Chromate and Arsenate Removal by Layered Double
Hydroxides-Polymer Beads
Nguyen Thi Kim Phuong Institute of Chemical Technology Vietnam
Academy of Science and Technology Layered Double Hydroxides
(LDHs):
Naturally occurring anionic clays: [M1-x2+ Mx3+ (OH)2]x+ (An-)x/n:
yH2O Due to their high specific surface areas, high anion exchange
capacities and flexible interlayer space remove negatively charged
species. - Use of LDHs in the fine powder forms requires follow-on
solid/water separation with substantially added cost. - Easy to
remove from the aqueous media LDHs may be one of the most potential
candidates. - So far, various forms of LDHs (LDHs coated
sand/zeolites, support on cellulose) have been developed. -
Recently, entrapment of Functional Materials within biopolymer
matrix are used very often because of their economic advantages,
high efficiency, easy handling and reusability Synthesis Mg-Al LDH
and Mg-Fe LDH
Cl- Mg2+Al3+/Fe3+ 450 oC 4h 65 oC, 24 h Preparation of LDHs beads
(LDHs = Mg-Al and Mg-Fe)
100 mL of polymer (1 g Alginate, 0.5 g PVA and 0.5 mL
Glutaraldehyde) + 8 g of LDHs CaCl2 solution LDHs beads (beads cure
24 h in CaCl2 solution The as-prepared beads (a) Blank, (b) 8%
Mg-Aland (c) 8% Mg-Fe (a) blank; (b) Mg-Al and (c) Mg-Fe (a) blank;
(b) Mg-Al and (c) Mg-Fe
XRD patterns of beads (a) blank; (b) Mg-Al and (c) Mg-Fe SEM study
of beads (a) blank; (b) Mg-Al and (c) Mg-Fe - For the initial conc.
of 100 mg/L of CrO42-, pH = 7
Removal efficiency = %, Adsorption capacity = mg/g LDHs beads (with
8% LDHs) mg/g LDHs powder - For the initial conc. of 100 mg/L of
AsO43-, pH = 8 Removal efficiency = %, Adsorption capacity = mg/g
LDHs beads (with 8% LDHs) mg/g LDHs powder - The adsorption
capacity of the LDHs beads decreased as the number of regeneration
cycles increases, however, the adsorption capacity of the LDHs
beads was decreasedabout % during a 5 adsorption-desorption cycle.
Adsorption kinetics Lagergren 1st: Pseudo 2nd: where
qt (mg Cr. g-1)- amount of chromate/arsenate removed at time t; qe
(mg Cr. g-1)- amount of chromate/arsenate removed at equilibrium;
k1 (h-1)-Lagergren first-order rate constant; k2 (g. mg-1. h-1)-
Pseudo second-order velocity constant. CrO42- AsO43- Mg-Al beads
Mg-Fe beads qe, exp(mg. g-1) 3.0575 3.1490 3.0032 2.5933 Lagergren
1st qe(mg. g-1) k1(h-1) 1.7531 0.2540 2.2070 0.3627 2.3126 0.3441
2.5200 0.4060 Pseudo 2nd k2(g. mg-1. h-1 ) 3.2373 0.2734 3.3124
0.3188 3.0750 0.3707 2.6350 0.3729 Adsorption kinetics Adsorption
isotherm Langmuir: Freundlich: where
qm (mg Cr. g-1)- monolayer surface coverage of adsorbents
bychromate/arsenate; Ce (mg Cr. L-1)- conc. of chromate/arsenate in
the solution at equilibrium; qe (mg Cr. g-1)- amount of
chromate/arsenate removed at equilibrium; KL (L. mg-1)- Langmuir
constant related to the binding energy; Kf (L. g-1)-the
distribution coefficient; n - Freundlich constant . CrO42- AsO43-
Mg-Al beads Mg-Fe beads Langmuir qm(mg. g-1) KL(L. mg-1) 3.2819
0.6266 3.0303 1.0436 3.2144 0.4574 3.0479 0.1464 Freundlich 1/n
KF(L. g-1) 0.2518 1.2679 0.2980 1.1484 0.4073 0.8478 0.4827 0.4694
Adsorption isotherm Despite of the adsorption in batch systems
understand the pollutants/adsorbents interaction and to select the
best operational condition. - The fixed-bed columns for the
adsorption application in the industrial scale-up once that the
process can be performed continuously. - This operational mode is
more appropriate for large-scale applications in industry than
other types of reactors as such agitated tanks, fluidized-bed
columns, etc. - The fixed-bed columns have a series of advantages:
simple operation, large yields and enhancement of effluent water
quality Initial conc. = 5.0 mg/L of Cr or As
Column ID = 2.5 cm; Bed depth = 40 cm; Flow rate (Q) = 3.0 L/min
where Ct and Co (mg/L) are the effluent and influent Cr or As
conc.; V (cm/h) is the linear flow velocity; x (cm) is the bed
depth; K (L/(mg.h)) is the kinetic constant; N is the maximum
adsorption capacity (mg/L); xo (cm) is the minimum column height
required to produce an effluent conc. Cb (breakthrough conc., 0.05
mg Cr/L or 0.01 mg As/L). Breakthrough curve - Column ID = 2.5 cm
and bed depth = 40 cm;
- Volumetric flow rate (V) = cm3/(cm2.h) or Q = 3.0 L/min - C0 =
5.0 mg/L of Cr or As - Cb = 0.05 mg/L for Cr and 0.01 mg/L for As -
CE = 4.5 mg/L of Cr or As Parameter Chromate Arsenate Mg-Al beads
Mg-Fe beads K (L/mg.h) 0.023 0.035 0.020 0.025 N (mg/L) 279.90
195.13 325.72 283.78 Breakthrough volume VBT (L) 2.7 1.8 1.44 1.08
Breakthrough timetBT (h) 15 10 8 6 xo (cm) 26.48 24.96 35.05 32.18
Analysis of column data
Total quantity of chromate/arsenate bound to adsorbents in a
fixed-bed column, qtotal (mg) where Q (mL/min) is volumetric flow
rate; ttotal (h) is total time of flow till exhaust; C0 (mg/L) is
initial conc. of chromate/arsenate; C (mg/L)is conc. of
chromate/arsenate in the effluent and m (mg) is the total amount of
adsorbents in column. Total amount of chromate/arsenate sent to
column, Mtotal (mg): % removal by column: Adsorbent Chromate
Arsenate Mtotal (mg) qtotal(mg) Total removal (%) Mg-Al beads 55.95
48.28 86.30 60.30 53.97 89.50 Mg-Fe beads 44.57 40.81 91.57 52.32
47.46 90.71 - To operate fixed-bed adsorption processes, the
concept of the Mass Transfer Zone (MTZ) proposed by Michaels was
applied. - MTZ is the layer between the equilibrium bed zone (used
bed zone) and the unused bed zone. During the process, as the feed
solution containing the chromate passes through the fixed-bed of
packed material, the MTZ moves in the direction of the flow and
reaches the exit. The height hz of the MTZ (cm):
where tz (min) is the time required for MTZ to move through its own
length up the bed; tE (min) is the time required for MTZ to become
established and move completely out of the bed; tf (min) is the
time needed for MTZ formation; Uz (cm/h) is the rate of the
movement of the MTZ along the length of bed. The rate of the
movement of the MTZ is a function of adsorption capacity of the
adsorbent. It is directly related to the height of MTZ. The times
tz, tE and tf are given by the following expressions: Parameter
Chromate Arsenate Mg-Al beads Mg-Fe beads tz (h) 44 37 60
F is the parameter measuring the symmetry of the breakthrough
curve: where, Sz (mg) is amount of chromate/arsenate that has been
removed by the adsorption zone from breakthrough to exhaustion,
Smax (mg) is amount of chromate/arsenate removed by the adsorption
zone if completely exhausted. The percentage of saturation of the
column in the breakthrough point is: Parameter Chromate Arsenate
Mg-Al beads Mg-Fe beads tz (h) 44 37 60 53 hz (cm) 33.80 39.93
38.13 39.29 Uz (cm/h) 0.77 1.08 0.64 0.74 Bed saturation (%) 86.70
73.20 91.97 90.65 Conclusions -Hybrid sorbent, LDHs beads satisfy
the need for a cost-effective, reliable, reusable materials and
easy to separate from the effluent water. This combines the
excellent handling and readily applied to fixed-bed adsorption
reactors in industry. -The removal efficiency was range % for
CrO42- and range % for AsO43-. The adsorption ability of LDHs beads
was decreased about 5-6 % during a 5 adsorption-desorption cycle. -
Adsorption mechanism follows the pseudo-second-order kinetic model
and adsorption data fitted well to a Langmuir isotherm. -In the
column study, the breakthrough time was found to be from h for
CrO42- and from 6-8 h for AsO43-. This results will be useful for
its further extension to field scale or for designing pilot plant
as future studies LDHs beads should be a promising adsorbent for
application to chromate and arsenate decontamination technology.
Thank you!!!!