Ceramic membranes from Moroccan natural clay andphosphate for industrial water treatment

L. Palacioa, Y. Bouzerdib, M. Ouammoub, A. Albizaneb, J. Bennazhab,A. Hernándeza, J.I. Calvoa,*

aDepartamento de Física Aplicada, Facultad de Ciencias, Universidad de Valladolid, 47071 – Valladolid,Spain, Real de Burgos, s/n, 47071–Valladolid (Spain)Tel. + 34 983 423 758: email: jicalvo@termo.uva.es

bLaboratoire Matériaux Catalyse et Environnement (L.M.C.E.), Département de Chimie, Faculté des Sciences etTechniques, Université Hassan II Mohammadia, BP. 146, 20650-Mohammadia (Maroc)

Received 30 June 2008; revised 01 December 2008; accepted 09 February 2009

Abstract

Ceramic MF membranes have been made from clays and phosphates, coming from Moroccan ores. Several flatdisk filters have been obtained by sintering the mixed materials with starch as organic additive. The resultingfilters have been characterized by using mercury porosimetry, air and water permeation, to choose the best fabri-cation parameters. Resulting optimized membranes have been used to treat dye containing solutions to check theutility of such filters in the waste water treatment for Moroccan textile industry. Results are quite promising, allow-ing these filters to be used as a previous clarification step in textile water treatment.

Keywords: Ceramic membranes; Natural clay, Micronized phosphate; Mercury porosimetry; Permeability; Dyeretention

1. Introduction

Membrane-based separation technologies

have found a widespread use in biotechnological,

pharmaceutical, and food industries or to treat

many other industrial effluents, interestingly by

the added value of the by-products or by envi-

ronmental reasons. Pressure based processes

such as Microfiltration (MF), Ultrafiltration (UF)

or Nanofiltration (NF) offer a wide range of sep-

aration procedures able to recover, concentrate,

or purify valuable solvents in all these industrial

fields. Nevertheless fouling is the main problem

in membrane processes as it diminishes the

membrane performance, leading to increased

Desalination 246 (2009) 128–134

*Corresponding author.

Presented at the conference Engineering with Membranes 2008; Membrane Processes: Development, Monitoring andModelling – From the Nano to the Macro Scale – (EWM 2008), May 25–28, 2008, Vale do Lobo, Algarve, Portugal.

0011-9164/0X/$– See front matter © 200X Elsevier B.V. All rights reserved.

doi: 10.1016/j.desal.0000.00.000

L. Palacio et al. / Desalination 246 (2009) 128–134 129

operational and energy costs but also to scaled

replacing costs, which are largely the biggest

investment costs in a membrane separation plant.

To minimize the influence of fouling on the

membranes, several procedures of aggressive

mechanical and/or chemical cleaning are usually

employed in membrane plants. These proce-

dures, along with the sterilization very often

needed in pharmaceutical industry, are prone to

cause damage in the membrane structure, espe-

cially for polymeric membranes. This is a reason

which has increased the interest on the use of

inorganic and, specially, ceramic membranes.

Nevertheless normally ceramic membranes

are more expensive than usual polymeric ones. In

that sense, the research on new membranes using

cheaper ceramic materials should benefit wide-

spread use of membrane technology, specially for

emerging countries, where many environmental

problems need to be solved at low cost.

Clay minerals and phosphates are a well-

known class of natural inorganic materials, but its

application to membrane making is still new. Only

some works deal with clay membranes in tubular

form, [1–5], but very few as flat disks, [6]. None

have successfully used micronized natural phos-

phate as base material for ceramic membranes.

The aim of this work was to develop new fil-

ters made from natural materials, abundant in

Morocco, and obtained with a simple and low

cost process. These filters are intended to be used

in the industrial waste water treatment, especially

in relevant Moroccan industries such as textiles.

2. Materials and Methods

2.1. Membranes

Several flat disks were made from natural clay

and micronized phosphate, both coming from

Moroccan ores. The plastic pastes were prepared

in a dry route, departing from ceramic powders

of source materials, homogeneously mixed with

organic additives and without water addition. The

chemical compositions of both powders are

shown in Tables 1 and 2, as determined by X-Ray

Fluorescence. Different contents of starch were

used as organic additives, being very adequate to

give resulting filters hydrophilic properties,

needed for materials treating water solutions. The

mixed powder was axially pressed to get flat

disks, which were then sintered at different tem-

peratures, using a programmable furnace.

Two heating programs were developed ade-

quate for each membrane material. In the case of

clay, the heating program ended at 950ºC, with

plateaus at 250, 750 and 950ºC, being enough to

get a stable structure. While for the case of

micronized phosphate new heating steps were

added to achieve a final temperature of 1250ºC.

This difference of temperature allowed avoiding

Table 1Mineralogical composition of claya

Oxide SiO2

Al2O

3Fe

2O

3CaO MgO SO

3K

2O

% 49.25 16.92 5.78 18.65 2.78 0.80 1.45

aThe chemical analysis of the clay and natural phosphate has been realised with X-Ray Fluorescence (Philips PW14000).

Table 2Mineralogical composition of natural phosphate

Oxide P2O

5CaO F– CO

2SiO

2Fe

2O

3Al

2O

3

% 30.7 49.57 3.56 6.35 5.40 0.19 0.26

130 L. Palacio et al. / Desalination 246 (2009) 128–134

phosphate tablets breaking. The temperature

increase between plateaus was made at speeds

ranging from 1–5ºC/min.

Finally some additional tablets were prepared

with 30% (w/w) of phosphate and 70% (w/w) of

clay, to check if the mixed membranes properties

are governed by the clay or the phosphate present

in the mixture. These samples will be named

CPM (mixed clay–phosphate) henceforth.

2.2. Mercury porosimetry and Scanningelectron microscopy

Mercury intrusion porosimetry was used to

determine the pore size distribution of the result-

ing tablets. An Autopore III from Micromeritics

was used with usual values of 130º for the contact

angle and 485 dyne/cm for the surface tension of

the mercury–membrane interface.

SEM surface micrographs were taken from

different samples using a JEOL apparatus, after

gold sputtering onto samples surfaces, and with

accelerating voltage of 15 kV.

2.3. Water and air permeabilities

The water permeability for distilled and Milli-

Q treated water was measured for several disks

of different base material and starch content. Per-

meability was measured at different pressures

ranging from 0 to 7000 Pa.

Air permeabilities were measured using a

Coulter Porometer II, which using a source of

clean and dry air, is able to determine the flow

of air for selected pressures in the range of 0–

3.5 bar.

2.4. Retention experiments

To determine the response of our membranes

to waste water filtration, some experiments were

conducted where the apparent retention of dye

containing solutions was measured as a function

of time. Filtration experiments lasted at least

2.5 h. and 2 mL of permeate were collected at

selected time spans, to measure the permeate con-

centration.

Three dyes were considered, all very com-

monly used in the textile industries of Morocco,

murexide (NH4C

8H

4N

5O

6, or C

8H

5N

5O

6 • NH

3),

also called ammonium purpurate; methyl

orange (C14

H14

N3NaO

3S) and potassium chro-

mate (K2CrO

4). For each dye, 2 L of water

solution were prepared and filtered at low pres-

sure (1500 Pa) through several disk of mem-

brane. All experiments were done at low

concentration to avoid signal saturation in the

spectrophotometer.

Initial and time filtered solution concentra-

tions were measured in a UV-Visible Spectropho-

tometer, UV-160A, from Shimadzu. For each dye

the measurement wavelength was adjusted for its

absorbance maximum and then absorbance was

measured with pure water reference. Finally

some experiments were conducted under contin-

uous recirculation mode, by using a peristaltic

pump, Gilson Minipuls. In this case, the samples

taken to measure the concentration were later put

back in the feeding solution to maintain the initial

volume of fluid.

3. Results

3.1. Mercury porosimetry and Scanningelectron microscopy

Mercury intrusion porosimetry was used ini-

tially to determine the influence of the starch

content on the final pore size distributions.

Results of mercury intruded volume versus pore

size, for clay and different starch contents are

shown in Fig. 1. It can be seen how the addition

of some starch leads to broader distributions

shifted to bigger pores, thus increasing the mean

pore size.

On the other side the SEM images (see Fig. 2)

for all the samples show a broad distribution of

voids and pores around some microns, confirm-

ing the mercury intrusion values.

L. Palacio et al. / Desalination 246 (2009) 128–134 131

amounts of fluid at lower costs. The water per-

meabilities for different samples made from clay

and phosphate are presented in Fig. 3, as a func-

tion of the starch content. Results of permeability

for mixed clay–phosphate (CPM) tablets are also

presented.

From the figure it is clear that higher contents

of starch render the filters more permeable due to

the increase on mean pore size yet commented

from mercury porosimetry results but also due to

an increase on the filter hydrophilicity. Probably

the reason for this behavior is that fast evaporation

of starch during sintering process gives rise to big-

ger voids and pores. On the other hand clay

appears to have more permeability than phosphate

while the mixed membranes appear to have behav-

ior closer to that of pure clay. In fact, and taking

into account experimental errors, it seems that

water permeability is governed by clay while phos-

phate has negligible influence on such a parameter.

d (µm)0.0 0.3 0.6 0.9 1.2 1.5 1.8 2.1 2.4

Vol

Hg

(m

L g–1

)

0.00

0.01

0.02

0.03

0.04

0.05

0.06Clay only+ 5 % starch+ 10 % starch+ 15 % starch

Fig. 1. Volume of mercury intruded in the samples ver-

sus pore diameter, for several samples of clay and differ-

ent contents on starch.

a) Clay

b) Phosphate

c) CPM

Fig. 2. Top-view SEM images of different samples made with 35% of starch as additive. (a) Clay, (b) Phosphate and (c)

Clay–phosphate mixed membrane.

3.2. Water permeability

Water permeability is always a key factor on

membrane applications, as allows treating higher

132 L. Palacio et al. / Desalination 246 (2009) 128–134

3.3. Air permeability

Similarly the air permeability to clean air was

measured for different samples of variable starch

content and different material (clay, phosphate

and CPM). Results are shown in Fig. 4 as a func-

tion of starch content.

As obtained for water permeation, results

show that increasing starch content lead to more

permeable membranes, with similar perform-

ances for clay and CPM-based membranes and

somehow lower for phosphate.

Water permeability

Starch content (%)

15 20 25 30 35 40

Per

mea

bilit

y (m

s–1

Pa–1

× 1

0–8)

0

2

4

6

8

10

12

14

16

ClayPhosphateCPM

Fig. 3. Water permeability (in m s–1/Pa) for each mem-

brane material with different starch contents.

Air permeability

Starch content (%)

15 20 25 30 35 40

Per

mea

bilit

y (L

min

–1 c

m–2

bar

)

1.8

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.0

ClayPhosphateCPM

Fig. 4. Air permeability (in L/min m2 bar) for each mem-

brane material with different starch contents.

Taking into account all the facts exposed in

the previous sections and figures, we decided to

fix best conditions of membrane making at 35%

(content of starch), so further results should be

related with such values.

3.4. Dye retention

Selected samples of each material were used

to filter water containing several dyes commonly

used in the traditional textile industry in Morocco.

The permeate concentration was measured at dif-

ferent times of filtration by UV spectrophotometry

and compared with initial concentration to obtain

total dye observed retentions, as follows:

where Robs

is the observed retention, Cp the perme-

ate concentration, and C0

the initial concentration.

Results on dye retention are shown for

murexide in Fig. 5 and for methyl-orange in Fig.

6, both for clay, phosphate, and CPM. The

behavior of both dyes (and also similarly for

potassium chromate are not shown here) show

interesting retentions, increasing with time to

achieve a plateau in 3–4 h (around 30–40% for

RCC

pobs

0

1= −⎛

⎝⎜

⎞

⎠⎟

T (min)0 50 100 150 200

Rob

s (d

imen

sion

less

)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

ClayPhosphateCPM

Murexide

Fig. 5. Observed retention of low concentration solutions

of Murexide versus time for each membrane material and

35% starch content.

L. Palacio et al. / Desalination 246 (2009) 128–134 133

T (min)

0 50 100 150 200 250

Rob

s (d

imen

sion

less

)

0.00

0.05

0.10

0.15

0.20

0.25

0.30

ClayPhosphateCPM

Methyl-orange

Fig. 6. Observed retention of low concentration solutions

of Methyl-Orange versus time for each membrane mate-

rial and 35% starch content.

methyl-orange and potassium chromate and even

higher for murexide).

Since the relative ratio between the dyes mole-

cules and expected pore sizes (higher than 0.3μm)

makes clear that retention is based more on spe-

cific affinity than in steric factors. This could make

retention properties to be lost with time, decreasing

performance of the filters for such application.

Hence some long-term experiments were done to

clarify such a fact. Several samples were used in

continuous recirculation experiments, an example

is shown in Fig. 7 for phosphate. The experiments

were conducted for 6 h changing the solution for a

fresh one after 3 h. The experiment showed that

retention sharply decreased after changing solu-

tions but later a new plateau was obtained similar

to the previous one after another 3 h.

4. Conclusions

Natural Moroccan clay and micronized natu-

ral phosphate are a good selection, as materials

for ceramic membranes, since they are cheaper

than usually employed materials. Also the

process of using flat filters makes it cheaper than

others membrane manufacturing processes. The

resulting filters present reasonable characteristics

of permeability for water and air.

Especially interesting and promising are the

results on retention of industrial tinctures which

allow envisaging these membranes to design a

first clarification step of wastewater treatment on

textile industries. Nevertheless more study would

need to be performed before a competitive appli-

cation can be proposed.

Acknowledgements

This work has been funded by the Spanish

International Cooperation Agency, AECI (PCI

Spain–Maroc Program), through projects:

A/3645/05 and A/5937/06.

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