WO1994022768A1

WO1994022768A1 - Removal of manganese from water

Info

Publication number: WO1994022768A1
Application number: PCT/GB1994/000703
Authority: WO
Inventors: Ralph Edmund Harris; Anne Marie Jacques
Original assignee: Archaeus Technology Group Limited
Priority date: 1993-04-02
Filing date: 1994-03-31
Publication date: 1994-10-13
Also published as: GB9424324D0; GB2282806A; GB2282806B; GB9306919D0

Abstract

A method of adsorbing manganese from an aqueous solution comprises (a) contacting a solid adsorbent with aqueous alkaline solution, (b) directly contacting the adsorbent resulting from step (a) with the manganese-containing solution, and (c) separating the adsorbent from the solution. Adsorbents may be chitin, chitosan, peanut skins, coconut peat, onion skins, peanut cases and wood chips.

Description

REMOVAL OF MANGANESE FROM WATER

This invention relates to a process for adsorbing manganese from an aqueous solution by treating with an adsorbent.

Manganese is a common contaminant in natural water sources where it often occurs in association with other metal contaminants, such as iron, aluminium or naturally occurring coloured compounds. The manganese may be present in aqueous solution in a variety of forms, for example as oxyanions, cations, hydrated oxides and organo-metallic complexes. It also occurs as a contaminant in industrial effluents. The prescribed admissible concentration of manganese in drinking water is 50μg/L and the National Rivers Authority sets limits for the amount of manganese which may be discharged to external water courses.

Manganese is conventionally removed from solution by oxidising with chlorine, ozone or potassium permanganate to form solid manganese compounds, which precipitate out of solution. In conventional water treatment, manganese removal is carried out in a second stage of treatment which requires the pH of the water to be raised by liming prior to chlorine addition and secondary filtration. The solid compounds are then removed from the solution by filtration, generating large quantities of sludge for disposal. Other metallic contaminants, such as iron and aluminium, and naturally occurring coloured compounds may be removed from aqueous solution by adsorption onto an adsorbent such as chitin (PCT/GB91/01369) , but attempts to remove manganese in the same way have been relatively unsuccessful, with removal rates being much lower for manganese than for other metals.

Surprisingly, it has been found that the adsorption of manganese may be improved considerably if the adsorbent is treated with an alkali solution prior to the adsorption step. The present invention therefore provides a method for removing manganese from an aqueous solution which comprises (a) contacting a solid adsorbent with aqueous alkaline ^■ solution, (b) directly contacting the adsorbent resulting from step (a) with the manganese-containing solution, and (c) separating the adsorbent from the solution. The present invention also provides a method which further comprises regenerating the adsorbent to which the manganese is adsorbed by contacting the adsorbent with an aqueous acid solution and subsequently recycling the regenerated adsorbent to step (a) .

The step (a) is generally referred to as the pre-wash step. The step (b) is generally referred to as the adsorption step. The adsorbent used in the method of the present invention is generally at least one of chitin, chitosan, alginic acid, peanut skins, coconut peat, onion skins, peanut cases and wood chips. Preferably the adsorbent is chitin, which is a low cost naturally occurring polymeric material. Chitin is a polysaccharide made up of molecules of N-acetylglucosamine linked together by 1,4-/3 glycosidic bonds. It occurs in the exoskeleton of shellfish and insects, and may be readily obtained as a by-product of shellfish industries. Thus, a source of chitin is from the exoskeleton of shrimps or crabs. Crab-derived chitin is preferred. Chitin is also found in the cell walls of fungi such as basidiomycetes, ascomycetes and zygomycetes.

Preferably the chitin used in the process of the present invention is in purified form. Chitosan is generally obtained by de-acetylating chitin. Coconut peat is a by-product from the coconut processing industry which contains cellulose, pentosan, furfural and lignin. Peanut skins, peanut cases and onion skins are by-products of the food industry. Alginic acid is a linear polysaccharide polymer made up of /3-(l,4)-D- mannosyluronic acid and α- (1,4) -L-gulosyluronic acid residues, which occurs in seaweed and is readily obtainable therefrom.

The adsorbent may be used in the process of the • present invention in any conventional form, for example as particles, beads, sheets or fibres. In particular, particles may be used which are typically 0.5-10 mm in size (i.e. they pass through an aperture of 10 mm but are retained by a sieve with apertures of 0.5 mm) , preferably from 0.5 to 2 mm in size. The step (a) (prewash) is carried out before the adsorbent is contacted with the manganese-containing solution in step (b) . The alkaline solution generally has a concentration of 0.005 to 0.5 M. The alkaline solution is generally a solution of KOH, NaOH, NH₄OH, or a metal carbonate such as Na₂C0₃ or NaHC0₃. Preferably the alkali is NaOH. Typically, NaOH at a concentration of 0.01 to 0.1 M, preferably 0.05 M, is used. The prewash is generally carried out at room temperature and pressure in a suitable apparatus. The adsorbent may be pre-treated prior to the prewash step, for example to remove contaminants or to enhance performance. When the adsorbent is chitin, a suitable pretreatment involves heating the chitin in an alkaline solution, e.g. sodium hydroxide. Suitably, a solution of 0.05 to 0.3 M sodium hydroxide may be used. The chitin may be heated in the alkaline solution at a temperature of for example, about 70°C. Typically, 3 litres of alkaline solution for 500 g of chitin may be used. After heating for a suitable period of time, e.g. about 2 hours, the chitin is rinsed with water.

The adsorption step (b) is carried out after the prewash step (a) . The manganese-containing solution is passed from the prewash step (a) to the adsorption step (b) without being subjected to any treatment between steps (a) and (b) . The manganese-containing aqueous solution may be obtained for example from industrial effluent or from a natural water source such as a spring or run-off water. The aqueous solution is typically naturally neutral or acidic and has a pH from 5 to 7. The pH may however be. specifically adjusted to optimise a particular step in water treatment such as flocculation, or aluminium or iron salt precipitation and may be outside this range.

The manganese-containing aqueous solution may contain other contaminants such as polyphenolic acids, particulate material and metal ions, e.g. aluminium or iron. Industrial effluents may also contain a wide range of organic compounds. Such additional contaminants may also be adsorbed by the adsorbent. Alternatively the solution may be substantially free of other contaminants such as metal ions. Depending on the nature of the contaminant and the adsorbent, the particles, metal ions or organic contaminants may be trapped by, or adsorbed onto the adsorbent.

The manganese-containing aqueous solution may be supplied to and contacted with the adsorbent by any suitable method using any suitable apparatus. The treatment may for instance be performed as a mixed batch process in which adsorbent is mixed with a batch of solution and is then removed for example by filtration, sedimentation or centrifugation and decantation of a supernatant.

Preferably however the manganese-containing aqueous solution is contacted with a bed, such as a packed bed, of adsorbent for instance in the form of a packed column of adsorbent. Other types of bed include a fluidised bed, a trickling filter bed or a slow filter bed. The type of arrangement which may be used can be selected by a person of skill in the art and will depend on factors such as the quantity of solution to be treated and the condition of the solution prior to treatment. The factors can be assessed by those of skill in the art familiar with filtration systems in the light of the present invention. Manganese adsorption may then be carried out in a continuous manner by passing the aqueous solution through or over a bed. For example a packed bed comprises packed adsorbent through which the solution to be treated is pumped or percolated under gravity to effect contact between the solution and the adsorbent.

In a situation where the turbidity of the solution to be treated is high, it is desirable to prefilter the solution prior to contact with adsorbent. In the case of a continuous filtering process, this may be achieved by insertion of a prefilter in line ahead of a bed in order to trap particulate material and prevent blinding of the bed. Such a prefilter may consist of coarse chitin (e.g. of a particle size of 1 to 10 mm) or coarse sand (e.g. mesh 16/30) or any other commercially available filtration unit or membrane filter, and will generally be shorter than the bed. It may be backflushed regularly with water to remove particulate material.

In a batch treatment the amount of adsorbent is preferably from 0.01 to 1.0 g, especially about 0.1 g, of dry adsorbent per 10 ml of solution. In a continuous treatment preferably 25 to 2500, more preferably 25 to 250, volumes of solution per volume of adsorbent, are treated by contact with a packed bed, without recycling of adsorbent. After the manganese removal treatment, the solution may be further treated, for example to render it suitable for use as drinking water, for example by chlorination, ozone treatment or ultraviolet sterilisation. The solution may also be further treated to render it suitable for industrial or agricultural use. Such treatment may for example include aerobic or anaerobic microbial treatment, activated carbon treatment, sand filtration, flocculation, ultrafiltration or reverse osmosis. These treatments may also take place prior to treatment with adsorbent. Following treatment of the solution with adsorbent, the adsorbent may be regenerated and recycled by treatment with an aqueous acid solution, comprising e.g. HCl, to release the adsorbed manganese and other adsorbed materials (e.g. other metal ions) from the adsorbent. The adsorbent may be treated as a batch following removal of adsorbent from a batch treatment or following removal of adsorbent from a packed bed after continuous treatment. Alternatively, in the case of a continuous treatment the adsorbent may be recycled by passing aqueous acidic solution through the bed of adsorbent. After treatment with acid the adsorbent may be reactivated by contacting with aqueous alkali prior to reuse of the adsorbent in the treatment of manganese contaminated water. This treatment may take the form of a prewash of the type described above for the preparation of fresh adsorbent .

The concentration of the acidic solution may for example be from 0.005 to 0.5 M, such as from 0.01 M to 0.1 M and preferably 0.025 to 0.05 M, in the case of HCl or may be an equivalent concentration of another acid e.g. sulphuric acid or acetic acid.

The amount of acidic solution used to recycle the adsorbent in a batch recycling treatment is typically from 0.2 to 2 ml of 0.05 M HCl per g of adsorbent. The amount of acid required for the recycling and the amount of alkali required for the regeneration of the adsorbent by passing through a packed bed is typically from 1 to 4 bed volumes, for example about 2 bed volumes acid followed by 2 bed volumes of alkali. For example 2 bed volumes of 0.05 M HCl followed by 2 bed volumes of 0.05 M NaOH may be used. The recycling process is preferably selected so that contaminants are eluted in one bed volume or less. The solutions of acid and alkali used in the recycling and regeneration steps may be discarded to waste but are preferably recharged to reach original strength and reused to minimise waste volumes.

The invention will now be illustrated by reference to the following Examples:

Example 1

Comparison of the Performance of Different Pretreatments for Chitin Beds Treating a Naturally Coloured Raw Water

Three chromatography columns (Sigma, 1.5 x 30 cm) were packed with 15 g of chitin prepared according to the method described in WO 92/02460. Each bed was washed with 1 1 distilled water. Two beds were conditioned with 100 ml each of 0.05 M hydrochloric acid followed by 500 ml water, while the third bed was washed with 500 ml 0.05 M sodium hydroxide, followed by water.

Raw water (1800 ml) at natural pH (4.8) was pumped through one of the beds washed with HCl and the bed washed with NaOH using a peristaltic pump (Pharmacia PI) . The same volume of water was adjusted to pH 9 using 2 M NaOH and pumped through the second HCl washed bed. Composite samples were collected from each bed at 30 minute intervals and analyzed for pH, colour, turbidity, iron, aluminium and manganese content.

The results in Table 1 show that pretreatment with HCl gave the most effective removal of colour, iron and aluminium, whereas pretreatment with NaOH gave the most efficient removal of manganese (93% removal of Mn after the passage of 13.5 bed volumes through the column). Raising the pH of the raw water increased slightly the amount of manganese adsorbed from the water, but this increase was small, when compared with the increase observed after running the water through a column prewashed with alkali. Example 2

Comparison of Regenerants for a Chitin Bed After Treating 56 Bed Volumes of a Naturally Coloured Raw Water

A pilot scale bed of chitin (0.025 x 1.2 m, bed volume 60 1) was washed with 0.05 M HCl, rinsed with water and then raw water (56 bed volumes) was pumped through the bed until colour breakthrough of 10 Hazen units occurred. The colour of water is expressed in terms of Hazen units and one Hazen unit is defined as the colour produced by 1 mg/1 of platinum in the form of chloroplatinic acid in the presence of 2 mg/1 of cobaltous chloride hexahydrate. Samples are compared against standards either visually, or photoelectrically at 400 nm using a previously constructed calibration graph. The bed was then regenerated using initially sodium hydroxide (0.048 M) and subsequently hydrochloric acid (0.035 M) .

The results in Table 2 show that colour, iron and aluminium were effectively released during regeneration with NaOH, while manganese was not released. In contrast Mn was released during regeneration with HCl . Fe and Al were also released to some extent in HCl .

Example 3

Effect of Varying Regeneration Conditions in a Pilot Plant containing Packed Beds of Chitin and Treating an Upland Raw Water

The pilot plant process consisted of three functional sections:

1. Raw water filtration (sand/anthracite) to remove gross suspended matter.

2. Three chitin packed beds connected in series. 3. Regenerant services.

Each of the chitin vessels was 250 mm diameter and 1500 mm between top and bottom distributing nozzle plates. Each vessel contained approximately 12 kg of chitin particles 0.5 to 2 mm in size. Filtered water was passed downwards through the three chitin beds in turn, then drained via a siphon beaker. In passing through the chitin, impurities such as humic acid colour, iron, aluminium and manganese were removed to different extents. The pilot plant treated between 5000 and 7000 litres of water per day.

During Period 1 the chitin was regenerated once daily using 0.05 M NaOH and then returned to an acid pH with ~ 0.035 M HCl or 0.025 M H₂S0₄ so that when the unit was returned to service the chitin bed was in an acid state. In this condition there was no removal of manganese from the raw water, whereas iron and aluminium were effectively removed (Table 3) .

During Period 2 the chitin was left in an alkaline state by rinsing with 0.05 M NaOH after regeneration with 0.025 M H₂S0₄. When the chitin bed was in an alkaline state manganese was effectively removed from the raw water down to its detection limit for some of the operating period. Control failures during Period 2 resulted in occasional large peaks of manganese and other contaminants appearing in the treated water. Iron and aluminium were also removed effectively while the chitin was maintained in an alkaline state in Period 2. TABLE 1

Pretreatment: Bed Rinsed with 0.05M HCl

Bed βH Colour Turbid Fe Al Mn X Mn Vol Hazen ntu tg/1 uα/1 uα/1 Removed

0 4.82 46 3.74 0.614 0.410 0.067 0

6.8 3.59 0 0.75 0.095 0.162 0.066 1

13.5 3.66 0 1.3 0.109 0.202 0.066 1

20.3 3.67 0 1.42 0.145 0.227 0.066 1

27.1 3.75 2 1.51 0.154 0.227 0.065 3

33.8 3.78 4 1.62 0.164 0.226 0.065 3

Pretreatment: Raw water DH ad.iusted from 4.8 to 9 using NaOH and bed rinsed with 0.05 M HCl

Bed βH Colour Turbid Fe Al Mn X Mn Vol Hazen ntu uα/1 uα/1 wq/1 Removed

0 8.14 55 3.03 0.540 0.415 0.067 0

6.8 3.93 0 0.91 0.084 0.116 0.057 15

13.5 3.81 0 1.26 0.132 0.123 0.059 12

20.3 3.86 2 1.49 0.176 0.144 0.059 12

27.1 3.91 2 1.61 0.215 0.150 0.053 21

33.8 3.99 4 1.78 0.254 0.164 0.048 28

Pretreatment: Bed Rinsed with 0.05M HCl

Bed βH Colour Turbid Fe Al Mn X Mn Vol Hazen ntu mn uα/1 mn Removed

0 4.68 51 3.59 0.530 0.405 0.067 0

6.8 6.38 30 3.67 0.294 0.213 0 100

13.5 6.38 23 2.35 0.250 0.172 0.005 93

20.3 6.09 16 2.25 0.243 0.167 0.027 60

27.1 6.21 19 2.42 0.244 0.165 0.049 27

33.8 6.25 18 2.27 0.247 0.166 0.060 10 TABLE 2

Comparison of the Reαenerants for a Chitin Bed After Treatinα 56 Bed Volumes of a Naturally Coloured Raw Water

Solution/Regenerant Hours Fe Al Mn Colour pumped Through Bed ( g/1) ⁽/ιg/1⁾ ( g/1) (A 400 nm x 10)

Treated water 0 83 24 5 0.30

NaOH (0.048M) 0.02 76 24 5 0.08

NaOH (0.048M) 0.18 114 47 5 0.55

NaOH (0.048M) 0.27 133 43 5 0.57

NaOH (0.048M) 0.35 6767 2679 7 40.00

NaOH (0.048M) 0.43 4027 1917 7 24.10

NaOH (0.048M) 0.52 2475 777 6 14.50

NaOH (0.048M) 0.60 2225 654 6 12.88

NaOH (0.048M) 0.77 2162 612 5 11.53

Treated water 1.02 2082 612 6 11.60

HCl (0.035M) 1.20 794 237 5 4.09

HCl (0.035M) 1.32 287 132 5 1.36

HCl (0.035M) 1.45 183 87 6 0.27

HCl (0.035M) 1.53 706 167 116 0.21

HCl (0.035M) 1.75 3668 714 85 0.26

Treated water 1.92 3275 722 91 0.22

Treated water 2.25 1129 283 36 0.10

Treated water 2.45 116 44 5 0.04

Treated water 3.23 75 45 5 0.07

Treated water 3.67 34 20 5 0.07

Treated water 3.88 32 19 5 0.09 TABLE 3

Comparison of Manganese. Aluminium and Iron Removal from

Raw Water bv Packed Beds of Chitin in a Pilot Plant with Acid

Pretreatment (Period 1) and Alkali Pretreatment (Period 2)

Claims

1. A method of adsorbing manganese from an aqueous solution which comprises (a) contacting a solid adsorbent with aqueous alkaline solution, (b) directly contacting the adsorbent resulting from step (a) with the manganese- containing solution, and (c) separating the adsorbent from the solution.

2. A method according to claim 1 wherein the adsorbent is at least one of chitin, chitosan, alginic acid, peanut skins, coconut peat, onion skins, peanut cases and wood chips.

3. A method according to claim 2 wherein the adsorbent is chitin.

4. A method according to claim 3 wherein the chitin is crab chitin.

5. A method according to claim 3 wherein the chitin is shellfish chitin.

6. A method according to any one of the preceding claims wherein the manganese-containing solution further contains metal ions, organic compounds and/or particulate materials which are adsorbed and/or trapped by the adsorbent .

7. A method according to any one of the preceding claims wherein the aqueous alkali solution is 0.005 to 0.5 M NaOH.

8. A method according to any one of the preceding claims which further comprises regenerating the adsorbent to which the manganese is adsorbed by contacting the adsorbent with an aqueous acid solution.

9. A method according to claim 8 wherein the aqueous acid solution is 0.005 to 0.5 M HCl.

10. A method according to claim 8 or 9 wherein the regenerated adsorbent is recycled to step (a) .