Chromate and Arsenate Removal by Layered Double Hydroxides-Polymer Beads

1. Chromate and Arsenate Removal by Layered Double Hydroxides-Polymer Beads Nguyen Thi Kim Phuong Institute of Chemical Technology Vietnam Academy of Science and Technology

2. 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.

3. - 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

4. Mg2+ Al3+/Fe3+ 450 oC Cl- Synthesis Mg-Al LDH and Mg-Fe LDH 4h 65 oC, 24 h

5. 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 LDHs beads (beads cure 24 h in CaCl2 solution CaCl2 solution

6. The as-prepared beads (a) Blank, (b) 8% Mg-Al and (c) 8% Mg-Fe

7. 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

9. - 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.

10. 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. Adsorption kinetics

11. Adsorption kinetics

12. 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 . Adsorption isotherm

13. Adsorption isotherm

14. 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

15. 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

16. where • Ctand 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).

17. 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

19. 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:

20. - 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.

21. 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 tfare given by the following expressions:

22. 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:

23. 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.

24. Thank you!!!!