Research Article, Issue 4

Analytical Methods in Environmental Chemistry Journal

Journal home page: www.amecj.com/ir

AMECJ

------------------------

Ahmad Ghozatloo

a,*

, Atefeh Enayatollahi

Research Institute of Petroleum Industry (RIPI), West Blvd. Azadi Sport Complex

P.O. Box: 14665-137, Tehran, Iran

significant amount of chemically contaminated

wastewater. The concentration of these chemicals

depends on the raw materials and the methods

of production. Due to the variety of different

processes and their demand for large amounts

of water, these industries are responsible for the

production of considerable contaminated water.

Production of textile requires several mechanical

processes such as spinning, weaving, knitting,

etc. [1]. Besides, there exist the “wet processes”

that include wool washing, bleaching, and dying.

During the production processes of fibers, cloth

and, clothes several contaminators will be added

Enhancing the effect of zinc oxide on the absorption of heavy

metals from wastewater by using silica in graphene bed

1. Introduction

During the past decades, due to population

growth and industrial development, the demand

for freshwater and wastewater treatment has

significantly increased. This includes the treatment

of industrial wastewater that contains considerable

amounts of heavy metals, such as; lead, copper,

chrome, cadmium, nickel, iron, zinc, arsenic,

manganese and mercury. Among the various

industries, the weaving industries produce a

*Corresponding Author. Ahmad Ghozatloo

E-mail: ghozatlooa@ripi.ir

DOI: https://doi.org/10.24200/amecj

A R T I C L E I N F O:

Received 2 Sep 2019

Revised form 26 Oct 2019

Accepted 16 Nov 2019

Available online 26 Dec 2019

Keywords:

Graphene,

Silica,

Adsorbent,

Heavy metals,

Wastewater

A B S T R A C T

In this study, the effects of nanostructure absorbent of zinc oxide (ZnO) in graphene

bed for wastewater treatment were studied. Initial analysis was undertaken to

identify the existing metals and their concentration in the prepared wastewater. It

was seen that the diluted solution consisted of the ambivalence ions of lead, copper,

nickel, cadmium, and silver with the concentration of 73.31, 81.19, 54.6, 98.1and

76.1 milligram per liter, respectively. Trivalent chrome, with a concentration of

98.1 milligram per liter was also observed. Therefore, by adding various amounts

of absorbent (20, 30 and 50 mg) to the wastewater sample and adjusting the pH

to 5 and 6, each metal was separately absorbed. Consequently, the concentration

of the remaining metals was measured, and it was observed that absorbent was

effective for the absorption of lead, copper and silver (with a reduction of up to

80%), however, the absorbent was weak in the absorption of nickel and chrome.

Hence, the silicon nanoparticles added to absorbent and the experiments repeated.

It was observed that the presence of silicone resulted in higher absorption of nickel

and chrome but negatively affected the absorption of copper and silver. Electrical

charges at lower pH’s have an inverse impact on the absorption of metal ions that

is due to the electrostatic repelling forces between the positive charges. In more

acidic solutions, the carbonyl groups in the surface of composite create positive

charges and hence repel the metal ions. Hence, the performance of the absorbent

improves by reducing the acidity of the solution. At the pH of six, the number of

hydroxide increases and the capacity of absorbing metal cations increase.

Absorption of heavy metals by nanotechnology Ahmad Ghozatloo et al

Analytical Methods in Environmental Chemistry Journal Vol 2 (2019) 27-38

Analytical Methods in Environmental Chemistry Journal; Vol. 2 (2019)

to the wastewater. As such, during the dying

process, heavy metals such as chrome and copper

contaminate wastewater. It is vital to reduce

and remove these metals, as these will disable

the bacteria’s in the biological treatment units.

Additionally, these metals can contaminate surface

and underground water sources, which due to their

toxicity can cause death by affecting nerves and

kidneys [2]. There are national and international

codes and standards that define a limit for disposing

of the wastewater containing heavy metals to the

water resources. Exceeding these limits could be

fatal and harm the environment. The main concern

in wastewater treatment in weaving industries

is the quantity of disposed wastewater. Dispose

of wastewater in these industries are often to the

absorbent wells, which will cause irreversible

damages to the environment. The sampling results

show high values of pH, BOD, COD and dye. The

quantity of COD, BOD and TSS was reported 750

- 3500 milligram per liter, 300-1800 milligram per

liter, 18-155 milligram per liter, respectively [3]. In

the same study, the pH was observed to be varying

from 5 to 12 and the quantity of dye was reported

to be from 30 to 550 units. The dying weaves

produce a significant amount of wastewater. If

these were disposed to the environment without

proper treatment, the damages to the environment

would be significant. This reveals a pressing need

for efficient methods of treatment. Cadmium and

nickel are examples of toxic elements, which

traces of these are seen in wastewater from

mining, alloying and battery production industries.

Adsorption is one of the methods used to reduce

these elements. Nowadays various adsorbents are

used with the capability of removing organic and

inorganic contaminators, which the most common

adsorbent is activated carbon. The activated carbon

is not an economical solution for large scale

treatment units, as there are significant losses of

carbon in the regeneration process [4]. The critical

elements for selecting the reduction methods are

environmental issues, regeneration and economic

matters. In the past few years, improvement in

nanotechnologies helped in the production of

nanostructures that are distinct due to their larger

surface. The unique structure of nano adsorbents

caused them to be high capacity adsorbents. On

this basis, wastewater treatment is considered one

of the main applications of nanotechnologies that

have the potential to considerably improve the

quality and capacity of the water and wastewater

treatment units.

2. Experimental

2.1. ZnO nanostructure synthesis in graphene

bed

Firstly, synthesis of the graphene sheets by oxidation

process in accordance with hummer’s method in the

concentrated acidic media that contains mixing ratio

of 1:2:46 of concentrated sulphuric acid, graphite

powder, and sodium nitrate, respectively in 2

temperature with continuous mixing. Afterward,

potassium permanganate to a ratio of 6 added

to mixture slowly and after oxidation reaction,

mixture temperature rose to 40

C and mixing

plateaued for 1 hour. Added distilled water and

sodium hypochlorite solution stopped the reaction

and trended the pH to neutral and then filtration,

washing and drying mixture, respectively. The

yellowish powder remained was graphene. In order

to extend synthesized graphene sheets completely,

pour 1mg graphene oxide powder in 100ml distilled

water and apply ultrasound for 3 hours. The

resulting solution, centrifuged for half an hour by

6500rpm in order to get out unexpended graphene

sheets by sedimentation process from the solution.

Then added 3 grams of zinc oxide salt powder

(6H

O*Zn(NO

)

) to remain solvent and apply

ultrasound spanned 1 hour then stirred it slowly

for more than 3 hours in 90 centigrade degrees.

Poured the produced mixture in an autoclave tank

and carried out a hydrothermal synthesis method

for 6 hours at 180 °C. Then cooled it down to

room temperature naturally and washed it with

extra ethanol. The amount of resulting graphene

was about 20 percent [1]. The final structure was

a hybrid form of zinc monoxide at the surface of

Absorption of heavy metals by nanotechnology Ahmad Ghozatloo et al

graphene (GO/ ZnO) that was used as the first type

of absorbent. Figure (1) illustrates the TEM of this

structure. As shown in Figure (1) use of graphene

sheets leads to produce an appropriate culture for

bonding nanoparticles of zinc oxide on it without

aggregation of zinc particles, in the second step

silica nanoparticles added on this hybrid structure.

Therefore, in order to build graphene/zinc/silica

nano-composite firstly applied ultrasound for

the solution of 0.5 grams synthesized GO/ZnO

in 100 ml of Dionysius water for half an hour in

room temperature. After that added 2 g of CTAB

and mixed completely, then by using caustic

soda adjusted pH around 9 and dropped 1mg of

tetraethyl orthosilicate solution and heated by 40°C

of temperature the mixture in a closed system with

magnetic agitator for 24 hours. The next step was

to dry out the solution after filtration the resulting

mixture and washed it with extra Dionysius water

in Avon with 60 °C for one day. This resulting

powder is the second type absorbent of this study.

Figure (2) illustrates the TEM of that. As shown in

Figure (2) darkness particles of SiO

bonded and

distributed correctly within ZnO nanoparticles that

stabilized on graphene culture.so it can be seen that

the second type absorbent structure formed as well.

2.2. Apparatus and Reagents

The furnace atomic absorption spectrophotometer

(GF-AAS, GBC 932 plus, Australia) were used for

the determination of Cu, Ni, Cd, Pb, Cr and Ag in

samples. First, the manufacturer’s manual book of

GF-AAS was prepared. The hollow cathode lamp

(HCL) with wavelength and current favorite for

Fig. 2. TEM nanostructure of GO /ZnO/SiO

Fig. 1. TEM nanostructure of GO /ZnO

Table 1. The instrumental conditions of heavy metals by GF-AAS

Element Current(mA) Wavelength(nm) Slit(nm) *LR *LOD *LOQ

Cu 4.0 327.4 0.5 1-30 0.3 1.0

Ni 9.0 229.0 0.2 1.5-60 0.4 1.5

Cd 3.0 228.8 0.5 0.2-6.5 0.05 0.2

Pb 3.0 283.3 0.4 2.0-70 0.5 2.0

Cr 7.0 357.9 1.5 1.0-15 0.3 1.0

Ag 4.0 328.1 0.5 0.2-4.2 0.05 0.2

*Linear range (LR, μg L

-1

); Limit of detection (LOD, μg L

-1

); Limit of quantification (LOQ, μg L

-1

)

Analytical Methods in Environmental Chemistry Journal; Vol. 2 (2019)

Cu, Ni, Cd, Pb, Cr and Ag was applied by 20 µL

of sample injection to graphite tube of GF-AAS.

For measuring pH, a Metrohm pH meter based on

glass electrode was used (E-744, Switzerland) in

wastewater samples. The instrumental conditions

are listed in Table1.

2.3. Analytical Procedure

The adsorption of heavy metals such as, Cu, Ni,

Cd, Pb, Cr and Ag based on GO/ZnO and GO/

ZnO-SiO2 as sorbent were studied. All metals

determined by electrothermal atomic absorption

spectrometry (GBC, 932; ET-AAS). The different

concentration heavy metals ( 1-50 ppm) in waters

was used based on GO/ZnO and GO/ZnO-SiO2 (

10-50 mg) for heavy meal adsorption at pH=5 and

6. After shacking, the heavy metals separated and

adsorbed by GO/ZnO or GO/ZnO-SiO2. Other

parameters such temperature, pH and sonication

time were studied and optimized. In optimized

conditions, temperature (25

C), pH=5-6, and

shacking of 5 min was achieved.

3. Results and Discussion

3.1. Dilution process

The wastewater used in this study belongs to an

active center of the dyeing and printing industry,

which kept motionless in a closed container

for 24 hours span until sedimentation had been

done. Then add an equal amount of Dionysius

water into solution in order to half the pollutants

concentration. Firstly, in order to assess the

wastewater condition, measured the levels of metal

ions by the atomic absorption method as listed in

a Table (2). According to Table (1) the despite the

dilution of prepared wastewater a large variety of

heavy metals in significant amounts exists. In order

to pursue the used absorbent function firstly added

0.2, 0.3 and 0.5 grams of it into 50 ml of selected

wastewater. For pH adjustment used 0.1solution of

NaOH. Each test carried out on 5-6 pH range.

Table (3) represents the samples and pH of

experiments. In every experiment samples put on

the ultrasonic device for 12 hours and afterward the

liquid part of solution separated from the solid part

by centrifuge equipment and then the concentration

of remaining metals in solutions measured by

atomic absorption machine. All experiments carried

out at room temperature.

3.2. Discussion

As represented in Table (3), the existence of

silica nanoparticles within the absorbent structure

generally improves the adsorption function and

increases adsorption efficiency that strongly

subordinates with pH level, so that, in general view,

the maximum efficiency of adsorption occurs when

pH equals 6. Results in adsorption rate deal with

pH value represent that it was rose up by increasing

of the adsorbent amount, that the main reason for it

is due to the total increase in adsorption valence for

each adsorbent unit, in other words, by increasing

of the adsorbent content, more adsorption bed

provided for heavy metals. Also, it can be seen

that the adsorption value varies for different metals

which process differently within the dissimilar

experimental conditions for each metal. Hence, in

the following, the adsorption conditions for metals

were investigated.

3.3. Lead adsorption

Results of Pb adsorption on used adsorbent shows

that in pH of 5 the rate of adsorption increases by

rising in amount of adsorbent, which is adsorbed

57.8% of Pb in case of 20 mg of adsorbent

existence and by increase this till 30 mg, adsorption

rate rises to 64.9%, in other way the adsorption rate

improves 12 percent. Also in the concentration of

Table 2. The initial quantity of metal ions in sample

wastewater

Initial quantityMetal ionsPollutant

19.8Cu

Copper

6.5Ni

Nickel

2.3Cd

Cadmium

31.7Pb

Lead

1.98Cr

Chromium

1.76Ag

Silver

Absorption of heavy metals by nanotechnology Ahmad Ghozatloo et al

50 milligrams of adsorbent, it reaches 75.4% that is

equivalent to 30 percent of enhancement. Therefore,

the first type adsorbent structure (graphene hybrid/

zinc oxide) represents an appropriate adsorbent

for lead adsorption. Although the existence of

silica within this adsorbent amplifies their ability

of adsorption, this may not be significant, in a

way, that adsorption improves 3% and 6% in the

concentration of 20 milligrams and 50 milligrams,

respectively, the results demonstrate in the Figure

(3).

According to Figure (3), the presence of

silica within the GO/ZnO adsorbent structure

was ineffective to reach more adsorption of

Table 3. Samples and pH status at the experiments

Adsorbent conc. pH

Remained concentration(mg L

-1

)

Non 0

31.73 19.81 6.54 2.32 1.98 1.76

GO/ZnO-20 20 11.10 4.36 6.24 1.23 1.86 0.49

GO/ZnO-30 30 9.19 3.37 6.24 1.25 1.84 0.29

GO/ZnO-50 50 6.34 2.18 6.18 1.23 1.82 0.19

GO/ZnO-SiO2-20 20 8.94 8.12 3.12 1.18 1.43 0.68

GO/ZnO-SiO2-30 30 7.39 6.93 2.75 1.13 1.20 0.46

GO/ZnO-SiO2-50 50 5.05 5.38 2.62 1.04 1.15 0.34

Non 0

31.73 19.81 6.54 2.32 1.98 1.76

GO/ZnO 20 13.39 4.82 6.37 1.78 1.92 0.90

GO/ZnO 30 11.14 3.63 6.37 1.80 1.89 0.67

GO/ZnO 50 7.79 2.19 6.31 1.78 1.87 0.55

GO/ZnO-SiO2 20 12.85 10.36 4.00 1.72 1.81 1.13

GO/ZnO-SiO2 30 10.02 8.95 3.56 1.66 1.74 0.87

GO/ZnO-SiO2 50 6.27 7.26 3.41 1.55 1.68 0.73

Fig. 3. Silica effect in GO/ZnO structure for Pb adsorption (pH=5)

Analytical Methods in Environmental Chemistry Journal; Vol. 2 (2019)

Pb. Therefore, in existence 20 mg of adsorbent

in wastewater 65%of Pb has adsorbed and by

increasing this quantity to 30 mg adsorption yield

rose up to 71%, in the other word adsorption

improved 9% and in 50 milligrams of adsorbent,

it reached 80%, equivalent 23% of increase. The

same results obtained in pH of 6. Although the

adsorption shows a little improvement in this pH,

it is not noticeable. Figure (4) demonstrate the pH

effect in different quantities of the second type of

adsorbent in which silica added to their structure.

As illustrated the adsorption process is better

in all amount of adsorbent (second type) in pH

of 6 than 5, but the effect of pH in lower amounts

of adsorbent (20 mg) is more significant, in a

way, in higher concentrations 4% improvement

has seen whereas in lower points of adsorbent’s

concentration it has been more than 20% (Fig.4).

3.4. Copper adsorption

Results of Cu adsorption on used adsorbent shows

that in pH of 5 the rate of adsorption increases by

rising in amount of adsorbent, which is adsorbed

75.7% of Pb in case of 20 milligrams of adsorbent

existence and by increase this till 30 milligrams,

adsorption rate rises to 81.7%, in other way the

adsorption rate improves 8 percent. Also in the

concentration of 50 milligrams of adsorbent, it

reaches 88.9% that is equivalent to 17 percent of

enhancement. Therefore, the adsorbent structure

(GO/ZnO) is considered as an approximately

appropriate adsorbent for copper adsorption. The

existence of silica within this adsorbent structure

reduces their function and abilities significantly,

in a way that in this situation,20 milligrams of

adsorbent low off about 37% in adsorption or

29% reduction for 50 milligrams of adsorbent’s

concentration. The results represent in Figure (5).

According to the bar chart in Figure (5) it

is considered that added silica nanoparticles to

GO/ZnO structure in order to more adsorption

of copper not only didn’t be efficient but also

reduced it. The same results obtained in pH of

6. Although the adsorption level is better in this

pH, it’s behavior and the proceeding similarly

continues, for 20 mg concentration of adsorbent in

wastewater solution,78% of copper adsorbed and

by its increase to 30 mg adsorption rose up to 83%,

in other words, adsorption efficiency improved

6% improved. Also, 50 milligrams of adsorbent

lead to 89% copper adsorption that is equivalent to

14% of improvement. Due to the function of both

adsorbent structure, the existence of silica within

this adsorbent structure strongly reduces the copper

Fig. 4. Comparison of the pH effect by the different quantity of adsorbent GO/ZnO/SiO2

Absorption of heavy metals by nanotechnology Ahmad Ghozatloo et al

adsorption which is subordinating to wastewater

pH. Figure (6) represents the effect of pH for

defined quantities of the second type adsorbent.

As represented in the Figure (6), the adsorption

process is better in all amount of adsorbent (second

type) in pH of 6 than 5. But, the similar to lead

adsorption the effect of pH in lower amounts

of adsorbent (20 mg) is more significant, in a

way, in higher concentrations 14% improvement

has seen whereas in lower points of adsorbent’s

concentration it has been more than 23%.

3.5. Nickel adsorption

Results of nickel absorption by the specified

adsorbents express that in the pH of 5, adsorption

rate rises by increasing adsorbent concentration.

Fig. 5. Effect of silica within the GO/ZnO adsorbent structure to copper adsorption(pH=5)

Fig. 6. Comparison of the pH effect on Cu adsorption by the different quantity of adsorbent GO/ZnO/SiO2

Analytical Methods in Environmental Chemistry Journal; Vol. 2 (2019)

The first type of adsorbent acts weakly in nickel

adsorption, in a way that due to the existence of 20

milligrams of adsorbent in wastewater, only 2.6%

of nickel adsorbed and by rising its concentration

to 30 milligrams adsorption plateaued and then

went up slowly till 3.5% for 50 mg of the adsorbent

concentration. Therefore, GO/ZnO structures have

known as an inefficient adsorbent for nickel. Chang

the adsorbent structure to GO/ZnO/SiO

strongly

improved its ability in nickel adsorption, in a way

that by added 20 milligrams of this structure into

wastewater adsorption yield reached to 38.8%,

despite of it is not acceptable yet but it shows that

silica is the main factor to the nickel adsorption from

wastewater. Also in 50 milligrams of adsorbent,

the adsorption amount raised up to 45.6% that

equivalent by 17% more nickel adsorption. The

results demonstrate a bar chart in Figure (7).

As shown in Figure (7) the existence of silica

nanoparticles within the GO/ZnO in order to

more adsorption of nickel act effectively and

strongly increased the adsorption results. Similar

proceeding to this has shown in the pH of 6

meanwhile the results are better. Results showed

that the adsorbent concentration of 20 milligrams

in per liter of wastewater, adsorbed 52.3% of its

nickel and by increasing that until 30 mg adsorption

range reach to 58%, means adsorption efficiency

improved 10 percent and also in 50 milligrams of

adsorbent concentration, the adsorption amount

was 59.9% that is equal to 14% increase in

adsorption yield. On the other hand, the existence

of silica in the adsorbent structure enhances nickel

adsorption efficiently. As considered in both types

of adsorbent function, it is obvious that silica leads

to a significant increase of nickel adsorption. it

is subordinated to the pH of wastewater. The pH

effect on nickel adsorption in different quantities of

the second type of adsorbent that is included silica

represented as a bar chart in Figure (8).

As represented in the Figure (8) adsorption

process is better in all amount of adsorbent (second

type) in pH of 6 than 5, but unlike with lead metal

adsorption, the pH’s effect shows off with the same

pattern in different quantities of the adsorbent,

which shows 14% of improvement for higher

concentration and 23% for lower ones.

3.6. Cadmium adsorption

The study results of cadmium adsorption on

defined adsorbent represent that in pH of 5

adsorptions weren’t changed by increasing of

Fig. 7. Effect of silica within the GO/ZnO adsorbent structure to nickel adsorption (pH=5)

Absorption of heavy metals by nanotechnology Ahmad Ghozatloo et al

adsorbent concentration. The first type of adsorbent

‘s function was weakness, in which,20 milligrams

of its concentration leads to adsorb 23.3% of

cadmium and by increasing that to 30 and then 50

milligrams per liter of wastewater the adsorption of

cadmium plateaued. So, the GO/ZnO structure had

seen as an adsorbent with moderate efficiency that

absorbs a few amounts of cadmium at low levels

and then remains constant. Even added silica to

adsorbent structure did not change their ability to

adsorb cadmium. The existence of 20 milligrams

of adsorbent with silica could adsorb 25.9 % of

cadmium that didn’t have noticeable difference by

the first type of adsorbent. so, it seems that not

only silica isn’t be adsorbing factor of cadmium

in wastewater, but also due to steric hindrance

of cadmium ions with atomic number of 48,

adsorption of this heavy metal is based on surface

of nanostructure that most of the time adsorption

quantities overmatch by steric competition on

adsorbent surface by increasing of adsorbent

concentration. Also in 50 milligrams of adsorbent,

the quantity of adsorption reaches 33.2% that it

is 42 percent more cadmium adsorption than the

case of silica absence. The results illustrated in

Figure (9).

According to Figure (9), it showed that the

existence of silica nanoparticles in the GO/ZnO

structure in order to more cadmium adsorption act

slowly and somewhat increases the adsorption.

Similar results with about double improvement

obtained that represents the strong effect of pH

on the adsorption process. For this number of pH,

20 milligrams GO/ZnO/SiO

،adsorbed 47% of

cadmium from wastewater and by increasing in

adsorbent concentration, the adsorption trends up

insignificantly. Moreover, silica in the structure

didn’t change the adsorption rate noticeable.

Figure (10) demonstrates the effect of pH in

different concentrations of the second type of

adsorbent and it represents that in all quantities

of GO/ZnO/SiO

adsorption yield is better in pH

of 6 than 5, in a way that in high concentration of

adsorbent 66% and in its lower concentration 89%

improvement is seen. Therefore, adsorption gets

better in the lower concentrations of adsorbent and

changes the pH number makes it double.

4. Conclusions

The most important parameter in absorbing heavy

Fig. 8. Comparison of the pH effect on Ni adsorption by the different quantity of adsorbent GO/ZnO/SiO

Analytical Methods in Environmental Chemistry Journal; Vol. 2 (2019)

metals in industrial wastewaters that contain a

high concentration of metals is the acidity of the

wastewater. The results of this experiment have

shown that metals often exist in wastewater as

ions, in which their interactions are controlled by

the acidity of the wastewater. The metal cations in

acidic environments are repelled by the cations in

the solution, and hence their effects are converged.

Under these circumstances, the adsorption

processes are emulative and hence the adsorption

efficiency diminishes [11]. Hence, one would be

able to enhance the efficiency by adjusting pH,

that is a function of metal ions in the wastewater.

However, it must be considered that both basic and

acidic environments are negatively affecting the

adsorption efficiency. Hence the type and quantity

of the existing metals and the adsorbent used can

affect the adsorption process. In this study, it was

also observed that the best adsorption process

takes place at pH of around 6. The effect of silicon

on the structure of nanocomposite, causes the

absorbing surface to be positively charged, which

are inappropriate for heavy metals. The electro-

statically force between anions and cations, at

increased pH, causes the efficiency of adsorbing

nickel and cadmium to improve [12]. Additionally,

the silicon nanostructure used in the adsorbent,

oddly resulted in higher adsorption in pH of

6 as compared to pH of 5. Presence of silicon

significantly increases the adsorption chrome but

reduces the copper adsorption, and has a neutral

effect on cadmium adsorption. From this, it can be

concluded that the adsorption processes depend on

chemistry of the adsorbs as well as the reaction of

the existing dissolved metals. Therefore, in order

to improve the performance of the graphene-

based adsorbents with ZnO, it recommended that

the heavy metals removed first, then reduced the

chrome, and nickel using silicon. Also in pH of

around 6, improves the performance of graphene

bases to some extent and enhances the efficiency.

Overall, the graphene-ZnO recommended for pre-

treatment of wastewater by adsorbing heavy metals.

Afterwards, there is a need for the main treatment

to reduce the metals to their acceptable limits

according to the standards. During the initial stages

of adsorption, several adsorbing sites are available.

However, as time passes, due to repelling forces

between the absorbed matter and the dissolved

molecules, the free sites are hardly usable. It was

also observed that increasing the concentration of

metals, causes the absorbing efficiency to drop.

In lower concentrations the chance of adsorption

increases and ions are able to react with the

Fig. 9. Effect of silica within the GO/ZnO adsorbent structure to cadmium adsorption (pH=5)

Absorption of heavy metals by nanotechnology Ahmad Ghozatloo et al

adsorbing surface, which improves efficiency.

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