyl
and hydroxyl (or phenolic) groups.
Cadmium shows fast kinetics of sorption for this material reaching equilibrium within
3 hours and having a sorption capacity similar to the cation exchange capacity (CEC)
and base neutralisation capacity data. On the other hand mercury shows slow kinetics
with 120 hours equilibrium time. Sorption capacity for mercury was high compared to
that of cadmium or the cation exchange capacity. Base neutralisation capacity shows
that the sorbent prepared at 200°C (C200) possesses more carboxylic and hydroxyl
groups than that prepared at 160°C (C 160) and agrees with the cation exchange
capacity data and cadmium sorption.
A comprehensive study for cadmium on C200 sorbent found that sorption increases
with pH and decreases with the presence of other metal ions in the aqueous solution.
Mechanism of sorption investigated by the ratio of [H+]/[Cd2+] on a molar basis gave
a value -2 indicating an ion exchange mechanism. Other metals such as Coe+, Cri+,
Cue+, Nie+, Pb2+ and Zn 2+ were found to behave in a similar way to cadmium. These
metals show fast kinetics with C200 reaching equilibrium within 3 hours. The ratio of
metal sorbed on C160 to that on C200 is similar to that of cadmium and also to the
ratio of CEC C 160/CEC C200. Such results suggest that these metal ions are sorbed
via an ion exchange mechanism onto sites such as carboxylic and hydroxyl groups.
Sorption was found to fit the Langmuir equation with an insignificant increase with
temperature. Column studies proved that C200 can be used several times to sorb Cd 2+
from aqueous solution and using 0.5 mol/dm3 sulphuric acid as a stripping solution.
On the other hand, mercury behaves differently showing, in addition to slow kinetics,
a much higher uptake. This suggests that mercury sorption is not only an ion
exchange mechanism. Such slow kinetics were found to follow a first order rate
equation and the sorption data also fits the Langmuir equation. Wet samples showed a
higher sorption than one that had been previously dried. Other metals such as Au (III),
Cr (VI), Pd (II), Ag (I), Pt (II) and Pt (IV) were found to behave similarly to Hg (II)
following the Langmuir equation with higher uptake with increasing temperature and
also showing slow kinetics. The kinetics of uptake of these metals follows the first
order rate equation and during the sorption process these metals were reduced. In
addition, Mn04_ was converted to Mn02 and Mn2+; Fe 3+ to Fee+and Cr6+ reduced to
Cr3+ which was then sorbed via an ion exchange process. Also, and depending on the
initial pH, the kinetics of sorption of Cr6+ was found to follow a second order rate
equation at pH 4.5, while starting at pH 1.5, chromium reduction follows a first order
rate equation. Sorption of this group of metals depends on the pH of the aqueous
solution. Hg 2+ sorption shows maximum uptake in the pH range 6-7 while Cr6+ shows
maximum sorption within pH range 2.2-2.6 depending on the sorbent state (wet, dry)
and the concentration of Cr6+. Reduction of Hg2+ to Hg2CI2 and elemental mercury
was confirmed with the observation of deposits on the carbon surface from SEM
photographs. X-ray powder diffraction (XRD) showed that the crystals formed on the
carbon surface are mercury (I) chloride. Some other metal species such as AuC14",
PdC12,A gNO3 and PtC142"showedre duction on the carbon surface to their elemental
forms as observed on the SEM photographs and confirmed by XRD. Reduction of the
metals was followed by the formation of new carbon-oxygen groups on the surface
and also evolution of carbon dioxide. PtC162' does not show any reduction on the
carbon surface suggesting the possibility that the carbon reduction potential is perhaps
very close to +0.7V.
This material seems to be an efficient alternative for activated carbon sorbents to
remove and recover heavy metals from waters, with added advantages for those
metals which can be reduced to the elemental form.