(427d) Speciation of Chromium Adsorbed On Activated Carbon and Stabilization of Lime Precipitated Chromium Residue With Activated Carbon

Speciation of Chromium adsorbed on Activated Carbon
and Stabilization of lime precipitated chromium residue with Activated Carbon

Chromium-containing waste residue is a
hazardous industrial solid waste due to the migration of toxic chromium into
the environment. It seems urgent for the further treatment of chromium-containing
waste residue in China
at present. Compared with other methods for treatment of chromium-containing
waste residue, stabilization/immobilization technique is characterized by
cost-effective, easier operation and in-situ application available by addition
of additives. In this work, the modified activated carbon was employed as an
additive to reduce the leaching of
chromium so as to achieve the stabilization of the lime precipitated
chromium residue(LPCR). Also, the leaching potential
of Cr adsorbed on modified activated carbon was evaluated by a five-step
sequential extraction procedure. The information will be helpful for the
establishment of a simple, economical and feasible disposal method for the chromium-containing waste residue.

The LPCR sample was collected from Wuhan Iron
and Steel (Group) Corporation. The contents of total Cr and hexavalent
chromium in LPCR were 24000mg/kg and 1856mg/kg. Activated carbon sample (AC)
was modified from commercial activated carbon with 20wt% HNO₃ at 20
ºC. The Cr-laden activated carbons for the sequential extraction were prepared by
static adsorption of Cr(III) ions and Cr(VI) ions
respectively from standard solution with modified activated carbon..

The extraction conditions of the five-step
sequential extraction procedure employed to investigate the fractions of chromium
in LPCR and those of trivalent chromium(Cr(III)) and hexavalent
chromium(Cr(VI)) adsorbed on activated carbon were listed in table 1. The
contents of total chromium and Cr(VI) in the leachate were determined by spectrophotometry
with diphenylcarbazide as chromogenic
agent at a wavelength of 540nm according to GB/T 15555.5-1995 and GB/T
15555.4-1995.

The distribution of Cr in LPCR and Cr(III) and Cr(VI) adsorbed on AC were listed in table 2. It
is considered that heavy metal present as the residual of the five fractions
can be expected to be the most stable. The results of the sequential extraction
showed that the residual fraction of total Cr and Cr(VI) in LPCR are 74.89% and
49.32%, which indicated that the unstable fractionations of chromium especially
Cr(VI) in LPCR have some environmental mobility. Nearly 94% of adsorbed Cr(III) on AC was present as the most stable residual
fraction while the fractions of exchangeable and bound to carbonates only accounted
for 0.44%, suggesting the possible chelation of Cr(III)
with the surface oxygen groups on the modified activated carbon. Adsorbed Cr(VI) on AC seemed less stable than Cr(III) with 71.35%
residual fraction. This maybe result from the selective
adsorption of the
surface binding sites to the different charged hydrolyzed Cr(III) and Cr(VI). Therefore, there
seems a need to study the chemical interaction between the surface functional
groups of activated carbon and heavy metals. The enhancement of adsorption via
chemical interaction is expected to enhance the stability of heavy metals.

Batch extraction and continuous column
leaching experiments were carried out for mixtures of LPCR with activated
carbon to investigate the leaching behavior of Cr and the effectiveness of
activated carbon as an additive. The batch extraction was performed in an
over-end-over rotator at 30rpm. The leaching experiments were conducted using a
series of LPCR/LPCR-AC packed columns (30mm-ID, 350mm-height). The leaching
solutions with different pH values flowed through the columns. A significant
decrease (54% for total Cr, 52% for Cr(VI) ) on Cr was
observed through activated carbon addition at 1:10 additive: LPCR ratio. The
results confirmed the positive effect of activated carbon on the stabilization
of Cr in LPCR.

Table 1 Extraction conditions of
the five¨Cstep sequential extraction procedure

Table 2 Distribution of Cr in LPCR
and AC by five-step sequential extraction (%)