Removal of cyanide from aqueous solutions by plain and

Removal of cyanide from aqueous solutions by plain and

metal-impregnated granular activated carbons

H.Deveci ?,E.Y .Yaz ?c ?,I.Alp,https://www.sodocs.net/doc/146091799.html,lu

Mineral Processing Division,Department of Mining Engineering,Karadeniz Technical University,61080Trabzon,Turkey

Received 10August 2005;received in revised form 27February 2006;accepted 2March 2006

Available online 27April 2006

Abstract

In this study,the removal of free cyanide from aqueous solutions by activated carbon was investigated.Effects of metal impregnation (Cu and Ag),aeration,and concentrations of adsorbent and cyanide on the rate and extent of the removal of cyanide were studied.The results have shown that the capacity of activated carbon for the removal of cyanide can be significantly improved (up to 6.3-fold)via impregnation of activated carbon with metals such as copper and silver.Silver-impregnated activated carbon was found to be the most effective at the reduction of cyanide level in solution.This appeared to be coupled with its comparatively high metal content after impregnation process where silver (5.07%)could be more readily loaded on activated carbon than copper (0.43%).Kinetics and equilibrium data for cyanide removal by plain and metal-impregnated activated carbons were determined to be consistent with the pseudo second-order kinetics and the Langmuir adsorption isotherms,respectively.Aeration (0.27l/min)was found to exert a profound effect on the process leading to a 5.5–49.1%enhancement in the performances of plain and metal-impregnated activated carbons.This enhancement could be attributed to the increase in the availability of active sites on activated carbon for adsorption and the catalytic oxidising activity of activated carbon in the presence of oxygen.Practical limiting capacity of plain,copper-and silver-impregnated activated carbons for the removal of cyanide were experimentally determined to be 19.7,22.4and 29.6mg/g,respectively.

?2006Elsevier B.V .All rights reserved.

Keywords:activated carbon;cyanide;effluent treatment;adsorption;environment

1.Introduction

All cyanides are classified as hazardous materials with acute and/or chronic characteristics of toxicity (Boening and Chew,1999;Hagelstein and Mudder,2001;Baskin and Brewer,2001;Mudder and Botz,2004).The effluents generated in gold/silver leaching and the metal finishing operations often contain free and metal-complexed cyanides and cyanide-related compounds at various levels (EPA,1994,2000;Mudder et al.,2001;Akcil,2002;

Zagury et al.,2004;Vapur et al.,2005).Treatment of these industrial effluents,prior to discharge,is therefore pre-requisite to meet environmental limits.

Several treatment processes based on natural degra-dation,chemical and biological oxidation,complexing/precipitation and recovery/recycling have been ex-ploited for the reduction of cyanide levels in waste solutions/slurries in compliance with environmental regulations (Young and Jordan,1995;?elik et al.,1997;Young,2001;Mudder et al.,2001;Akcil,2003;Akcil and Mudder,2003).Natural degradation is inherently a slow process depending intimately on the climate conditions (Botz and Mudder,2000).

The

Int.J.Miner.Process.79(2006)198–

208

https://www.sodocs.net/doc/146091799.html,/locate/ijminpro

?Corresponding author.

E-mail address:hdeveci@https://www.sodocs.net/doc/146091799.html,.tr (H.Deveci).

0301-7516/$-see front matter ?2006Elsevier B.V .All rights reserved.doi:10.1016/j.minpro.2006.03.002

oxidative treatment processes including SO2/air,hydro-gen peroxide,alkaline chlorination,Caro's acid,ozone and biological oxidation are widely practiced at pilot and industrial scale(Mudder et al.,2001;Akcil,2002, 2003;Whitlock and Mudder,2001;Castrantas et al., 1993).However,these processes are,in general,costly and ineffective for the removal of strong acid dissoci-able cyanides(CN SAD;p K N30)and,hence,further treatment of their effluents with complexing/precipita-tion methods may be required prior to discharge(Young, 2001).The processes such as A VR(Acidification–V olatilisation–Reneutralisation),based on the recovery and recycling of cyanide,may offer a less expensive option than the destruction of cyanide,particularly for the waste solutions/slurries containing high levels of cyanide;albeit,such processes often fail to fulfil strict regulatory requirements(Stevenson et al.,2001;Vapur et al.,2005).In addition to the aforementioned processes, cyanide compounds can be removed from waste solu-tions by adsorption on various organic and inorganic adsorbents including minerals,agricultural by-products, resins and activated carbon(Adams,1994;Young and Jordan,1995;Kurama and?atalsarik,2000;Young, 2001;Bose et al.,2002;Yaz?c?,2005).

Due to the particular affinity of gold and silver cyanides for adsorption,activated carbon is utilised extensively for the recovery of these metals from loaded cyanide leach solutions(McDougall and Fleming,1987; Marsden and House,1993).In addition to gold and silver, activated carbon can also adsorb cyanide complexes of other metals such as copper,zinc,nickel,mercury etc. from cyanide leach liquors with the extent of their adsorption being dependent on the conditions such as pH, free cyanide level,temperature,relative concentrations of metals and,speciation,solubility and charge of metal cyanide complex concerned(Bailey,1987).Botz and Mudder(1997)reported the successful use of activated carbon for polishing of effluents(0.15–5.53mg/l CN W AD or CN T)with over95%removal of cyanide as well as metals.Not withstanding this,the application of activated carbon for the treatment of cyanidation effluents has attracted comparatively less attention probably due to the limited capacity of activated carbon for the adsorption/ removal of free cyanide as noted by Adams(1994), Williams and Petersen(1997)and Adhoum and Monser (2002).These authors also indicated that the removal of cyanide could be improved using the activated carbons impregnated with metals such as Ag,Cu and Ni.Williams and Petersen(1997)reported that56.5%of free cyanide (20mg/l)could be removed using silver impregnated carbon(1g/l)compared with plain carbon(～11%CN removal).They attributed higher capacity of metal impregnated carbons for cyanide removal to the formation and adsorption of cyanide metal complexes on activated carbon.In column tests,Adhoum and Monser(2002) observed that silver and nickel impregnated activated carbons could adsorb up to26.5and15.4mg CN per unit mass of adsorbent respectively compared with7.1mg CN/g for plain carbon.In this regard,activated carbon has great potential for use as an alternative process for the removal of cyanide and metals from waste solutions and, therefore,could be particularly favourable for the plants where carbon adsorption technology is already employed (Adams,1994;Botz and Mudder,1997).Although many studies have concentrated on its adsorptive properties, activated carbon is also known to function as an oxidation catalyst in the presence of oxygen(Ahumada et al.,2002). Despite the fact that oxygen is also essential for the effective adsorption of precious metals on activated carbon from leach solutions(Bailey,1987;McDougall and Fleming,1987),the effect of aeration on the adsorp-tion of cyanide on activated carbon is often overlooked.

In this study,the performance of plain and metal-impregnated activated carbons for the removal of free cyanide from solutions was studied.The effects of ae-ration,activated carbon dosage and concentration of cyanide on cyanide removal were examined.Consider-ing their prime importance for the design of an adsorp-tion process,kinetics and equilibrium parameters for the removal of cyanide were determined.

2.Experimental

2.1.Reagents

Reagent grade sodium cyanide(NaCN),copper chloride(CuCl2·2H2O),silver nitrate(AgNO3)and sodium hydroxide(NaOH)were used to prepare stock solutions in distilled water.The coconut shell activated carbon(?4+1mm,BET:546m2/g)used in this study was kindly provided by Ovacik Gold Mine(Bergama, Turkey).Prior to the use in the experiments,the activa-ted carbon sample was washed with distilled water and then treated with acid(1%HCl).Following acid treat-ment,the sample was thoroughly rinsed with distilled water prior to heat treatment for30min in an oven maintained at700°C.Resulting activated carbon was then impregnated with copper and silver using the fol-lowing procedures.

2.2.Preparation of metal-impregnated activated carbons

The impregnation of activated carbon samples was performed by contacting a known portion of the

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pretreated activated carbon samples (21–23g)with a 200ml stock solution of copper (1g/l CuCl 2)and silver (1–10g/l AgNO 3)in Erlenmeyer flasks (250ml)over a period of three days.During the impregnation period,a number of samples were removed and analysed for copper and silver to monitor the impregnation process (Fig.1).The activated carbon was found to have a remarkably higher equilibrium capacity for the loading of silver than copper with rapid and almost complete depletion of silver from the solution.Following solid –liquid separation using a sieve (1mm),the impregnation procedure was completed by drying the metal loaded activated carbons in a furnace for one hour at 350°C.Although the impregnation of silver was carried out in solutions containing 1and 10g/l AgNO 3,the activated carbon impregnated from 10g/l AgNO 3solution was used in the experiments due to its higher silver content which resulted in higher capacity for the removal of cyanide.The metal content of activated carbons was determined to be 5.07%Ag and 0.43%Cu by weight.2.3.Adsorption experiments

Adsorption tests were performed in Pyrex beakers (600ml)in the confines of the laboratory.A 30-ml aliquot of cyanide stock solution (1000mg/l)was transferred into each reactor filled with deionised water to produce an initial cyanide concentration of 100mg/l in a final volume of 300ml.A required amount of plain and metal-impregnated activated carbons was added to achieve an adsorbent concentration of 0.2–4.5g/l.Over the experimental period,the top of the reactors were kept covered to minimise the transfer of atmospheric air from solution interface.The reactor contents were agitated using magnetic stirrers to ensure off-bottom suspension of activated carbon particles.In the experi-ments where the effect of aeration on the cyanide re-moval was investigated,air was supplied using a sparger to the reactors at a flow rate of 0.27l/min.

Sampling was performed by removing 5-ml aliquots which were then analysed for cyanide by titrating against standardised silver nitrate solution (0.001M)in the presence of p -dimethylaminobenzylrhodanine (0.02%w/w in acetone)as indicator (Patnaik,1997).The residual concentration of cyanide in solution was used to determine the extent of cyanide removal at a particular interval where change in volume and cyanide removed by samples were also accounted for.During the experiments,pH was maintained consistently at 10.5–11.0by the addition of 1M NaOH.2.4.Equilibrium experiments

These experiments were undertaken to collect the equilibrium data (adsorption isotherms)for the removal of cyanide by plain (AC),copper (AC –Cu)and silver (AC –Ag)impregnated activated carbons.Equilibrium tests were performed in 34-ml PTFE bottles with an operating volume of 15ml.Cyanide stock solution (1000mg/l)was aerated to increase the dissolved oxygen level and then diluted to prepare the cyanide solutions of different strength in phosphate buffer (pH 11).Bottles were filled with the solutions of varying concentrations of cyanide (10–200mg/l)prior to the addition of the required amount of adsorbent (1g/l)and placed on a reciprocal shaker operating at 180rpm.Having been allowed to equilibrate over a period of 72h,the bottles were sampled and the equilibrium concentration of cyanide in solution was analysed to determine the amount of cyanide adsorbed on each adsorbent (mg CN ?per g adsorbent)at different cyanide concentrations.The data from duplicate experiments were then used to construct the adsorption isotherms.3.Results and discussion

3.1.Effect of adsorbent concentration on cyanide removal

Fig.2illustrates the concentration of cyanide removed from the solution at different dosages (0.2–4.5g/l)of the plain activated carbon (AC).The solid lines in Figs.2,3and 4were produced using the pseudo second-order kinetic model (Section 3.2for more details).The removal of cyanide by AC was initially rapid;thereafter,it tended to slow down approaching equilibrium.This could be ascribed to the initially high availability of active sites,which gradually saturated

Time in hours

m m o l A g p e r g o f A C

0.00

0.05

0.10

m m o l A g o r C u p e r g o f A C

Fig.1.The loading of copper and silver on activated carbon from the solutions containing CuCl 2(1g/l)and AgNO 3(1–10g/l)over an equilibrium period of three days.

200H.Deveci et al./Int.J.Miner.Process.79(2006)198–208

with cyanide until the equilibrium conditions were established.Although the extent of cyanide removed increased with increasing the dosage of AC (Fig.2),the capacity of AC for cyanide removal appeared to be limited in that,even at 4.5g/l AC,the concentration of cyanide in solution was reduced by 14.3%corres-ponding to a cyanide removal of only 4.43mg CN ?(i.e.2.95mg CN per g AC).

A similar general trend for the time-dependency of cyanide removal at the adsorbent dosages tested was also observed for metal-impregnated activated carbons (Figs.3and 4).However,it can be deduced from Figs.3and 4for copper-(AC –Cu)and silver-impregnated (AC –Ag)carbons,respectively,that the capacity of activated carbon for the removal of cyanide could be significantly enhanced through impregnation with metals.To illustrate,the extent of cyanide removal by AC –Cu and AC –Ag was recorded to be 2.4-and 6.3-fold higher than that by AC at an adsorbent dosage of 4.5g/l (Fig.5).The results also indicated that silver impregnated carbon (AC –Ag)had the highest capacity with the removal of 5.7–92.3%CN ?compared with 1.5–14.3%and 4.4–35.4%CN ?for AC and AC –Cu,respectively,at the corresponding adsorbent dosages of 0.2–4.5g/l (Fig.5).The current findings were consis-tent with the earlier reports (Adams,1994;Williams and Petersen,1997;Adhoum and Monser,2002)that the impregnation with metals such as copper,nickel and silver improved markedly the performance of activated carbon for the removal of cyanide from solutions with silver leading to the highest increase in performance.The silver-and copper-impregnated activated car-bons were found to contain 5.07%Ag and 0.43%Cu,respectively,after the impregnation process (Fig.1).The amount of cyanide removed from solution was

Time in hours

C o n c . o f C N - o n A C (m g /l )

Fig.2.Removal of cyanide from solution using plain activated carbon (AC)at the adsorbent dosages of 0.2–4.5g/l ([CN ?]0:100mg/l,pH 10.5–11).

Time in hours

C o n c . o f C N - o n A C -C u (m g /l )

Fig.3.Removal of cyanide from solution using copper-impregnated activated carbon (AC –Cu)at the adsorbent dosages of 0.2–4.5g/l ([CN ?]0:100mg/l,pH 10.5–11).

Time in hours

C o n c . o f C N - o n A C -A g (m g /l )

Fig.4.Removal of cyanide from solution using silver-impregnated activated carbon (AC –Ag)at the adsorbent dosages of 0.2–4.5g/l ([CN ?]0:100mg/l,pH 10.5–

11).

20406080100

0.2123 4.5

Adsorbent Dosage (g/l)

C N - r e m o v a l (%)

https://www.sodocs.net/doc/146091799.html,parative performance of plain (AC),copper-(AC –Cu)and silver-impregnated (AC –Ag)activated carbons for the removal of cyanide at different adsorbent dosages (0.2–4.5g/l)over an equilibrium period of 22h ([CN ?]0:100mg/l,pH 10.5–11).

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determined to vary between 1.74–27.88and 1.36–10.45mg for AC –Ag and AC –Cu,respectively,compared with 0.44–4.43mg for AC at 0.2–4.5g/l adsorbent.It may be inferred from these data that the enhanced removal of cyanide by metal-impregnated activated carbons could be attributed probably to the

formation of metal cyanide complexes (Me(CN)n ?(n ?1)

;Me:Ag or Cu)on the activated carbon.The molar ratio of CN ?/Ag and CN ?/Cu on the metal-impregnated activated carbons was calculated to be 1.42and 3.61,respectively.This suggests that cyanide is removed by

AC –Ag in the form of AgCN and Ag(CN)2?

,consistent with the speciation of silver calculated using MEDUSA software (2004)indicating that silver would probably be present in both forms in a solution of 100mg/l CN ?at pH 10.5–11(Fig.6).Similar calculations (Fig.7)also

showed that Cu(CN)32?

would be dominant species of copper,which has poorer affinity for adsorption on

activated carbon than Cu(CN)2?

(Bailey,1987).Adams (1994)reported that the release of copper from activated carbon during adsorption of cyanide was minimal (0.1%)and cyanide was removed,to some extent,by the catalytic oxidative activity of activated carbon particularly in the presence of copper.This would result in a high molar ratio of CN ?removed to Cu probably as noted in this study.

3.2.Kinetic modelling of cyanide removal

Kinetics of cyanide removal by plain and metal-impregnated activated carbons were examined using the pseudo first-order and second-order kinetic models,which are extensively used to describe the rate of ad-sorption of inorganic and organic pollutants on various

adsorbents (Aksu and Tezer,2000;Aksu,2005;Parab et al.,2005).Based on the capacity of adsorbent,the pseudo first-(Eq.(1))and second-order (Eq.(2))kinetic models can be expressed as:d q

d t

?k 1eq e ?q Te1T

d q

?k 2eq e ?q T2e2T

where q and q e (mg/l)are the quantity of adsorbate removed from solution by the adsorbent at time t =t and t =∞(equilibrium),respectively;k 1(h ?1)and k 2(l mg ?1h ?1)are the first-and second-order rate constants,respectively.In linear form,the integration of Eqs.(1)and (2)yields:ln q e ?q q e ??k 1t

e3T

t q ?1k 2q 2e

t1q e t e4T

The kinetic parameters for the removal of cyanide (Table 1)were determined using the linear forms of the pseudo first-(Eq.(3))and second-order (Eq.(4))expressions by plotting Ln(q e ?q /q e )or t /q against t producing a straight line with the slopes of k 1or 1/q e

(intercept:1/k 2q e 2

),respectively.In the pseudo first-order model,q e is often difficult to procure from the experimental data with certainty since the extrapolation of data to infinity is required.Hence,the least-square analysis (using Excel 2000)was utilised to determine q e

.

Fig.6.pH-dependent speciation of silver (2.48mM Ag,analogous to

the amount of silver on activated carbon at 4.5g/l)in cyanide solution (3.84mM CN ?

).

Fig.7.pH-dependent speciation of copper (0.25mM Cu,analogous to the amount of copper on activated carbon at 4.5g/l)in cyanide solution (3.84mM CN ?).

202H.Deveci et al./Int.J.Miner.Process.79(2006)198–208

It can be deduced from Table 1that the experimental data were consistent with the pseudo second-order ki-netic model as indicated by high correlation coefficients (R 2)and by predicting q e values in close agreement with the experimental results.It has been reported that the pseudo first-order model can be most suitably used to describe the adsorption kinetics over short periods of b 30min (Aksu,2005).Table 1also illustrates that the rate of removal of cyanide decreased with increasing dosage of adsorbent apparently due to the concentration effect.Furthermore,the impregnation of activated car-bon with metals appeared to enhance the kinetics of cyanide removal at high concentrations of adsorbent (i.e.at N 1g/l).

3.3.Effect of aeration on cyanide removal

Compared with the data presented in Fig.5,the re-moval of cyanide appeared to be more extensive in the presence of air (0.27l/min)as shown in Fig.8.Over the same period of 10h,a 5.5%to 49.1%improvement in the extent of the removal of cyanide by plain and metal impregnated carbons at 0.2–4.5g/l adsorbent was noted to occur with the aid of aeration (Figs.5and 8).Figs.9and 10compare the removal of cyanide from solution by AC and AC –Ag,respectively,with and without aeration at the adsorbent dosages of 0.2and 4.5g/l.A similar pattern for the effect of aeration to those presented in Figs.9and 10was also produced by AC –Cu (hence not shown).The level of enhancement in the capacity of AC for the cyanide removal was similar at 0.2–4.5g/l adsorbent

(Fig.9).However,the contribution of aeration to the removal of cyanide by AC –Cu and AC –Ag in particular tended to decrease with increasing the adsorbent dosage (e.g.Fig.10for AC –Ag).This could be attributed to the limited availability of cyanide in solution for adsorption on AC –Cu and AC –Ag rather than the reduction in the contribution of aeration at high adsorbent concentrations.In CIP/CIL operations,the aeration of leach pulps is essential for the effective recovery of gold and silver on activated carbon (Bailey,1987).Although the actual role of oxygen in the adsorption of gold/silver is not completely elucidated,it is believed to derive from the presence of chromene type functional groups on

Table 1

Kinetic parameters of the pseudo first-and second-order models for the removal of cyanide by plain (AC),copper-(AC –Cu)and silver-impregnated (AC –Ag)activated carbons at the adsorbent dosages of 0.2–4.5g/l ([CN ?]0:100mg/l,pH 10.5–11)(g/l)

Pseudo first-order AC AC –Cu AC –Ag k 1(h ?1)

q e (mg/l)R 2k 1(h ?1)q e (mg/l)R 2k 1(h ?1)q e (mg/l)R 20.27.500 1.4210.9818.133 4.0460.946 5.196 5.4070.98810.264 2.1720.9750.311 6.1770.902 1.09719.1970.98120.164 4.7500.9130.71411.0380.976 2.61529.2390.98230.17911.8950.8310.56019.0440.930 2.36453.8740.9814.50.210

12.848

0.964

1.050

29.260

0.980

2.693

78.684

0.978

(g/l)

Pseudo second-order AC AC –Cu AC –Ag k 2(l/mg/h)

q e (mg/l)R 2k 2(l/mg/h)q e (mg/l)R 2k 2(l/mg/h)q e (mg/l)R 20.2 3.708 1.469 1.0000.320 4.4970.9880.497 5.803 1.00010.173 2.3670.9880.102 6.3350.9540.12420.167 1.00020.046 5.4890.9390.08712.083 1.0000.11131.207 1.00030.02512.5200.9380.04320.8980.9970.05757.6680.9984.50.018

15.108

0.987

0.041

32.389

1.000

0.025

85.199

0.998

20406080100

0.212 4.5

Adsorbent Dosage (g/l)

C N - r e m o v a l (%)

Fig.8.Removal of cyanide from solution using plain (AC),copper-(AC –Cu)and silver-impregnated (AC –Ag)activated carbons (0.2–4.5g/l)in the presence of air (0.27l/min)over a period of 10h ([CN ?]0:100mg/l,pH 10.5–11).

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H.Deveci et al./Int.J.Miner.Process.79(2006)198–208

activated carbon (McDougall and Fleming,1987;Bailey,1987).These functional groups are presumed to be oxidised in the presence of molecular oxygen leading to the formation of hydrogen peroxide (Bailey,1987;Ahumada et al.,2002):

O 2t2H 2O t2e ?Y Act :Carbon

H 2O 2t2OH ?

e5T

This could also account for the increase observed in pH when activated carbon is suspended in deionised water (pH 7)and for the catalytic oxidising activity of activated carbon (Eq.(6)):CN ?t0:5O 2Y Act :Carbon

CNO ?

e6T

Since electrons are supplied by activated carbon,this mechanism assumes the generation of positively-charged active sites with the concomitant increase in adsorption capacity of activated carbon for negatively charged ions such as cyanide and metal cyanide com-plexes as observed in this study.However,since free cyanide was weakly adsorbed,the catalytic oxidation of free cyanide (Eq.(6))should be also taken into account in that the oxidation product,cyanate,could desorb from the active sites allowing further adsorption/oxidation of free cyanide.It is also pertinent to note that the for-mation of H 2O 2(0.17mg/l over 0.5h at pH 10.5–11)by AC (10%wt/vol)in the presence of air (0.27l/min)was confirmed in a separate experiment.This also suggests that a portion of CN ?could have been removed via catalytic oxidation by activated carbon (Eq.(6))in the current tests.Analysis of the oxidation products (CNO ?,

NO 3

?and CO 32?)are required to quantify the extent of catalytic oxidation of cyanide.

Adams (1994)found that the appearance of CNO ?in solution was linked closely with the addition and subsequent loading of copper on activated carbon.The author also noted that the loading of copper (added as CuCN and CuSO 4into solution)on activated carbon increased when the solution was sparged with oxygen from which the author inferred that copper-impregnated carbon acted also as redox catalyst (Cu +/Cu 2+)with the resultant improvement in the removal of cyanide.In case of silver-impregnated activated carbon,aeration would be expected to promote the oxidation and com-plexing of silver with cyanide.Despite the inconclusive evidence available in the literature and in the current study,the enhanced removal of cyanide by AC,AC –Cu and AC –Ag in the presence of air could be attributed to a mechanism similar to that operative in the enhanced adsorption of precious metals from aerated leach solu-tions (e.g.the increased availability of active sites for adsorption on activated carbon and the catalytic oxi-dation by AC,particularly in the presence of copper).Furthermore,detailed studies appear to be required to explicate the complex role of air/oxygen in the enhance-ment of the capacity of activated carbon for the adsorp-tion/removal of free and metal-complexed cyanides.

3.4.Equilibrium modelling of cyanide removal In adsorption processes,the capacity of an adsorbent can be determined from the equilibrium data,which are essential for the design of these processes.The Lang-muir and Freundlich models are most widely used to describe adsorption isotherms for the removal of organic

Time in hours

C o n c . o f C N - o n A C (m g /l )

https://www.sodocs.net/doc/146091799.html,parison of the performance of plain activated carbon (AC)(0.2and 4.5g/l)for the removal of cyanide with and without aeration (0.27l/min)over a period of 10h ([CN ?]0:100mg/l,pH 10.5–11).

Time in hours

C o n c . o f C N - o n A C -A g (m g /l )

https://www.sodocs.net/doc/146091799.html,parison of the performance of silver-impregnated activated carbon (AC –Ag)(0.2and 4.5g/l)for the removal of cyanide with and without aeration (0.27l/min)over a period of 10h ([CN ?]0:100mg/l,pH 10.5–11).

204H.Deveci et al./Int.J.Miner.Process.79(2006)198–208

and inorganic constituents from wastewaters (Mantell,1951;V olesky,2003;Aksu,2005).Monolayer sorption onto the surface of an adsorbent,which is homogenous with an infinite number of identically active sites avail-able for adsorption is assumed in the Langmuir isotherm (Eq.(7)).The empirical Freundlich isotherm (Eq.(8))is essentially Langmuir adsorption at low surface cover-age,but,with a distribution of free energy for the ad-sorption sites and surmises that the surface of an adsorbent is heterogeneous.q e ?

QbC eq 1tbC eq

e7Tq e ?KC 1=n

eq

e8T

where q e (mg/g)and C eq (mg/l)are the concentration of adsorbate on a unit mass of adsorbent and in solution,respectively,under the equilibrium conditions;Q (mg/g)and b (l/mg)are the practical limiting (i.e.maximum)capacity of adsorbent and the Langmuir constant,res-pectively;K and n are the Freundlich constants related to adsorption capacity and intensity,respectively (Aksu,2005).

Equilibrium data were collected by contacting cya-nide solutions of different strength (10–200mg/l CN ?)with the activated carbons (1g/l AC,AC –Cu and AC –Ag)over an equilibrium period of 72h (Fig.11).The Langmuir and Freundlich models were tested to describe the resultant adsorption https://www.sodocs.net/doc/146091799.html,ing the linear forms of Eqs.(7)and (8),the parameters of both models with the correlation coefficients (R 2)were esti-mated and presented in Table 2.Although the equilib-rium data were found to be consistent with both models (R 2N 0.94),the Langmuir model appears to describe better the adsorption isotherms produced under the experimental conditions with consistently higher corre-lation coefficients (Table 2).The Langmuir parameter,Q ,indicated the maximum capacity of AC,AC –Cu and AC –Ag with the latter having the highest capacity for the removal of cyanide.Being related to the distribution of free energy for the adsorption sites,the Freundlich constant,n ,is empirical in nature and tends to increase with an increase in the intensity of adsorption since the amount adsorbed becomes proportional to a fractional power of the concentration of adsorbate in solution at high concentrations (Mantell,1951;Toth,2002).At still higher concentrations,adsorption increases only slightly and eventually becomes independent of the concentra-tion of adsorbate (https://www.sodocs.net/doc/146091799.html,ngmuir).These were consistent with the increasingly higher value of n with the respec-tive adsorption capacity of AC,AC –Cu and AC –Ag and apparently with the better correlation of the Lang-muir model with the experimental data.

Free energy of adsorption (ΔG =?RT Ln K e )for the experimental data was determined based on the assumptions of both models for surface coverage (Eqs.(9)and (10))(Mantell,1951;Toth,2002):C eq K e ?

h 1?h

Langmuir eTe9TC eq K e ?h

eFreundlich T

e10T

where C eq (mg/l),K e and θare the equilibrium concentration of adsorbate,the equilibrium constant

CN eq (mg/l)

m g C N -/g A d s o r b e n t

Fig.11.Adsorption isotherms for cyanide removal using plain (AC),copper-(AC –Cu)and silver-impregnated (AC –Ag)activated carbons ([CN ?]0:10–200mg/l,pH 11,Adsorbent:1g/l).

Table 2

Adsorption parameters of Langmuir and Freundlich isotherms for the removal of cyanide by plain (AC),copper-(AC –Cu)and silver-impregnated (AC –Ag)activated carbons (([CN ?]0:10–200mg/l,pH 11,Adsorbent:1g/l)Adsorbent

Langmuir Freundlich Q (mg/g)

b (l/mg)R 2ΔG (kcal/mol)K n R 2ΔG (kcal/mol)AC 24.100.030.976?3.79 1.82 2.080.970?2.93AC –Cu 23.950.060.980?4.40 5.20 3.390.955?3.04AC –Ag

29.88

0.17

0.994

?4.55

13.33

6.34

0.943

?3.10

205

H.Deveci et al./Int.J.Miner.Process.79(2006)198–208

and the surface coverage,respectively.For all the activated carbon systems,the free energies(Table2) were determined to be negative with an increase in negativity in the order of AC,AC–Cu and AC–Ag.It can be deduced from the free energy data that the adsorption of cyanide occurs spontaneously with relatively higher affinity of cyanide for AC–Ag.

Guo et al.(1992)used the Freundlich isotherm to describe the adsorption of cyanide on activated carbon from dilute solutions(1mg/l CN?)with the values of K and n to be2.3and7.5.They reported a practical limiting capacity(Q)of2.21mg CN per g of activated carbon,which was approximately8.2-to9-fold lower than that reported by Bernardin(1973)and in the current study where it was determined from the solutions containing high concentrations of cyanide,i.e.225and 200mg/l,respectively.Adhoum and Monser(2002) reported the cyanide removal capacity of plain and silver-impregnated activated carbons to be7.1and 26.5mg/g,respectively,in the column studies;the latter is close to the experimental value(Q exp)of29.6mg/g determined in the current study.The significantly lower adsorption capacity of plain activated carbon reported by Guo et al.(1992)and Adhoum and Monser(2002) compared with the current findings(Q exp:19.7mg/g) could be attributed to the differences in retention(i.e. equilibrium)time,cyanide concentration and oxygen content of solutions used.McDougall and Fleming (1987)have argued that initially aurocyanide is rapidly adsorbed on the macro/mesopores of carbon establish-ing a pseudo-equilibrium in≥4h;yet,it continues to load indefinitely diffusing slowly into the carbon micropores until the size of pores confines the diffusion of aurocyanide ion.They have also stated that it is extremely difficult to establish experimentally the true equilibrium and true equilibrium capacity could be more than10times higher than the pseudo-equilibrium capacity.Free and metal cyanide ions would be expected to behave in a similar manner to aurocyanide during adsorption onto activated carbon.

It may be inferred from the current findings that activated carbon as plain or impregnated with metals can be suitably used to remove free and metal-complexed cyanides from the effluents,particularly in the presence of aeration.Botz and Mudder(1997)provided the pilot plant data for the application of activated carbon in the polishing stage to further reduce cyanide and metal levels in the effluents prior to discharge.Furthermore, cyanide adsorbed can be readily stripped and recovered from plain and metal impregnated activated carbons using the conventional A VR procedure presumably with a likely reduction in acid consumption(Adams,1994).This would also allow the regeneration of plain or metal impregnated activated carbons on acidifying to the appropriate pH where copper and silver would be expected to remain as CuCN and AgCN,respectively, on the activated carbon as predicted by the speciation diagrams(Figs.6and7).Adams(1994)proposed a conceptual flowsheet for the integration of activated carbon process for the recovery of cyanide into the existing CIP facilities.

4.Conclusions

This study has shown that the capacity of activated carbon for the removal of cyanide from solutions can be significantly enhanced by impregnation with metals such as copper and silver.Silver can be readily loaded onto activated carbon with comparatively high adsorp-tion capacity of the resultant activated carbon for cyanide due to high metal content.Activated carbon impregnated with silver was shown to remove5.7–92.3%of cyanide compared with1.5–14.3%and4.4–35.4%CN?by plain and copper-impregnated activated carbons at the adsorbent dosages of0.2–4.5g/l.The increased capacity of metal-impregnated activated carbons could be attributed to the formation/adsorption of metal cyanide complexes(Me(CN)n?(n?1))on acti-vated carbon as also suggested by speciation calcula-tions for copper and silver cyanide complexes. Aeration was found to have remarkable effect on the removal/remediation of cyanide increasing(by up to 49.1%)the capacity of plain and metal-impregnated activated carbons.This enhancement in capacity with aeration could be ascribed to the formation of positively charged active sites on the activated carbon for adsorption and to the catalytic oxidation of cyanide by AC,particularly in the presence of copper. However,further detailed studies are probably required to firmly establish the role of oxygen in the adsorption process and the relative contribution of catalytic oxidation to the removal of cyanide.The kinetic analysis of the data has shown that the rate of removal of cyanide by plain and metal impregnated activated carbons is consistent with the pseudo second-order kinetic model.Equilibrium capacity of plain,copper-and silver-impregnated activated carbons was experi-mentally determined to be19.7,22.4and29.6mg per g of adsorbent,respectively,at the initial cyanide concentrations in the range of10–200mg/l.Equilib-rium data can be described by Langmuir adsorption isotherm.These findings suggest that,having already proved effective for the recovery of precious metals from leach pulps/solutions,activated carbon as plain or

206H.Deveci et al./Int.J.Miner.Process.79(2006)198–208

impregnated with metals can be suitably exploited for the treatment of the effluents containing free and metal-complexed cyanides with its potential for the recovery/ recycling of adsorbed cyanide.

Acknowledgement

The authors would like to express their sincere thanks and appreciation to the Research Foundation of Karadeniz Technical University for the financial support (Project No:2002.112.8.3),to Dr.Celal Duran(Kar-adeniz Technical University)and Mr.Erol Yilmaz (University of Quebec in Abitibi-Temiscaminque)for their technical support and to the Newmont Mining Co. for kindly providing the activated carbon samples. References

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操作系统第四版-课后习题答案

操作系统第四版-课后习题答案

第一章 作者：佚名来源：网络 1、有一台计算机，具有IMB 内存，操作系统占用200KB ，每个用户进程各占200KB 。如果用户进程等待I/O 的时间为80 % ，若增加1MB 内存，则CPU 的利用率提高多少？ 答：设每个进程等待I/O 的百分比为P ，则n 个进程同时等待刀O 的概率是Pn ，当n 个进程同时等待I/O 期间CPU 是空闲的，故CPU 的利用率为1-Pn。由题意可知，除去操作系统，内存还能容纳4 个用户进程，由于每个用户进程等待I/O的时间为80 % , 故： CPU利用率＝l-（80%)4 = 0.59 若再增加1MB 内存，系统中可同时运行9 个用户进程，此时：cPu 利用率＝l-（1-80%)9 = 0.87 故增加IMB 内存使CPU 的利用率提高了47 % : 87 ％/59 ％=147 % 147 ％-100 % = 47 % 2 一个计算机系统，有一台输入机和一台打印机，现有两道程序投入运行，且程序A 先开始做，程序B 后开始运行。程序A 的运行轨迹为：计算50ms 、打印100ms 、再计算50ms 、打印100ms ，结束。程序B 的运行轨迹为：计算50ms 、输入80ms 、再计算100ms ，结束。试说明（1 ）两道程序运行时，CPU有无空闲等待？若有，在哪段时间内等待？为什么会等待？( 2 ）程序A 、B 有无等待CPU 的情况？若有，指出发生等待的时刻。 答：画出两道程序并发执行图如下： （1）两道程序运行期间，CPU存在空闲等待，时间为100 至150ms 之间（见图中有色部分） （2）程序A 无等待现象，但程序B 有等待。程序B 有等待时间段为180rns 至200ms 间（见图中有色部分） 3 设有三道程序，按A 、B 、C优先次序运行，其内部计算和UO操作时间由图给出。

操作系统第四章作业讲解

1、“整体对换从逻辑上也扩充了内存，因此也实现了虚拟存储器的功能”这种说法是否正确请说明理由。 答：上述说明法是错误的。整体对换将内存中暂时不用的某个程序及其数据换出至外存，腾出足够的内存空间以装入在外存中的、具备运行条件的进程所对应的程序和数据。虚拟存储器是指仅把作业的一部分装入内存便可运行作业的存储器系统，是指具有请求调入功能和置换功能，能从逻辑上对内存容量进行扩充的一种存储器系统，它的实现必须建立在离散分配的基础上。虽然整体对换和虚拟存储器均能从逻辑上扩充内存空间，但整体对换不具备离散性。实际上，在具有整体对换功能的系统中，进程的大小仍受到实际内存容量的限制。 2、某系统采用页式存储管理策略，拥有逻辑空间32页，每页为2KB，拥有物理空间1MB。 1）写出逻辑地址的格式。 2）若不考虑访问权限等，进程的页表有多少项每项至少有多少位 3）如果物理空间减少一半，页表结构应相应作怎样的改变 答：1）该系统拥有逻辑空间32页，故逻辑地址中页号必须用5位来描述，而每页为2KB，因此，页内地址必须用11位来描述。这样，可得到它的逻辑地址格式如下： 2）每个进程最多有32个页面，因此，进程的页表项最多为32项；若不考虑访问权限等，则页表项中只需给出页所对应的物理块号。1MB的物理空间可分成29个内存块，故每个页表项至少有9位。 3）如果物理空间减少一半，则页表中项表项数仍不变，但每项的长度可减少1位。 3、已知某系统页面长4KB，每个页表项为4B，采用多层分页策略映射64位的用户地址空 间。若限定最高层页表只占1页，则它可采用几层分页策略 答：方法一：由题意可知，该系统的用户地址空间为264B，而页的大小为4KB，故作业最多可有264/212（即252）个页，其页表的大小则为252*4（即254）B。因此，又可将页表分成242个页表页，并为它建立两级页表，两级页表的大小为244B。依次类推，可知道它的3、4、5、6级页表的长度分别是234B、224B、214B、24B，故必须采取6层分页策略。 方法二：页面大小为4KB=212B，页表项4B=22B，因此一个页面可以存放212/22=210个面表项，因此分层数=INT[64/10]=6层 4、对于表所示的段表，请将逻辑地址（0，137）、（1，4000）、（2，3600）、（5，230）转换 成物理地址。 答：[0,137]：50KB+137=51337； [1，4000]：段内地址越界；

新八年级下册英语课文语法填空和短文改错

一、 HanselandGretel______(live)nearaforestwithhisfatherandstepmother.Oneyear,theweather______(be)sodry thatnofood_____(grow).Thewifetoldherhusbandthatunlesshe________(leave)hischildren______(die)inthe forest,thewholefamilywoulddie.Gretel_________(hear)thattheirstepmotherplanned________(kill)herandh erbrother.ButHanselhadaplan________(save)himselfandhissister.Hewenttogetsomewhitestonesbeforehew enttobedthatnight.Thenextday,thewife_________(send)thechildrentotheforest.Hansel___________(drop)t https://www.sodocs.net/doc/146091799.html,terthatnight,they________(see)thestonesbecauseoftheshiningmoon.Thestones__ ace)forthisistheHimalayas.TheHimalayasrunalongthe______________(southwest)partofChina.Ofallthem ountains,Qomolangma________(rise)thehighestandis____________(famous).Itis8,844.43metershighands oisverydangerous________(climb).Thickcloudscoverthetopandsnow__________(fall)veryhard.Evenmore serious_______(difficulty)includefreezingweatherconditionsandheavystorms.Itisalsoveryhard_____(take) inairasyougetnearthetop. Thefirstpeople_____(reach)thetopwereTenzingNorgayandEdmundHillaryonMay29,1953.ThefirstChi neseteam__(do)soinI960,whilethefirstwoman_____(succeed)wasJunkoTabeifromJapanin1975. Whydosomanyclimbersrisktheir_____(life)?Oneofthemain_______(reason)isbecausepeoplewant______(c hallenge)themselvesinthefaceofdifficulties. Thespiritoftheseclimbers_____(show)usthatweshouldnevergiveup____(try)toachieveourdreams.Itals

中国文化常识-中国传统艺术I 中国民乐与戏剧(英文讲授中国文化)Chinese Folk Music and Local Operas

Folk Music 中国民乐 Beijing Opera 京剧 Local Operas 地方戏 Outline ?China, a nation of long history with profound culture, ?inherited and developed a great variety of traditional art forms which suit both refined and popular tastes 雅俗共赏. ?From the melodious and pleasant folk music to the elaborate and touching local dramas, ?from the simple but elegant inkwash painting(水墨画) to the flexible and powerful calligraphy, ?one can always discern the light of sparkling wisdom. ?Traditional Chinese arts have tremendously impressed the world. ?Chinese folk music, with strong nationalistic features, is a treasure of Chinese culture. ?As early as the primitive times, Chinese people began to use musical instruments, ?which evolved today into four main types categorized by the way they are played 吹拉弹打.materials they are made of （丝竹管弦）. stringed instrument 丝：弦乐器 wind instrument 竹：管乐器 ?吹：The first type is wind instrument 管乐器, as show in xiao （a vertical bamboo flute ）箫，flute 笛子, suona horn 唢呐, etc. ?拉：The second type is string instrument 弦乐器, represented by urheen 二胡, jinghu 京胡（a two-string musical instrument similar to urheen ）, banhu fiddle 板胡, etc.

计算机操作系统第四版试题(卷)与答案解析

操作系统期末考试（一） 一、单项选择题（在每小题的四个备选答案中，只有一个是正确的，将其号码写在题干的括号中。每小题2分，共20分） 1、文件系统的主要组成部分是（） A、文件控制块及文件 B、I/O文件及块设备文件 C、系统文件及用户文件 D、文件及管理文件的软件 2、实现进程互斥可采用的方法（） A、中断 B、查询 C、开锁和关锁 D、按键处理 3、某页式管理系统中，地址寄存器的低9位表示页内地址，则页面大小为（） A、1024字节 B、512字节 C、1024K D、512K 4、串联文件适合于（）存取 A、直接 B、顺序 C、索引 D、随机 5、进程的同步与互斥是由于程序的（）引起的 A、顺序执行 B、长短不同 C、信号量 D、并发执行 6、信号量的值（） A、总是为正 B、总是为负 C、总是为0 D、可以为负整数 7、多道程序的实质是（） A、程序的顺序执行 B、程序的并发执行 C、多个处理机同时执行 D、用户程序和系统程序交叉执行 8、虚拟存储器最基本的特征是（） A、从逻辑上扩充内存容量 B、提高内存利用率 C、驻留性 D、固定性 9、飞机定票系统是一个（） A、实时系统 B、批处理系统 C、通用系统 D、分时系统 10、操作系统中，被调度和分派资源的基本单位，并可独立执行的实体是（） A、线程 B、程序 C、进程 D、指令 二、名词解释（每小题3分，共15分） 1.死锁: 2.原子操作: 3.临界区: 4.虚拟存储器: 5.文件系统: 三、判断改错题（判断正误，并改正错误，每小题2分，共20分） 1、通道是通过通道程序来对I/O设备进行控制的。（） 2、请求页式管理系统中，既可以减少外零头，又可以减少内零头。（） 3、操作系统中系统调用越多，系统功能就越强，用户使用越复杂。（） 4、一个进程可以挂起自已，也可以激活自已。（） 5、虚拟存储器的最大容量是由磁盘空间决定的。（） 6、单级文件目录可以解决文件的重名问题。（） 7、进程调度只有一种方式：剥夺方式。（） 8、程序的顺度执行具有顺序性，封闭性和不可再现性。（） 9、并行是指两个或多个事件在同一时间间隔内发生，而并发性是指两个或多个事件在 同一时刻发生。（） 10、进程控制一般都由操作系统内核来实现。（） 四、简答题（每小题5分，共25分） 1、简述死锁产生的原因及必要条件。 2、什么是多道程序技术，它带来了什么好处？ 3、有结构文件可分为哪几类，其特点是什么？ 4、分时系统的基本特征是什么？ 5、分页系统与分段系统的区别主要在于哪些方面？