Key factors in chemical reduction by hydrazine
for recovery of precious metals
J.Paul Chen *,L.L.Lim
Department of Chemical and Environmental Engineering,The National University of Singapore,10Kent Ridge,
Crescent,Singapore 119260
Received 23January 2002;received in revid form 11June 2002;accepted 11June 2002
Abstract
Most of the commonly ud metal waste treatment approaches only allow removal of metals which are ultimately discarded as sludge and do not permit the reu of the metals,resulting in a waste of raw materials.In this study,the recovery of precious metals of sliver and copper in a synthesized wastewater in batch reactors was investigated using a reduction method by hydrazine as the reducing agent.Recovery of metal ions was greatest at pH >11.The prence of humic acid did not have negativ
e effects on the recovery process.Varying dissolved oxygen levels in the hydrazine solution did not significantly affect the recovery of both metals while eding and ageing process resulted in an in-crea in the particle size of the solid obtained.Under competitive conditions between Cu 2þand Ag þions,the recovery of silver remained the same,while that of copper was enhanced.Ó2002Elvier Science Ltd.All rights rerved.
Keywords:Metal;Recovery;Chemical reduction;Hydrazine;pH;Humic acid
1.Introduction
Increasing industrialization and urbanization world-wide had substantially ravaged our aquatic environment through the discharge of industrial and domestic wastes.The wastewaters are frequently laden with toxic heavy metals such as copper,silver and mercury in which sig-nificant amounts are deposited into the natural aquatic and terrestrial ecosystems.For humans,poisoning by the metals can result in vere dysfunction of kidney,reproduction system,liver,brain and central nervous systems.
The toxicity of heavy metals had received much concern,which had triggered numerous studies focusing on their removal.Techniques,such as precipitation,adsorption,biosorption,ion exchange,and
membrane
technology,exist for metal removal from waste streams (Volesky,1989;Xie et al.,1996;Chen and Peng,1999;Chen and Lin,2001).Selection of the techniques is bad on factors such as metal concentration,pH and operational efficacy.However,the methods only allow removal of metals which are ultimately discarded as sludge and do not permit the reu of the metals,re-sulting in a waste of raw materials.This is especially rious for the electroplating industry.
As the natural sources for metals are dwindling,it is becoming more economical to recover heavy metals if possible to their metallic state and reu the metals obtained.The u of reducing agents such as sodium borohydride,hydrazine and formaldehyde for metal recovery is a relatively simple method,which is similar to chemical precipitation except that the different soluble metal cations are recovered (by chemical reduction)as uful insoluble elemental metal instead of metal hy-droxide sludge.The metal solids can then be parated from the solution by filtration,dimentation and cen-
trifuge.
Chemosphere 49(2002)
363–370
/locate/chemosphere
*
Corresponding author.Tel.:+65-6874-8092;fax:+65-6779-1936.
E-mail address:paulchen@nus.edu.sg (J.P.Chen).
0045-6535/02/$-e front matter Ó2002Elvier Science Ltd.All rights rerved.PII:S 0045-6535(02)00305-3
Sodium borohydride(NaBH4)was utilized to treat copper and cobalt metals(Gomez-Lahoz et al.,1992). A1000l/h continuous pilot-plant operation with the influent metal concentrations in the25–40mg/l range was tested.Residual concentrations lower than0.1mg/l were achieved.Metal reduction generally proceeds ac-cording to the following reaction(Fanning et al.,2000):
M mþþBHÀ
4þH2O!M0#þBðOHÞ
3
þH2ð1Þ
where M is the ,copper).
Prior addition of sodium dithionite is required to avoid reoxidation problems arising from dissolved oxy-gen(DO).Due to immediate formation of ultrafine copper metal particles,flocculation–dimentation and sandfiltration have to be ud for sludge paration. Meanwhile,a new impurity(B(OH)3)is produced in the effluent,which might be toxic,and thus defeating the purpo of removing toxic heavy metals.High cost of sodium borohydride retards its application.Becau of the disadvantages,this reductant ems less favourable in metal recovery.
Hydrazine(N2H4)is a powerful strong reductant widely ud in various chemical operations.A ries of striking results has been obtained where hydrazine is ud as a reducing agent for the production offinely divided metals,of metal-on-glassfilms,and of metallic hydrosols,and electroless plating(Simpson,1985).In a rearch conduced by Ducamp-Sanguesa et al.(1993), monodisper spherical palladium particles were pro-duced by the hydrazine reduction of[Pd(NH3)4]2þin ethylene glycol in the temperature range from20toÀ9°C.Degen and Ma c ek(1999)ud hydrazine as a re-duci
ng agent to prepare nickel powders in the submi-crometer size range from nonaqueous solutions of nickel salts.Paraffin oil,ethylene glycol,di-and tri-ethanol-amine were ud as the reaction media.The rate and yield of the reaction were both enhanced at higher re-action temperatures but were limited by the relatively low boiling point of water.Nickel powders with mean particle sizes ranging from0.1to veral l m and with up to99.8%purity were obtained by this method.Nickel et al.(2000)studied the production of a silver colloid by reduction with hydrazine as a support for highly nsitive surface-enhanced Raman spectroscopy.The reduction of aqueous silver nitrate by hydrazine di-hydrochloride in weakly alkaline solution results in a polydisper colloid that is stable for many months without addition of stabilizing compounds.The average size of the predominantly spherical particles depends on the initial concentration of silver ions and ranges from 40to70nm in diameter.
An important half reaction involving hydrazine is: 4OHÀþN2H4¼N2þ4H2Oþ4eÀE0¼1:17Vð2ÞIt can effectively be employed in reduction of various metal cations(M nþ)to the elemental state(M0)ac-cording to the following reaction(Lee,1996):
M mþþN2H4!M0#þN2þHþð3aÞBad on the discussion by Tobe and Burgess(1999), metal ions can also be reduced:
M mþþN2H4þOHÀ!M0#þN2þNH3þH2O
冠状沟炎
ð3bÞHydrazine can react with DO in water according to (Audrieth and Ogg,1951),
N2H4þO2!N2þ2H2Oð4ÞIt can undergo lf-oxidation and reduction in both al-kaline and acidic solutions,
3N2H4!N2þ4NH3ð5ÞThe application of hydrazine in recovery of precious metals from wastewater however is less available in lit-erature.As one can e from the above reactions,pre-cious metals can be easily recovered by using this powerful reductant.Since metal cations are immediately reduced to metallic state,there is very limited amount of metal ions prent in the solution.Complexation be-tween metal and ammonia(due to Eqs.(3b)and(5)) therefore unlikely occurs.By using air stripping,am-monia is easily removed from the solution.In addition, unutilid hydrazine can be removed by aeration(Eq.
(4)).
The objective of the study was to explore the per-formance of hydrazine as a reducing agent for copper and silver recovery from synthesized wastewater streams.Factors controlling the reduction process,such as pH,metal concentration,DO,effect of eding and ageing process,as well as prence of humic acid(HA) and competitive metal ions were investigated.
2.Experimental
Copper sulphate,silver nitrate and sodium hydroxide from Merck(Germany),nitric acid from Ashland Chemical(USA),HA(sodium salt)from Aldrich(Mil-waukee,USA)and hydrazine hydrate from Sigma Chemical(USA)were ud in this study.The hydrazine hydrate ud contained85%aqueous solution of hy-drazine hydrate,which is equivalent to54.4%hydrazine. All chemicals ud are of reagent grade.A stock copper solution of5000ppm was prepared by dissolving copper sulphate in a1000ml volumetricflask and various copper concentrations were obtained by dilution of the stock solution.A stock silver solution of5000ppm was prepared by the same method.
364J.P.Chen,L.L.Lim/Chemosphere49(2002)363–370
Metal concentrations were analyzed by an induc-tively coupled argon plasma atomic emission spectro-scopy(ICP-AES)(Perkin Elmer Optima3000DV,USA). Solution pH was measured by an Accumet Basic pH meter from Fisher General Scientific.All samples were filtered using Whatman Autovial syringeless0.45l m PTFEfilters(Clifton,NJ)before analysis.Particle size of the solids obtained was analyzed using a LS230 Coulter Particle Analyzer.DO was measured using a Model50B Dissolved Oxygen Meter from YSI Incor-porated(Ohio,USA).A JEOL JSM-5600LV scanning electron microscope(SEM)was utilized to analyze morphology of the solids.
pH of each metal sample was adjusted using1M sodium hydroxide or0.1M nitric acid.A10ml hydra-zine solution with pH of9was then poured into a10ml metal sample in a centrifuge tube(withfixed pH or different pH depended on experimental objectives).(The metal concentrations in thefigures are referred to the
concentrations of the mixed solutions.)The tubes were shaken in a controlled water bath at25°C.A reaction time of2–4min was employed as the reduction reactions occurred quickly.The mixed solutions were sub-quently centrifuged at5000rpm in order to parate the resultingfine metal solids from the solution.The su-pernatant was analyzed by the ICP-ES and the pH meter.Some of the solids were dried up at50–60°C in an oven before they were nt for the analysis of particle size as well as the surface morphology by the SEM. Experiments for effect of initial metal concentration, HA,competitive reduction and DO were similarly con-ducted with an exception that the solution pH>11was controlled.
The experiments to enhance the sizes of the chemi-cally reduced copperfine particles through eding and ageing process were carried out.Hydrazine was slowly added to the metal solution controlled at different tem-perature.The resultingfine particles were allowed to stay in an oscillating water bath for a period of time so that they can grow into large ones.
3.Results and discussion呦组词
3.1.Effect of pH
As solution pH can change the solution chemistry of the aqueous systems,it could significantly affect the treatment results.Experiments for pH effect on the re-duction of Cu2þby N2H4werefirst carried out.When pH was high,some copper precipitates were obrved before hydrazine was added.However,they were dis-solved immediately when hydrazine was poured.The variation of solution pH and thefinal copper concen-trations were illustrated in Fig.1.
It shows that all points were linearlyfitted(1/1)in initial pH of2–5and9–13.Thefinal pH was around9 when the initial pH ranged6–9,which could cau by the weak basicity of hydrazine according to the follow-ing reaction(Audrieth and Ogg,1951):
N2H4þH2O¼N2Hþ
5
þOHÀð6aÞK¼
½N2Hþ
5
½OHÀ
½N2H4
¼8:5Â10À7ðat25°CÞð6bÞ
Hydrazine rembles ammonia as it can be regarded as an ammonia derivative in which one of the hydrogen atoms in NH3is replaced by the more negative NH2 group.
Fig.1shows that thefinal copper concentration significantly decread as the initial pH was incread from2to6.At initial pH>6:0,the copper concentra-tions ranged from9.1to15.3ppm;namely,the recovery percentage of97–98%was achieved.The dramatic in-crea in the recovery was due to the change in the so-lution chemistry.The speciation of hydrazine versus pH bad on Eq.(6b)indicates that percentage of N2H4 reaches100%at pH>11.An important half reaction
involving hydrazonium(N2Hþ
5
)is:
伤感说说心情短语N2Hþ
5
þ3Hþþ2eÀ¼2N2Hþ
4
E0¼1:27Vð7ÞOne can e that hydrazonium is a strong oxidizing re-agent and cannot recover metal ions.
The reactions between Cu2þions and hydrazine proceed according to the below reaction:
2Cu2þþN2H4¼2Cu#þN2þ4Hþð8aÞCu2þþ2N2H4þ2OHÀ¼Cu#þN2þ2NH3þ2H2O
ð8b
Þ
电子表格怎么求和
脚发热什么原因J.P.Chen,L.L.Lim/Chemosphere49(2002)363–370365
On the basis of Eqs.(6a),(8a)and(8b),we can conclude that thefinal pH has to be controlled above11.There-fore,in the subquent experiments,the pH was always adjusted to11.
Interestingly,one can obrve from Fig.1that the final concentration begins to achieve its minimum value at initial pH of6(correspondingfinal pH of8.7).Hy-drazine–hydrazonium speciation diagram sho
ws that the former occupies80%,resulting in incomplete copper reduction.Therefore,precipitation reactions shown below could play some roles in the copper removal since the initial copper concentration was quite high(Schecher and McAvoy,2001):
Cu2þþH2O¼CuOþ2Hþp K¼7:64ð9aÞ
Cu2þþ2OHÀ¼CuðOHÞ
2
p K¼8:67ð9bÞBy using a centrifuge,the metal precipitates were pa-rated from the bulk solution.
In addition,Cu2þcould be reduced to Cuþ,which instantly formed metal crystal.This phenomenon has widely been reported in various literatures on electroless plating.For example,Lim et al.(2001)found from X-ray photoelectron spectroscopic measurements that during their electroless plating process,the Cu2þions werefirst reduced to Cuþon the polypyrrolefilm sur-face.The Cu(I)was subquently reduced to its ele-mental form.In their study,formaldehyde was ud as a reductant.
3.2.Effect of initial concentration
The effect of initial metal concentration was investi-gated with afixed hydrazine concentration of3:1Â10À3 M.The respective copper and silver concentrations were 9:0Â10À4–8:0Â10À3M and4:7Â10À4–4:7Â10À3M. For each metal,two experiments were carried out.pH was adjusted to11for one experiment,while the pH was not controlled for the other.
宝星棋牌
As shown in Fig.2a,thefinal copper concentration
for‘‘nofixed pH’’ca(pH ranging from4.1to7.8for copper)was much higher than that for‘‘pH11’’ca. Thefinal copper concentration of0–1:8Â10À4M was obrved for the initial concentration of9:0Â10À4–8:0Â10À3M at pH11.Bad on the stoichiometry(Eq. (8a)),final copper concentration can be calculated.As illustrated in thefigure,the calculatedfinal concentra-tions(termed as theoretical values)match very well with the measured ones when the initial concentrations were less than6:25Â10À3M.When the initial copper con-centration was higher than6:25Â10À3M,the theoret-ical values were however much higher than the measured ones becau the precipitation reactions and/or addi-tional reductions(Cu2þ!Cuþ)were not taken account in the calculation.It was also noted that for lower initial copper concentrations,the copper solids formed were veryfine and tended to be suspended in the solution, taking an extremely long time to ttle down.
Effect of initial silver concentration on its recovery was illustrated in Fig.2b.Hydrazine reacts with silver ions to give elemental silver through the below reactions: 4AgþþN2H4¼4Ag#þN2þ4Hþð10aÞ
2Agþþ2N2H4þ2OHÀ¼2Ag#þN2þ2NH3þ2H2O
ð10bÞAs shown in thefigure,thefinal silver concentration at pH11was lower than2:3Â10À5M for the concentra-tion ranging from5:0Â10À4to4:7Â10À3M.The人格测试题
final
366J.P.Chen,L.L.Lim/Chemosphere49(2002)363–370
concentration for‘‘nofixed pH’’ca was also lower than2:3Â10À5M for the initial concentration<3:7Â10À3M;however,a sharp jump was obrved at the initial concentration>3:7Â10À3M,which was due to the incomplete reduction of silver at the lower pH.
Similarly,final silver concentrations were calculated bad on the stoichiometry as shown in Eq.(10a). Generally speaking,the theoretical values matched the obrved ones well.There are however slight deviations due to the simplification in the calculations.
3.3.Effect of HA
HA is one of the many naturally occurring dissolved organic matters found in waste streams.In natural water,about50%of the dissolved organic materials con-sists of HAs and fulvic acids.The environmental significance of metal sorption onto humic substances derives from the fact that humic substances provide an important source of organic ligands such as carboxylic and hydroxyl groups(–COOH and–OH)and,therefore, are expected to influence the bioavailability and mobility of metals in
soil,diments and aquatic systems(Liu and Gonzalez,2000).Humic substances can exist in precious metal waste streams.Hence,experiments were carried out to investigate the effect of the prence of HA on the recovery of the metals.
It was obrved in Fig.3a that HA did not in anyway hinder the recovery of copper,but rather,the recovery was much higher than when there was no prence of HA.This could be due to the immediate formation of Cuþand subquent production of Cu–HA and/or Cuþ–HA crystals.The silver recovery,however,exhibits a slightly different trend.Fig.3b shows that at lower initial silver concentrations,the addition of HA thwarted the recovery of silver.At higher initial concentrations,the recovery was similar to that when no HA existed.It can be speculated that in low silver concentrations,there are sufficient amount of HA to bind a substantial amount of Agþions,thus preventing from undergoing reduction by
hydrazine.
3.4.Effect of DO
Though the recovery of silver and copper by hydra-zine is successful,it is still not satisfactory as hydrazine could be harmful when its concentration in aqueous pha reaches a certain level.Physiol
ogical effects of aqueous solution of hydrazine include corrosion to eyes, skin and mucous membranes upon contact and it is a probable human carcinogen.Furthermore,hydrazine fumes with air and its vapour attacks the no and throat upon inhalation and can also irritate the eyes, causing temporary blindness(Audrieth and Ogg,1951). Hence,it is important to control the amount of hydra-zine added for the metal reduction,as an excess amount of hydrazine will result in the above problems in the treated water.
Dilute solutions of hydrazine are particularly sus-ceptible to attack by atmospheric oxygen and the de-composition of hydrazine generally proceeds according to Eq.(4).In order tofind out whether DO level can significantly affect the metal recovery,a ries of experi-ments with various DO levels was carried out(results not shown here).It was obrved that the increasing levels of DO in hydrazine solution did not have a very significant effect on thefinal concentration of aqueous Cu2þsolution.It can be due to the rapid reduction re-action kinetics listed in Eqs.(3a)and(3b).
A study showed that an alkaline solution of hydra-zine containing0.1M was found to have undergone decomposition to the extent of2%during a
10min
月暗
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