Clay Minerals

INTRODUCTION

 Clay minerals are the characteristic minerals of the earths near surface environments.  They form in soils and sediments, and by digenetic and hydrothermal alteration of rocks.  Water is essential for clay mineral formation and most clay minerals are described as hydrous alumino silicates.  Structurally, the clay minerals are composed of planes of cations, arranged in sheets, which may be tetrahedrally or octahedrally coordinated (with oxygen), which in turn are arranged into layers often described as 2:1 if they involve units composed of two tetrahedral and one octahedral sheet or 1:1 if they involve units of alternating tetrahedral and octahedral sheets.  Additionally some 2:1 clay minerals have interlayers sites between successive 2:1 units which may be occupied by interlayer cations, which are often hydrated.  The planar structure of clay minerals give rise to characteristic platy habit of many and to perfect cleavage, as seen for example in larger hand specimens of micas.  

Clay Minerals are Phyllosilicates

All have layers of Si tetrahedra and layers of Al, Fe, Mg octahedra, similar to gibbsite or brucite

The kaolinite clays are 1:1 phyllosilicates

The montmorillonite and illite clays are 2:1 phyllosilicates

The classification of the phyllosilicate clay minerals is based collectively, on the features of layer type (1:1 or 2:1), the dioctahedral or trioctahedral character of the octahedral sheets (i.e. 2 out of 3 or 3 out of 3 sites occupied), the magnitude of any net negative layer charge due to atomic substitutions, and the nature of the interlayer material.

The basis on which clay minerals are classified is shown below; see Hillier (2003) for a more detailed introduction to clay mineralogy.

Analyses: 

Some types of clay minerals such as mixed-layered clay minerals can only be identified precisely by techniques such as XRD.  Although it is not unusual to have to use a variety of techniques such as XRD, infrared spectroscopy, and electron microscopy to characterize and more fully understand types of clay minerals present in a sample.  We have extensive experience of the identification of clay minerals in both soils and rocks.  Our XRD work is based is backed up by our ability to compare clay mineral diffraction data with calculated diffraction data.  This is a particularly important technique for the precise identification of mixed-layer clay minerals.  Our track record in the Reynolds Cup round robin on quantitative clay mineral analysis is testimony to the quality of our work on the identification and quantification of clay minerals. We also have wide experience of the use of electron microscopy to study the texture and petrographic relationships of clay minerals. A useful gallery of clay

mineral images, showing some of the different morphologies in which clay minerals can occur can be found 

The importance of clays and clay minerals in the drilling industry is evident from two different viewpoints. First, commercially mined clays are added to drilling fluids to build viscosity, control fluid loss, etc. These clays include bentonite, attapulgite and sepiolite. Second, one or more clay minerals are present in nearly all sedimentary rocks and their reaction to the drilling process can produce major problems, such as hole enlargement, sloughing, shale hydration and the gumbo phenomenon. The magnitude of these potential problems can be seen if one considers that argillaceous rocks (shales, siltstones, mudstones, etc.) comprise more than half of sedimentary rocks and are composed of more than 50% clay minerals. In addition, many sandstones have as much as 20-30% clay minerals and even limestones can have small amounts of clay minerals.

STRUCTURE

The clay minerals are hydrous aluminum silicates of a layer type lattice structure with Magnesium, iron and potassium either between the layers or substituted within the Lattice. The exception to this is the attapulgite-sepiolite minerals which have achain type structure.

The basic structural components are silica tetrahedrons and alumina octahedrons, Figures 1 and 2, arranged in a sheet structure and bound together by shared oxygens between the sheets, resulting in the layer type lattice. A crystal structural classification of the layer type lattice clay minerals divides them into three major categories based on the arrangement of the structural units.

The first and most abundant type is referred to as the 2:1 clay minerals and includes illite and smectite. The structure is constructed of an alumina sheet between two silica sheets.

The second is the 1:1 clay minerals which includes kaolinite and has one silica sheet and one alumina sheet.

The third category is the 2:1:1 clay minerals which includes chlorite. The structure consists of a 2:1 lattice with a layer of magnesium hydroxide in the interlayer position.

ILLITE

Illite is by far the most abundant clay mineral type found in sediments. It is a segment of a complex 2:1 structural series of the mica family (Figure 3).

Illite has less substitution of aluminum in the silica sheet, less potassium in the interlayer positions,

and finer crystal size than muscovite. However, illite is essentially inert to hydration and is the most common nonswelling component of mixed layer-clays formed by the diagenetic alteration of smectite.

Illite is similar to muscovite and is the most common clay mineral, often composing morethan 50 percent of the claymineral suite in the deep sea.

They are characteristic of weathering in temperate climates or in high altitudes in the tropics, and typically reach the ocean via rivers and wind transport.

The Illite clays have a structure similar to that of muscovite, but is typically deficient in alkalies, with less Al substitution for Si. Thus, the general formula for the illites is: KyAl4(Si8-y,Aly)O20(OH)4, usually with 1 < y < 1.5, but always with y < 2.

Because of possible charge imbalance, Ca and Mg can also sometimes substitute for K.

The K, Ca, or Mg interlayer cations prevent the entrance of H2O into the structure. Thus, the illite clays are non-expanding clays.

Illite type clays are formed from weathering of K and Alrich rocks under high pH conditions. Thus, they form by alteration of minerals like muscovite and feldspar. Illite clays are the main constituent of shales.

SMECTITE (Montmorillonite)

smectite is family of expansible 2:1 phyllosilicate clays having permanent layer charge because of the isomorphous substitution in either the octahedral sheet

(typically from the substitution of low charge species such as Mg2+, Fe2+, or Mn2+ for Al3+)

The most common smectite is Montmorillinite, with a general chemical formula : (1⁄2Ca,Na)(Al,Mg,Fe)4(Si,Al)8O20(OH)4.nH2OMontmorillinite is the main constituent of bentonite, derived by weathering of volcanic ash. Montmorillinite can expand by several times its original volume when it comes in contact with water. This makes it useful as a drilling mud (to keep drill holes open), and to plug leaks in soil, rocks, and dams.

Montmorillinite, however, is a dangerous type of clay to encounter if it is found in tunnels or road cuts. Because of its expandable nature, it can lead to serious slope or wall failures.

Smectite is the least stable clay mineral; it is the most susceptible to hydration and diagenetic alteration. It has a 2:1 lattice structure with magnesium and iron substituted for aluminum and one of several cations in the interlayer region along with one or more layers of water (Figure 4).

The interlayer region is highly susceptible to hydration, although interaction between interlayer ions and adsorbed water limits the level of hydration, depending on the ion present. When water is abundant, continued swelling to many tens of layers of water in the interlayer region is common for smectite with sodium as the interlayer ion, whereas calcium in the interlayer position seems to restrict expansion.

Swelling Clays

The interlayer in montmorillonite or smectites is not only hydrated, but it is also expansible; that is, the separation between individual smectite sheets varies with the amount of water present in the soil. Because of this, they are often referred to as "swelling clays". Soils having high concentrations of smectites can undergo as much as a 30% volume change due to wetting and drying or these soils have a high shrink/swell potential and upon drying will form deep cracks.

MIXED LAYER CLAYS

Mixed layer clays are formed either by degradation by weathering or by diagenetic alteration of clay minerals. The interlayering may be considered to occur in two basic configurations: (1) regular alternation of layers in a rational repetitive sequence (2) a non-rational alternation sequencewhere layers are neither regularly repetitive nor are likely to occur in equal proportions.Although any combination of two or more clay minerals can form a mixed layer clay,the most common occurrence is a random sequence of illite type layers and smectite type layers. The proportion of swelling to non-swelling layers determines the expandability of these clay minerals.

KAOLINITE Al2Si2O5(OH)2

Kaolinite has a 1:1 structure with one silica sheet and one alumina sheet (Figure 5), with most of the oxygen ions in the alumina sheetbeing replaced by hydroxyl ions. This results in a neutral composition with no surface charges to attract interlayer cations or water and therefore no possible expansion of the structure.

used in the ceramic industry, especially in fine porcelains, because they can be easily molded, have a fine texture, and are white when fired.These clays are also used as a filler in making paper.

CHLORITE

Chlorite is classified as a 2:1:1 mineral. Its structure is identical with the 2:1 minerals except for a layer of magnesium hydroxide fixed in the interlayer region (Figure 6)

This magnesium hydroxide layer neutralizes the surface charge of the 2:1 lattice so that the chlorite structure is non-expandable.

Vermiculite

Vermiculite is a high-charge 2:1 phyllosilicate clay

mineral. It is generally regarded as a weathering product of micas. Vermiculite is also hydrated and somewhat expansible though less so than smectite because of its relatively high charge.

Vermiculite possesses the special property of expanding to between six and twenty times its original volume when heated to approximately 1,000 degrees Celsius. This process, called exfoliation, liberates bound water from between the mica-like layers of the mineral and literally expands the layers apart at right angles to the cleavage plane.Vermiculite is used to loosen and aerate soil mixes. Mixed with soil, it improves water retention and fertilizer release, making it ideal for starting seeds. Also used as a medium for winter storage of bulbs and flower tubers.

BENTONITE

Contrary to popular usage in our industry, bentonite is not a mineral. Bentonite is a rock, which in most instances would beclassified as a clay stone . This rock was formed by post depositional alteration of volcanic ash and its composition is dependent on the environment in which the alteration took place.Bentonite is composed primarily of smectite along with varying amounts of quartz, feldspar, cristobalite, zeolites, gypsum, kaolinite, micas, carbonates and volcanic glass. Composition of the smectite varies in the type of lattice substitution and, more important, the ionic population of the interlayer region. The outstanding property of smectite which is so important to drilling fluids is its ability to swell to many times its volume when sodium is the interlayer ion.This type of smectite is predominant in the bentonites found in the Wyoming area as well as other bentonites used in drilling fluids.

There are four main groups of clay minerals:

Kaolinite group - includes kaolinite, dickite, nacrite, and halloysite; formed by the decomposition of orthoclase feldspar (e.g. in granite); kaolin is the principal constituent in china clay.

Illite group- also includes hydrous micas, phengite, brammalite, celadonite, and glauconite (a green clay sand); formed by the decomposition of some micas and feldspars; predominant in marine clays and shales.

Smectite group- also includes montmorillonite, bentonite, nontronite, hectorite, saponite and sauconite; formed by the alteration of mafic igneous rocks rich in Ca and Mg; weak linkage by cations (e.g. Na+, Ca++) results inhigh swelling/shrinking potential

Vermiculite

Uses of Clay

1) Oil & Water Drilling

2) Drilling Mud

3) Cooling andcleaning the drill Gushers” used to be common until the use of drilling mud was implemented

4) Contaminant RemovalClay slurrys have effectively been used to remove a range of comtaminants, including P and heavy metals, and overall water clarification.

Filtering: Clays are used to decolorize, filter, and purify animal, mineral, and vegetable oils and greases due to their high absorbing properties.

Environmental Sealants: Bentonite is used to establish low permeability liners in landfills, sewage lagoons, water retention ponds, golf course ponds, and hazardous waste sites.

Pharmaceuticals/ Cosmetics:Bentonite is used as a binder in tablet manufacturing and in diarrhea medications. Clays are used as thickeners in a wide variety of cosmetics including facial creams, lipsticks, shampoos and calamine lotion.

Pelletizing: Bentonite is used to bind tiny particles of iron ore, which are then formed into pellets for use as feed material for blast furnaces.

Paints: Finely ground clays are used in the paint industry to disperse pigment evenlythroughout the paint. Without clays, it would be extremely difficult to evenly mix the paint base and color pigment.

Clay Rocks in Egypt

There was clay deposits in Egypt in the following main areas:

(1The Sinai Peninsula

The South-western part of Central Sinai stored huge heat and raw materials the clays. One of the most important raw materials in the Sinai Peninsula kaolin in the area of Abu zenima, where Tina alkaolinih in many locations, some of them in the northeastern area of about 200 km2 limited between 10 am – 16.5 33 East and latitudes 03 29-13.5 29 North, approximately 15 km from the city of Abu zenima. Others are located in the South-East of Abu zenima in an area some 150 km 2 are between longitudes 15 33 23 33 East and latitudes 52.5 28 – 58.5 28 North.There are layers of kaolin in Abu zenima with thick layers of sandstone, ranging from fish can be exploited economically between 1, 4.5 meters, thickness ranges from sandstone layers alternating with 10, 20 m. Tina reserves amount to alkaolinih North and South Abu zenima about 16.5 million tonnes.

The new deposits are found kaolin in the early 1980s, the plateau wandering, about 25 km east of Abu zenima, this area is determined by the intersection of a latitude 10.5 29 North and longitude 15.5 33 East. And reserves that was explored in the plateau of wandering about 88 million tons.

Notes for kaolin described their varying proportion of aluminum oxide from area to 20, 36% and the proportion of impurity mica and iron oxides, and titanium, necessitating the need for treatment and processing of the raw material to raise the proportion of aluminum oxide and reduce impurities to meet local market needs in various industries, especially for alumina refractories and ceramics, export possible export. Kaolin is used currently in the Sinai produced alumina refractories and ceramic tiles. Refractories industry experts should address this material, especially since the price of a tonne of kaolin imported approximately $ 180 while the price of a tonne of kaolin Egyptian about 40 pounds.And targeted projects in Sinai, depending on availability of TINA kaolin and pure limestone is white cement production project is being created now Mt. Sadat in Suez with a production capacity of 250,000 tons per year with an estimated investment cost of around 500 million pounds. This project will provide 600 jobs from various disciplines. There is currently only one factory for the production of white cement in Minya province card design 250,000 tons, enough to meet the demand of the local market.

(2The Aswan region:

After the cessation of the production of kaolin from the Sinai Peninsula following the 1967 aggression was discovered about 16.5 million tons of kaolin in the Kalabsha southwest of Aswan has been exploited in the manufacturing of refractories. And the proportion of alumina from 29% to 35%, but production from the region has largely focused on areas of the Sinai Peninsula after liberation for technical and economic reasons. Kalabsha kaolin needs further technical evaluation and economic studies of the reserves.

And the exploitation of thermal albolklay mud and Tina from Abu Rish areas before me and Abu Rish and Abu Virginia North of Aswan, where in large quantities in an area 70 km 2 and a thickness of 2 to 4 m. Take advantage of the Egyptian company for refractories Tina extracted from its mines in Virginia, Abu refractories, where it is marketed under the name "Egyptian 32, 30" (numbers indicate percentage of alumina), characterized by raw materials with low Abu Virginia bug which reflected its impact on refractories production specifications, and these serve as raw materials for the production of ceramics and ceramics produced by the company mentioned in the four factories in Helwan and Alexandria, Aswan and discern how malleable and dry.

There are several albolklay photo Tine were the following:• Clay loam: alumina up to about 22% and iron up to 19%.• Green Clay: alumina ratio ranges from between 22, 26% and between 3.9%.• Clay gray: about 29% alumina and iron 3, 16%.• Green italics for the Tan clay: the ratio of around 29% alumina and iron about 7%.

Used mud brick industry in albolklay drainage pipes, pottery products. Can then address this material and access to appropriate use of ferric oxide in the production of refractories and ceramic tile and porcelain with high plasticity help ease of composition. And treatment can get varying qualities of excellent and good and to draw all the appropriate production type so don't outlaw this raw products do not need to type.

CONCLUSIONClay minerals are probably the most abundant minerals in sedimentary rocks.They are also the most unstable minerals.Their composition and structure determine their properties, which to a large extent govern the properties of rocks in which they occur. Therefore, it is important to understand these minerals in order to properly assess and predict potential drilling problems.

Bentonite products for water purification

Bentonite products provide versatile applications for waste water treatment and purification.The existing smectite clay-minerals in the bentonite (i.e. montmorillonite) generate a huge reactive surface when disperged. The clay minerals´ weak negative charge enables it to adsorb changeable cationic components.

Fig. 1 shows the structure of smectites exhibiting two tetrahedron-layers and one octahedron layer. By exchange of ions with higher valence against ions with weaker charge the silica layers of montmorillonite show a negative charge. This is compensated by adsorption of counter ions in the interlayers whereby the TOT-layer corpuses are electrostatically held together. The charge of the layers is but so weak that the interlayer cations are exchangeable. Furthermore the weak charge effects an expansion of the interlayers, enabling the counter ions to be integrated with their hydrate shell (inner crystalline swelling).

Fig 1: crystalline structure of montmorillonite

The original earth alkaline cation compound in the interlayers of the most market commonactivated bentonites have been exchanged by Na+-ions during a technical process (alkaline activation). Calcium bentonites are not activated and exhibit less swelling capacity. They contain smectites with interlayers being predominantly occupied with Ca2+ or Mg2+-ions. Furthermore there are natural sodium bentonites(Wyoming-Bentonites, but also in other deposits) comprising smectites that are predominantly occupied with Na+-ions between their interlayers, often in coexistence with Ca2+ or Mg2+ -ions in various quantities.

What kind of advantages do S&B Industrial Minerals GmbH bentonites offer forthe waste water industry?

With AQUAMONT S&B Industrial Minerals GmbH offers powdery means for flocculation precipitation and adsorption based on natural minerals.We offer diverse products matching special requirements that are determined by the different consistencies of the waste water.

AQUAMONT based on high swellable and fine dispersable bentonite generates flocculating colloidal dispersions and effects a strong adsorption due to its high specific particle surface and interlayer charge.

By the negative surface charge of the dispersed clay particles positively charged particlesare closely and partly irreversibly bound, agglomerated, flocculated and embedded.

Due to its cation exchange capacity AQUAMONT particularly binds cationic tensides,softeners based on quaterny ammonium compounds and heavy metals.

AQUAMONT is also applicable as a means for flake enrichment effecting an acceleratedsedimentaion or in combination with appropriate polyelectrolytes as a means for

flotation.

AQUAMONT being a chemically inert silica mineral does not contribute to any additionalstrain of the waste water.

Fabrication and dosage

AQUAMONT optimally develops its effect when added to the waste water in a hydratedand dispersed condition.

Therefore it is advantageous to prepare AQUAMONT in pure tap water under highshearing rates to generate a suspension showing a solid concentration of 5 %. Beforeuse the suspension should be swollen while slightly stirred for at least 1 hour. Forthe preparation of the suspension especially powerful mixers or even dispersing unitsshould be used. At less intense preparation a higher water temperature supports andaccelerates hydratizing, swelling and delamination.

To achieve an optimal effect the AQUAMONT-suspension has to be pured homogeneousinto the waste water by intense mixing or strong turbulences.

If that is not possible lower concentrated suspensions should be applied which caneasily be poured homogeneously into the waste water.

Most appropriate for dosing are piston membrane pumps, eccentric worm pumpsand, if necessary, centrifugal pumps.

Typical proportions are 50 to 150 g AQUAMONT per m3 wate water depending onpollution.

The flocculation of AQUAMONT is best initiated by high molecular and weak anionicPolyacrylamides (molar mass > 107), if not yet being induced by calcium-, aluminum orIron salts. The proportion is at 0, 5 to 1 % of the AQUAMONT quantity while the additionIs to be done in form of a 0,2 to 0,5 % suspension after AQUAMONT has beenadded. Laboratory tests have to determine the minimum time between addingAQUAMONT and polymers. After intense mixing a sufficiently long time should bekept for a better macro flocculation.

Water purification using clay pot water filters and copper mesh

AbstractLack of clean water for use by rural communities in developing countries is of great concern globally. Contaminated water causes water-borne diseases such as diarrhea, which often lead to deaths, children being the most vulnerable. Therefore, the need to intensify research on point-of-use (POU) water purification techniques cannot be overemphasized. In this work, clay pot water filters (CPWFs) were fabricated using terracotta clay and

sawdust. The sawdust was ground and sieved using 300, 600 and 900 μm sieves. The clay and sawdust were mixed in the ratios 1:1 and 1:2, by volume. Pots were then made, dried and fired in a furnace at 850oC. Raw water collected from nearby rivers was filtered using the pots. The raw and filtered water samples were then tested for E. coli, total coliforms, total hardness, turbidity, electrical conductivity, cations and anions. The 600 μm pot had the capacity to destroy E. coli completely from the raw water, whereas the 900 μm pot reduced it by 99.4%. The 600 μm and 900 μm pots could reduce the total coliform concentration by 99.3% and 98.3%, respectively. An attempt was also made to investigate the germicidal action of copper on the coliforms in raw water, with a view to utilising it in the CPWFs. Results showed that 10 g of copper, in the form of mesh made of thin wire of diameter 0.65 mm, had the capacity to completely eliminate E. coli, by immersing it in 300 mℓ of raw water for 5 h, and total coliforms, by immersing it for 10 h. Subsequently, copper was added to the CPWF by placing the mesh in the receptacle of the CPWF. Tests showed that copper could destroy any remaining E. coli in the filtered water, rendering the CPWF a completely viable POU technique for producing clean water. All other critical parameters such as total hardness, turbidity, electrical conductivity and ions in the filtered water were also within acceptable levels for drinking water quality. The filtration rate of the pot was also measured as a function of grain size of the sawdust and height of the water column in it. The filtration rate was found to increase with grain size and height in all of the pots

Water treatment describes industrial-scale processes that make water more acceptable for an end-use, which may be drinking, industrial, or medical. Water treatment is unlike small-scale water sterilization that campers and other people in wilderness areas practice. Water treatment should remove existing water contaminants or so reduce their concentration that their water becomes fit for its desired end-use, which may be safely returning used water to the environment.The processes involved in treating water for drinking purpose may be solids separation using physical processes such as settling and filtration, and chemical processes such as disinfection and coagulation.

Industrial water treatment

Two of the main processes of industrial water treatment are boiler water treatment and cooling water treatment. A lack of proper water treatment can lead to the reaction of solids and bacteria within pipe work and boiler housing. Steam boilers can suffer from scale or corrosion when left untreated leading to weak and dangerous machinery, scale deposits can mean additional fuel is required to heat the same level of water because of the drop in efficiency. Poor quality dirty water can become a breeding ground for bacteria such as Legionella causing a risk to public health.

With the proper treatment, a significant proportion of industrial on-site wastewater might be reusable. This can save money in three ways: lower charges for lower water

consumption, lower charges for the smaller volume of effluent water discharged and lower energy costs due to the recovery of heat in recycled wastewater.

Corrosion in low pressure boilers can be caused by dissolved oxygen, acidity and excessive alkalinity. Water treatment therefore should remove the dissolved oxygen and maintain the boiler water with the appropriate pH and alkalinity levels. Without effective water treatment, a cooling water system can suffer from scale formation, corrosion and fouling and may become a breeding ground for harmful bacteria such as those that cause Legionnaires ' disease. This reduces efficiency, shortens plant life and makes operations unreliable and unsafe.

Water treatment: Disinfectants and Other

Disinfectants: Ozone, as a very strong oxidant, is one of the main disinfectants when purifying water. As ozone breaks down in the water, a complex chain reaction mechanism occurs under the effect of the various solutes in the water or released during purification treatment. Its ability to inactivate living cells can be extended to the point of provoking their lysis.

Ultraviolet (UV) radiation is produced using ultraviolet lamps with quartz covers. UV produces a minimum of by-products when treating the water.

Other: An advanced oxidation process (AOP) is a system to purify water by chemical oxidation to deactivate residual organic pollutants. AOPs are capable of generating a more powerful and less selective secondary oxidant in the reaction medium by activating an available primary oxidant. AOP has been only gradually used in the water treatment industry. One of the many AOP systems, the combined O3/H2O2, is the most widely used one especially for the purpose of destroying pesticides in order to produce water for human consumption.

ExperimentalFabrication of porous clay pots

Porous pots were made using terra-cotta clay and sawdust. The sawdust was ground and sieved using 300 μm, 600 μm and 900 μm sieves. The clay and sawdust were mixed in the ratios 1:1 and 1:2 to make the pots, which were then dried for a week and fired in an electric furnace at 850oC. Firing time was 8 h and cooling time was 20 h. For initial tests, small pots of capacity 1 ℓ and thickness of approximately 1.0 cm were made. (Throughout this paper, the pots are labelled with grain size of the sawdust and clay/sawdust ratio. For example, 300 μm (1:2) refers to a pot made of clay and 300 μm grain size sawdust, in the ratio 1:2).

Schematic diagram of a clay pot water filter (CPWF)

Testing of potsRaw water, collected from Lobamba and Mtilane Rivers in Swaziland, was filtered using the pots. Both the raw and filtered water were then tested for E. coli, total coliform, total hardness, turbidity, electrical conductivity, cations and anions. Filtration rates of the pots were measured as a function of the grain size as well as the height of the water column in the pots.

Germicidal effect of copper

A detailed study was carried out to determine the germicidal effect of copper in combating coliforms in raw water. This was done by counting

the number of coliform colonies remaining: (i) after immersion of different masses of copper in a fixed volume (300 mℓ) of raw water, and (ii) by immersing the same amount (10 g) of copper for different time intervals. Coliform tests were carried out on both raw water and copper-treated water samples.

Design and testing of the clay pot water filter (CPWF)Finally, for fabrication of clay pot water filter for point-of-use

application, 600 μm (1:1 and 1:2) and 900 μm (1:1 and 1:2) pots were made. They had a capacity of 2 ℓ and a thickness of approximately 1.5

cm. The pots were suspended inside plastic receptacles with lid and tap, as shown in Fig. 1. Copper mesh made of thin copper wire of mass 2.5 g and diameter 0.65 mm was placed at the bottom of the receptacle. Raw water and filtered water, with and without copper mesh, were tested for E. coli, total coliform, total hardness, turbidity, electrical conductivity,

cations and anions.

Results and discussion

Table 1 shows the effect of filtering raw water on E. coli, using 300 μm, 600 μm and 900 μm grain size pots. The 300 μm and 600 μm pots had the capacity to remove E. coli in the raw water completely. The E. coli level was as high as 9 600 CFU per 100 mℓ. However, the 900 μm pots could not reduce the E. coli level to zero, even though a decrease from 9 600 to 1 CFU/100 mℓ (~ 99.99%) was achieved. The 1 CFU/100 mℓ of E. coli that remained in the filtered water may presumably be due to the larger pore sizes of these pots, which allowed the bacteria to pass through.

The presence of E. coli indicates that the water could have been contaminated with animal or human waste and could cause waterborne

diseases such as diarrhoea, which often lead to deaths, particularly among children. Because of the potential disease-causing characteristics of certain E. coli, removal of E. coli from raw water is a major step in all water purification systems. It is worth noting that the accepted level for potable water quality is zero CFU per 100 mℓ for coliforms.

Table 2 shows the effect of grain size on the filtration rate of the pots. The data show that the larger the grain size, the higher the filtration rate. Filtration rates of the 600 μm and 900 μm pots were found to be about 2 to 3 times the filtration rate of the 300 μm pot. This is expected because larger grain size presumably results in larger pore size and hence higher filtration rate. In comparison with the 300 μm pot, the 600 μm pot would be ideal for small-scale water filters because of its higher filtration rate. Even though the 900 μm pot had the highest filtration rate, it had the disadvantage of not being able to remove the E. coli completely .

Figure 2 shows the variation of filtration rate with the height of the water column in a typical pot. Being gravity-driven, filtration rate is expected to increase linearly with height. However, the rate was found to increase non-linearly with height, as shown. For example, an increase in the height of the water column by a factor of 5 (from 40 to 200 mm) led to an increase in the filtration rate by a factor of about 15 (from 70 to 1 100 mℓ/h). This non-linearity can be explained by the fact that water filtration takes place not only through the base of the pots but also through its sides. From a practical point of view, it is therefore advisable to design pots that are as tall as possible in order to get the maximum filtrate.

Figure 3 shows the germicidal effect of copper, in 300 mℓ raw water, as a function of mass. As seen, the coliform colonies remaining in the water decreased with the mass of copper added.. For example, 1 g of copper reduced the total coliform level from an initial count of 2 413 to 1 046 CFU/100 mℓ (~ 56.7%) in 10 h, while 9 g of copper had the capacity to reduce the level to 18 CFU/100 mℓ (~ 99.3%) within the same time interval. A similar trend was seen for E. coli .

The germicidal effect of copper as a function of time is shown in Fig. 4. It is observed that the number of coliforms destroyed increases with time. For example, 10 g of copper can reduce the E. coli level from 186 to 4 CFU/100 mℓ (~ 98 %) in 1 h and to zero (100%) in 5 h. Total coliform was reduced to zero after a period of 10 h. A similar decrease in E. coli concentration in river water isolates, using copper pots, has been reported (Shrestha et al., 2009). To the knowledge of the authors, no work has been found in literature on the use of copper in CPWFs for its germicidal action .

It noted that in both cases (Fig. 3 and Fig. 4) the activity of copper is much stronger initially. This is because an essential condition for the

inactivation of the bacteria is that they come into contact with the metal. Hence, the larger the number of coliforms present in the vicinity of the copper at a particular instant, the greater the chance that the coliforms will come into contact with it and be killed, since the coliforms move

about randomly in the water.

Table 3 shows the coliform counts in raw and filtered water samples using 600 μm and 900 μm pots. The initial count for total coliforms and

E. coli in the raw water was 5 475 and 697 CFU per 100 mℓ, respectively. The 600 μm pot reduced the total coliform count to 38 CFU per 100 mℓ (nearly 99.3%) and the E. coli count to zero. Incorporation of copper in

the receptacle reduced the total coliforms to zero as well. The 900 μm pot reduced the total coliform and E. coli levels by 99.3% and 99.4%, respectively, and addition of copper in the receptacle reduced their

concentrations by 99.98% and 100%. These results suggest that use of copper in the CPWF is an effective means to eliminate coliforms in raw water. Thus, there are two mechanisms responsible for the removal of impurities from contaminated water – the physical process, using the

pores in the structure of the pot, and the biological process, through the germicidal effect of copper. Silver is another metal known to have

germicidal action on microorganisms (Blanc et al., 2005; Landeen et al., 1989; Lin et al., 1996; Jayesh et al., 2008), and has been effectively used

in CPWFs in colloidal form and as nanoparticles (Hwang et al., 2007; Halem et al., 2007). However, fabrication of such silver-impregnated

filters can be cumbersome if it is to be carried out by a rural community, as compared to using copper mesh. Moreover, silver is expensive and not

easily available.

Table 4 shows the effect of filtering raw water samples on total hardness, turbidity and electrical conductivity of the filtrate, using a 600 μm pot.

Total hardness in the raw and filtered water was within the RSA standard for potable water. Filtration could reduce it by nearly 55%. Total

hardness in the range 0–200 mg/ℓ CaCO3 has no adverse health effects (RSA guidelines: WRC, 1999; John and Trollip, 2009).

Turbidity in the raw water was much higher than the RSA and WHO limits. Filtration reduced it from 8.31 to 1.13 NTU (~ 86%), which is an acceptable level for potable water. Turbidity does not have any direct adverse health effects.However, it provides information about the presence of suspended solids, and is a measure of the cloudiness of water. Electrical conductivity of the raw and filtered water was found to be within the RSA standard. Filtering reduced it from 51.78 to 35.05 μS/cm (~ 32%). Electrical conductivity has no adverse health effects below 700 μS/cm, but indicates the total dissolved salt content in water.

Table 5 shows the effect of filtering raw water samples, using a 600 μm pot filter, on the concentration of various anions. The sulphate ions were reduced to 21.84 mg/ℓ (~ 44%) whilst the chloride ions decreased to 1.25 mg/ℓ (~ 54%). The rest of the anions, namely, ammonia, nitrite, nitrate, fluoride and phosphate ions, showed an increase in concentration after filtering. This could be a result of leaching as water passed through the pot. Even though the concentration of these anions increased, their levels in the filtered samples were still within acceptable limits for potable water.

Table 6 shows the effect of filtering raw water, on the concentration of various cations. Calcium, iron and magnesium ions were reduced, while the concentration of nickel and vanadium ions increased, presumably because of the presence of these ions in the clay. There was no noticeable change in the concentration of the other cations.

Conclusions

Porous pots were made from terracotta clay and sawdust. The clay and sawdust, ground and sieved using 300 μm, 600 μm and 900 μm sieves, were mixed in the ratios 1:1 and 1:2 to make the pots. They were then dried and fired in an electric furnace at 850oC. The pots were suspended inside plastic receptacles to make clay pot water filters (CPWFs) for point-of-use (POU) application. The filters were tested for their ability to purify raw water obtained from local rivers. The 600 μm filter yielded

water that was completely free of E. coli, and reduced the total coliform concentration by 99.3%. The 900 μm filter could reduce the E. coli levels by only 99.4% and the total coliform levels by 98.3%, but had the advantage of having a higher filtration rate compared to the 600 μm filter. Tests showed that copper is a suitable antibacterial agent to combat coliforms in contaminated water. Copper mesh made of thin wire of diameter 0.65 mm was placed inside the receptacles of the filters. This resulted in the 600 μm filter being able to eliminate the total coliform completely from the filtered water, whilst the 900 μm filter reduced it by 99.9% and eliminated the E. coli completely. The use of copper as an antibacterial agent in the CPWF is therefore an effective and reliable means for producing clean water for domestic use. The filtration rate was found to increase with the grain size of the sawdust and height of the water column in the pot. These factors need to be taken into account when designing pot filters for practical use.

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