Environmental Engineering Reference
In-Depth Information
equivalent to its CeC. The amount of the modified mmT adsorbed was maximum at pH ≤ 1 and decreased to almost half the
quantity at pH 2, and remained constant up to pH = 6. Above pH = 8, the amount adsorbed was negligible. And the pH dependence
of chromate adsorption was attributed to the pH-dependent equilibria HCr
2
O
7
−
⇋Cr
2
O
7
2−
⇋CrO
4
2−
. HdTmA was also used for
the modification of various minerals for the removal of chromate. The optimum pH for Cr(VI) adsorption was 4 in all the exam-
ined cases. The maximum adsorption capacity followed the order vermiculite (27 mg/g) > BnT (24 mg/g) > attapulgite (ATP)
(15 mg/g) > zeolite (13 mg/g) [35].
With regard to the Cr(VI) ion, several works were reported to compare the adsorption property of the clay minerals after
different treatments. It was found that the adsorption capacity of organo-modified clay was higher than that of acid- and heat-
treated ones [36, 37]. It indicated that organo-modified clay could be used as the adsorbent for Cr(VI) removal.
natural rectorite (ReC) was modified with the surfactant of dodecyl benzyl dimethyl ammonium chloride, HdTmAB, and
octadecyl trimethyl ammonium bromide via the cation exchange reaction. The three kinds of organic modified rectorites
(OReC) were used as adsorbents for Cr(VI) removal in aqueous solution [38]. They showed better adsorption of Cr(VI) at pH
range of 2-6, with maximum adsorption near 100% with the octadecyl trimethyl ammonium bromide-modified ReC within
40 min. And its maximum adsorption capacity (
q
m
) was calculated to be 3.57 mg/g.
stearyl trimethylammonium chloride (sTAC) was used for the modification of ReC via a similar procedure [39]. The
adsorption capacity of the modified ReC (sTAC-ReC) reached as high as 21 g/kg for Cr(VI), or 400 mmol/kg of chromate.
Higher Cr(VI) adsorption on the sTAC-ReC occurred in acidic solutions (pH 4), and the amount of adsorbed Cr(VI) decreased
rapidly with increasing pH. so the formation of the Cr(VI)-sTAC complex was inhibited during adsorption.
The halloysite nanotubes (HnTs) were modified with the surfactant of HdTmAB to form a new adsorbent. The modified
HnTs exhibited a rapid adsorption rate for chromates and approached 90% of the maximum adsorption capacity within 5 min
[40]. The regeneration of modified HnTs could be realized by an eluent (mixed solution of nanO
3
and naOH), and the recov-
ered adsorbent could be used again for Cr(VI) removal.
naidu et al. modified BnT with a commercially available, low-cost alkyl ammonium surfactant Arquad
®
2HT-75 (Aq) or
adsorbing Cr (VI) from aqueous solution [41]. It was found that more ordered solid-like conformation of surfactant molecules
was obtained in the organo-clay as the surfactant loading increased. And higher surfactant loadings provide better adsorption
efficiency.
Two hydrous phyllosilicates, a mmT, and a vermiculite were organo-modified with hexadecylpyridinium (HdPy),
HdTmA, and benzethonium (Be), and the Cr(VI) adsorption properties were investigated [42]. After the organic cation
treatment, the surface charge of the clays shifted from negative to positive values up to 269 mmol
c
/kg; it was highly affected
by the specific surface area, layer charge, and pH. depending on the kinds of the organic cations used, the adsorption capacity
of Cr(VI) was shown to have a comparable effect of 0.38 and 0.30 mol/kg for organo-vermiculites HdPy and HdTmA and
0.37 and 0.26 mol/kg for the corresponding BnTs at pH 4.5, respectively. for all organo-clays, the adsorbed amount of
Cr(VI) decreased with increasing pH. The effect of competing anions, Cl
−
, nO
3
−
, and sO
4
2−
, on Cr(VI) adsorption appeared
to be small.
Atia modified BnT with cetylpyridinium bromide (CPBr) for the removal of the oxyanions of Cr(VI) and mo(VI) from
aqueous solutions [43]. It was found that the surfactant was adsorbed on mmT in a bilayer structure, and the adsorption was an
ion exchange mechanism.
The cationic surfactant octadecyl benzyl dimethyl ammonium was used for the modification of BnT to remove As(V) and
As(III) from aqueous solution [44]. Its adsorption capacities were 0.288 mg/g for As(V) and 0.102 mg/g for As(III), which
were much higher than 0.043 and 0.036 mg/g for the unmodified BnT. The adsorption of As(V) and As(III) was strongly
dependent on solution pH. Addition of anions did not impact As(III) adsorption, while they clearly suppressed adsorption of
As(V).
majdan et al. modified BnT with octadecyltrimethylammonium bromide (OdTmA) for the sorption of u(VI) [45]. It was
reported that OdTmA-BnT exhibited high sorption affinity for u(VI) ions in a pH range of 6-10, and the most probable
anionic complex was the (uO
2
)
3
(OH)
7
−
ions.
Besides the quaternary ammonium salts mentioned earlier, alkylamine, diamines, and the amphoteric or other surfactants
containing an amino group have also been used for the organo-modification of clay minerals as adsorbents for HmI. guerra et
al. modified the BnT via an intercalation process with polar n-alkylamine molecules of general formula H
3
C(CH
2
)
n
-nH
2
(n = 1-4) for mercury removal from water [46]. It was found that the Hg(II)-nitrogen interactions was favorable for the adsorp-
tion of Hg(II).
dodecylamine-intercalated na-mmT was prepared as a novel adsorbent for the removal of chromium from tannery waste-
water [47]. The adsorption involved the electrostatic interaction of hydrogentetraoxochromate(VI) anion with the protonated
dodecylamine and the surface hydroxyl groups of the clay material with a adsorption capacity of 23.69 mg/g. The adsorbent
could be regenerated using naOH solution.
