heavy metal precipitation

8/12/2019 Heavy Metal Precipitation

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Experiment 2 Elimination of heavy metals bymeans of a precipitation process

1. Introduction [1]

Heavy metals are critical elements for human health and the environment because they show a

distinctive capability for accumulation in organisms and other environmental spheres.

The maximum limits for some important heavy metals, following the regulations on safe

drinking water is given in table 1 sind, [, !].

Tab. 1" #aximum limits of important heavy metals in drinking water

Nr. Parameter regulation on safe drinking water

[mg/L]

1 antimony, $b %,%%&

arsenic, 's %,%1

! lead, (b %,%1

) cadmium, *d %,%%&

& chromium, *r %,%&

+ copper, *u ,%

nickel, -i %,%

mercury, Hg %,%%1

/ selenium, $e %,%1

*urrent problems with trace metals in drinking water appear exclusivly from the output of

materials 0copper and lead which belong to the distribution system

The elimination of heavy metals is an elementarily process step in waste water treatment..

2eside the avoidance of emissions in the a3uatic environment it is utili4ed to recycle

valueable products [) 5 +].

Heavy metal containing waste waters are derived from the chemical industrie, the metallurgy,

the mining and especially the metal5working industry.

(ossible processes for removal of heavy metals in drinking water treatment is summariced in

Tab. . 6locculation and precipitation are the most important techni3ues.

1


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Tab. " review of processes for heavy metal removal 0drinking water treatment [1]

process Al Sb As Pb Cd Cr Ni g Se !n

precipitation/floccu-

lation

77 77

08886e

77

096e

77 77 77 77

:alk

7 7 7

oxidation/reduktion 7

;x.

7

;x.

77

1% are

re3uired for 3uantitative precipitation. $trong bases like sodium hydroxide solution and lime

milk 0calcium oxid5solution are normally used.

The advantages of sodium hydroxide solution are caused by a high solubility and good

proportioning properties. ?isadvantages are represented in higher costs and in the worsend

filterability.The solubility of suspended lime milk @ust amounts by 1, gAB at % C* and decreases with

higher temperatures. Bime milk dissolves and neutrali4es very fast if its mixed with acidic

waste water. 'dvantages of the lime milk utili4ation are not only caused by lower costs 0&,

times cheaper then -a;H, but also by a better process engineering. $o the bivalent

calciumion influences the coagulation in a convinient way and its precipitation products have

a better filterability.

Heavy metals react to nearly dissoluble hydroxids or dissoluble basic salts. The formation and

the stoechiometry of this products depend on the precipitation5pH5value. The constitution of

these products can verify in big range. $everal modifications of the hydroxids, the basic salts

and of common mixtures can originate in dependance of the precipitation conditions. The

basicity of the different hydroxids effectuats different pH5values. ' 3uantitative precipitationprocess needs a sufficient number of hydroxide ions [1.1].


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0 zz

Me zOH Me OH+ + [1.1]

The pH5value for 3uantitative precipitation has to be higher when the hydroxid is more basic.

$o a ph5range between !,& for 8ron 0888 and 1%,& for *admium is observed.

' decrease of pH5values in the solutions is determined after 3uantitative precipitation. This

effect is caused by metal oxidation 06e886e888 or by slow transformation of basic salts into

hydroxids. Hydroxids of the trivalent metals are highly voluminous. Therfore they are used in

waste water treatment for the process step flocculation.

'n oversaturation of the precipitant in the solution is necessary before the precipitation can

start.

't first nuclei have to form. They further grow to bigger particles until a filterable precipitate

is originated. The ratio of concentrations is ad@usted according to the corresponding solubility

product after the precipitation. The precipitat properties strongly depend on the rate of nuclei

formation and the rate of cristallisation. The nuclei formation is substantially faster then the

cristal growth. 8f the oversaturation is formed very fast the rate of nuclei formation will be

increased by formation of many nucleis. The cristal growth is ended when the concentrationgradient in the limiting diffusion layer is low. 's a result many small particles are formed

which create a fine disperse participate with an unsufficient filterability.


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tab. !" important parameters for precipitation of metals with sodium hydroxide [)]

metal p"#alue solubilit$

product

%issociation

constant of

$dro&ide

comple&

begin of

precipitation

'uantitati#e

precipitation

redissolution

6e0888 , !,& 5 , 1%5! 5

$n088 !, ),! /, + 1%5& 1%5&

'l ),! ), , 1%5! ) 1%5!)

*r &, +, ,! ! 1%5 &,+ 1%5!

*u &, ,+ 5 1%51/ 5

6e088 ,% ,/ 5 1%51& 5

Dn ,+ /,% 1%, ) 1%51 + 1%51/

-i , /,/ 5 &, 1%51& 5

(b +,& 1%,! 5 ca. 1%51! 5

*d /,1 1%,) 5 1,! 1%51) 5

The consumption of neutrali4er is negatively influenced by certain water ingredients, e.g."

the buffer capacity,

the presence of complexing agents.

Especially the influence of complexing agents can generate a high transgression of the

limiting values. -evertheless a 3uantitaive precipitaion is reached by using of an excess of

lime milk 0overbased. 2eside the higher pH5values the yielded calcium5ions can influence

the precipitation positively by"

creation of poorly soluble calcium compounds with the complexing agents 0e. g. poly5

phosphates,

replacement of heavy metal complexes by creation of calcium complexes,

reduction of the complex stability by an increase of total ion concentration.

*opper can precipitate with lime milk in presence of E?T' by ad@usting pH5values of more

then 11,. The residual concentrations are higher by reaching pH5values over 1,&. 'lso good

results with citrate or tartrate containing water were achieved by using of higher

concentrations of lime milk []. 'ddition of drop chalk to lime milk is able to moderate the

alkali4ation effect [].

The 6erro5Bime milk5(rocess can furtherly improve the removal of copper in complexing

agents containing water. The success of this process is based on the reduction of the bivalent

copper ion with iron 088. The precipitation of the generated monovalent copper ion is more

effective. 'lso copper ions can incorporate into the isomorph iron0885hydroxid5lattice [/]. $o

residual concentrations of copper under %,& mgAB by an ad@usted pH5value of 1%, areachieved. 'dding of 6e0888 supports the *opper elimination via flocculation and

)


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(. Aim

2asic processes for the treatment of heavy metal containing waste water 0neutrali4ing

precipitation should carry out. The influence of complexing agents on the removal of heavy

metals is analy4ed.

). Sub*ect8n this practical course a two5stage process for waste water treatment is demonstrated. The

neutrali4ing precipitation with sodium hydroxide represents the main step for the elimination

of the heavy metal copper. 6urther treatment of the residual copper should carry out by using

iron08885salt, lime and drop chalk. The influence of complexing agents on the removal of

heavy metals by precipitation is demonstrated by the example of the copperAtartrate5system.

+. Instruments

measuring graduated flask 01B,

beakers 01&%% mB, %% mB, 1&% mB

Erlenmeyer flask %% mB,

graduated cylinder,

8mhoff funnel,

transfer pipettes 0& mB, 1% mB, & mB,

dropping pipette,

funnel,

tap funnel 0&% mB with precision divison,

filters 0hard,

magnetic stirrer,

funnel rack,

stop watch, pH5meter.

,. C-emicals

a) Synthetic waste water

$tock solution 8 mit % gAB *u70F %,!1& molAB"

,&/ g *u$;)=&H; 0#G )/,+ gAmol dissolved in destilled water and fill

up to 1 B

$tock solution 88 mit & gAB tartrate 0F %,1+ molAB"

),% g :5-a5tartrate=)H; 0#G ,+ gAmol dissolved in destilled water

and fill up to 1 B

b) Chemicals for waste water treatment

sodium hydroxide 1 #

iron chloride5solution 0c F !% gAB 6e*l=+H;

calcium carbonate 0drop chalk

Bime milk &% gAB *a0;H

hydrochloric acid 01% ig

c) Atomic Absorption Spectroscopy (AAS)

copper5standard solution 01%%% mgAB

dilution water 0 mB kon4. H-;!AB


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$ynthetic waste water will be produced from stock solutions 8A88 according to the descriptions

in tab. ) for a total volume of %,& B 0pH ) I &, H*l.

Tab )" production of the waste water models

odel"number. 0 ( ) + ,

$tock solution 8

[mBAB]]

& & & & &

$tock solution 88

[mBAB]

% ,& ,& 1& )%

*opper0885concentration [mgAB] 1%%% 1%%% 1%%% 1%%% 1%%%

*opper0885concentration [mmolAB] 1&,& 1&,& 1&,& 1&,& 1&,&

tartrate5concentration [mgAB] % 1& !& &% %%%

tartrate5concentration [mmolAB] % %,! ,& & 1!,!

mole ratio tartrate " *u7 % %,%& %,1+ %,! %,&

The influence of complexing agents will be tested by using five several tartrate5

concentrations.

't first the chief portion of copper will be reduced by using neutrali4ing precipitation untill

a pH5value of .

The residual copper is eliminated by using of an additional treatment step. Therefore the

copper complexes have to destabili4ed.

1. possibility" ?estabili4ation can reached by addition of iron08885ios. The treatment

will be finnished by addition of lime milk and drop chalk.

. possibility" The complexing agents can be destroyed by using of advanced oxidation

processes like o4oni4ation, J9, H;, 6enton5process etc.

The waste water models will be treated by a precipitation with sodium hydroxide 0

molAB until a pH5value of is reached. ' dropwise addition of the base will be performed

to record a titrion curve.

Then the mixture will be filled in an 8mhoff funnel. The settled sludge volume will be

determined after half an hour. %% mB of destilled water should be given to the sample

before a filtration will be carried out.

*opper concentration will be determined by using 'tomic 'bsorption spectroscopy. '

calibration should be performed before the measurement of the samples begin.

&% ml will be taken from the sample and mixed with 8ron chloride5solution 0!% gAB

6e*l!=+H;. The concentration of iron will be originated by +% mgAB. The pH5value will

be under &. ;therwise it will be ad@usted with H*l 01% . Then 1%% mgAB drop chalk 01

tip of a spatula will be added to the solution. ' pH5value of I / will be ad@usted by

addition of lime milk 0& . 't the end the solution will be mixed carefully for ! minutesand then has to settle.

+


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' small sample will be filtrated and the copper concentration is determined by 'tomic

'bsorption spectroscopy.

work flow for precipitation of waste water model ( 1record titration cur#es model ( 2 +34

&%% mB5volumetric flask

7 & mB stock solution 8 0copper0885sulfate5solution

7 ,& mB stock solution 88 0:5-a5tartrate5solution

5pH5ad@ustment with hydrochloric acid 01% between ) I &

transfer in a beaker

in %.& mB intervals addition of -a;H 0c F 1 molAB until pH

caution" at pH &,& dosing carefullyKK

please note the pH after every intervalKKJse this pair of values to generate a titration curve

5after precipitation transfer the solution complete in an 8mhoff cone

after !% min measure the volume of the settled sludge

filtrate nearly %% mB of the supernatant solution over a filter paper

5

&% mB of the filtrate is to analy4ed by atomic absorption spctrometrie

5postprecipitation of the filtrate by dosing of an iron08885solution that in the filtrate is a

c[6e0888] F +% mgAB at pH L &

add 1 spit of spatula of precipitated chalk to the filtrate

ad@ust the pH by dosing lime milk 0& between I /

agitate carefully for ! min and settle

5filtrate nearly %% mB of the supernatant solution over a filter paper

copper residues is to analy4e by ''$ L %, & mgAB

6. 7#aluation

1. The measured titration curves should be depictedK Explain the curves in dependance of the

content of the complexing agent.

. The measured contents of copper should be represented in tables and should be discussed

under consideration of the process steps and the complexing agent concentrations.

!. Mhy is the removal of heavy metals from waste water so importantN


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). *all several chemicals for the precipitation of heavy metals which are applied in waste

water treatment.

&. Mhy is the metal precipitation with sulfide criticalN

+. The residual concentration of metals in the solution increase with higher contents ofneutral salts. Mhat is the reason for this effectN

. Explain the measuring principle of 'tomic 'bsorption $pectroscopyK

8. Literature

[1] Gimbel,

heavy metal precipitation

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