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-Al and (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 = 90.0 - 92.5 %, Adsorption capacity = 3.0575– 3.1490 mg/g LDHs beads (with 8% LDHs)  38.2188 – 39.3625 mg/g LDHs powder - For the initial conc. of 100 mg/L of AsO43-, pH = 8 Removal efficiency = 79.1 - 91.2 %, Adsorption capacity = 2.5933 – 3.0032 mg/g LDHs beads (with 8% LDHs)  32.4163 – 37.5400 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 decreased about 5 - 6 % 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 by chromate/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) = 36.69 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 time tBT (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 90.0 - 92.5 % for CrO42- and range 79.1 - 91.2% 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 10 -15 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.

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