Anal. Methods Environ. Chem. J. 6 (4) (2023) 52-64

Research Article, Issue 4

Analytical Methods in Environmental Chemi s try Journal

Journal home page: www.amecj.com/ir

AMECJ

Design of sludge drying package pilot for removal of

excess sludge in petrochemical indus tries: Heavy metals

determination in sludge by polarography and atomic

absorption spectrometry

Mos tafa Hassani a,b* and Bahareh Azemi Motlaghc

a Department of Applied Chemis try, Faculty of Science, Islamic Azad University,

South Tehran Branch, Tehran, Iran

b Member of the Research Council of Sablan Petrochemical Indus tries Company, Assaluyeh. Iran

c Mas ter of Environmental Management, Faculty of Natural Resource and Environment, Science and Research

Branch, Islamic Azad University, Tehran, Iran

ABS TRACT

In this research, according to the high amount of sludge in a

petrochemical company, an iron package type of drying sludge bed

was made/designed with carbon s teel. Then, the drying sludge pond

was lled with layers of sand with dierent mesh sizes. The excess

sludge from the sedimentation pond was passed over this bed, and

the amount of sludge removed by the bed was obtained at %96. The

values of heavy metal and microbial forms were determined using

the proposed method based on activated sludge after was tewater

treatment. For the validation process, 10 mL of deionized water

(DW) was mixed with 1.0 g of dried sludge with pure nitric acid (2%

HNO3), and then the solid phase was ltered with the Whatman lter

(WF). The concentration of heavy metals (As, Cd, Cu, Pb, Hg, Mo,

Ni, Co, Se, Zn) in the remaining solution of sludge (mg kg-1) and

was tewater (µg L-1) was extracted/ separated based on sulfur-doped

graphene oxide adsorbent (SDGO) by solid-phase microextraction

procedure (SPME) before being determined by the ame and hydride

generation atomic absorption spectrometry (F-AAS; HG-AAS) which

had similar range to the polarography analysis. The limit of detection

(LOD), linear range (LR) and preconcentration factor (PF) for (As,

Hg) and (Cd, Cu, Pb, Mo, Ni, Co, Se, Zn) were obtained (0.016 µg

L-1; 3.3 µg L-1), (0.05-10 µg L-1; 10-1000 µg L-1), and 10.0 by HG-

AAS and F-AAS, respectively.

Keywords:

Sludge,

Bed design,

Was tewater,

Heavy metals,

Atomic absorption spectrometry,

Polarography

ARTICLE INFO:

Received 16 Aug 2023

Revised form 20 Oct 2023

Accepted 24 Nov 2023

Available online 29 Dec 2023

*Corresponding Author: Mos tafa Hassani

Email: dr.hasani.2023@gmail.com

https://doi.org/10.24200/amecj.v6.i04.315

1. Introduction

Was tewater treatment is always associated with

producing two parts: sewage and sludge. Euent after

secondary treatment is often of favourable quality

for discharge into the environment, while sludge

requires treatment and s tabilization due to its high

pollution load. Was tewater treatment concentrates the

impurities and pollutants in them and separates them

from the liquid phase. The detached part contains a

high concentration of contaminants and undesirable

subs tances that mus t be puried appropriately. This

part is generally called sludge. In a was tewater

treatment plant, sludge treatment and s tabilization

facilities are far more sensitive, specialized, and

expensive than other units. So, sludge treatment

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

and disposal devices usually account for 40 to 60

percent of the cons truction cos t and up to 50 percent

of the operation cos t of a treatment plant, creating a

signicant share of the operational problems related

to this facility. Based on this, special attention should

be paid to the technical and economic optimization

of sludge purication and s tabilization methods [1].

The main technology in was tewater treatment is

removing organic matter by biological oxidation. The

nal products of this process are new cells (sludge),

carbon dioxide, soluble microbial products, and

water. The activated sludge process is widely used

worldwide in municipal and indus trial was tewater

treatment. The daily production of excess sludge from

the conventional activated sludge process is about 15

to 100 litres per kilogram of BOD5 removed, which

contains more than 95% water [2]. Since the sludge

produced as a was te material mus t continuously

be discharged into the environment economically,

biomass production is of economic importance.

Nowadays, the solutions to minimize excess sludge

production in the activated sludge process are

becoming very practical. Therefore, it seems necessary

to review the methods used to reduce the sludge

production from the activated sludge process on an

indus trial scale [3]. Until now, the main techniques are

the aerobic process with anaerobic sedimentation [4],

activated sludge process combined with ozonation

[5], sludge retention time control and its biological

decomposition [6], and high dissolved oxygen process

[7] for the management of excess sludge produced

in the activated sludge process has been reported.

Since the disposal of excess sludge from purication

sys tems in the environment is done either as compos t

(fertilizer) or as landll, the presence of heavy metal

pollution plays an important and inuential role in

its disposal because the entry of heavy metals into

the soil and the s tructure of plants, it will eventually

appear in the human body through diusion in water

and dus t. Heavy metals such as arsenic (skin cancer)

[8-9], cadmium (lung cancer) [10-11], copper (kidney

and liver failure) [12], lead (des troying the nervous

sys tem) [13], mercury (congenital abnormalities)

[14-15], molybdenum (hyperactivity and convulsion

factor) [16], nickel (cardiovascular abnormalities)

[17] and selenium (kidney and liver des truction)

[18] which are considered a serious threat to human

health. There are many methods for determining

and extracting heavy metals from dierent matrixes.

Recently, some adsorbents such as nano graphene

oxide modied phenyl methanethiol nanomagnetic

composite, Fe3O4-supported naphthalene-1-thiol-

functionalized graphene oxide (SH-GO), CysSB/

MetSB@MWCNTs, task-specic ionic liquid

immobilized on multi-walled carbon nanotubes

(TSIL-MWCNTs), functionalized multi-walled

carbon nanotubes (F-MWCNTs), MWCNTs@

DMP, nitrogen-doped porous graphene adsorbent

(NDPG), immobilization of N-acetylcys teine on

MWCNTs, thiol modied bimodal mesoporous

silica (HS-UVM7), and palladium embedded on

the mesoporous silica (Pd-MSN) were used for

extraction and determination heavy metals in dierent

matrixes[19-31].

Therefore, in this research, the design and cons truction

of a pilot sludge drying reactor to dispose of the

excess sludge of petrochemical indus tries (methanol)

and measure the amount of fecal coliforms, and heavy

metals such as cadmium, copper, lead, nickel, cobalt

and zinc (Cd, Cu, Pb, Ni, Co, Zn were determined

by polarography (VT). Also, after treatment in the

resulting sludge, Hg, As, Se, Mo plus Cd, Cu, Pb, Ni,

Co, and Zn were determined by F-AAS and HG-AAS.

2. Experimental

2.1. Ins trumental

Polarography is an electrolysis technique in which

micro-electrodes are performed at a solution’s

dropping mercury electrode (DME). The polarography

device measured the heavy metals (Metrohm model

VA Computrace 797, Swiss) by the anodic s tripping

voltammetry (ASV) mode. The results are achieved

as a current–voltage curve (CVC). Polarography or

voltammetry technique (VT) determines ion

species based on loss or take electrons at the surface of

a DME at an optimized potential. Electrode surfaces

are used for inorganic and organic compounds.

Polarography is used for heavy metal analysis (Cd, Cu,

Pb, Ni, Co, Zn) in high sludge concentrations (mg kg-

1), but it cannot determine the low concentration of less

Sludge drying package and heavy metals analysis in sludge Mos tafa Hassani et al

than ppm. The Flame atomic absorption spectrometry

had more sensitivities (DL<0.1 mmol L-1) compared

to polarography (DL>0.1 mmol L−1). In analytical

applications, the alternating current (AC) mode has

more sensitivities than the direct current (DC)

polarography, but it is better to use the FAAS for a low

detection limit. In this work, we used polarography/

VT and F-AAS /HG-AAS for high and low values

of heavy metals in sludge. The cold vapor-atomic

absorption spectrometer (CV-AAS) was used for

trace determination of mercury in the water samples

(HG-3000, GBC, Aus). The NaBH4 reagents and a

reaction loop were used for mercury determination

by CV-AAS. The linear ranges (LR) and the detection

limit (LOD) of mercury were obtained between 1-60

μg L-1 and 0.25 μg L-1, respectively, by open quartz

cell of CV-AAS. The wavelength, the current lamp

and the slit of mercury HCL were tuned at 253.7 nm,

3 mA and 0.5 nm, respectively, by AAS (peak area).

The HG-AAS was also used for arsenic determination

in water samples. Other heavy metals such as Cd,

Cu, Pb, Mo, Ni, Se, Co and Zn were determined by

ame atomic absorption spectrometer (F-AAS, GBC

932 plus, Autosampler, Air/Acetylene). The mean

LOD and LR for Cd, Cu, Pb, Mo, Ni, Se, and Zn

were obtained from 50-100 µg L-1 and 0.1-10 mg L-1,

respectively. A pH meter determined the sample pH

(Metrohm AG 744, Switzerland).

2.2. Reagents and Materials

The dierent layers used in the designed bed consis t

of sand with dierent meshes, which are very

economical. All reagents, such as nitric acid (Sigma,

CAS No.: 7697-37-2, 65%) and sodium hydroxide,

were purchased from Sigma and Merck (Germany).

The s tandard solutions of arsenic, cadmium, copper,

lead, mercury, molybdenum, nickel, selenium, and

zinc (As, Cd, Cu, Pb, Hg, Mo, Ni, Co, Se, Zn) were

purchased from Sigma, Germany. The dierent

s tandard solutions of heavy metals were prepared

by amounts of metal salt as 1000 mg L-1 solution by

dissolving in HNO3 (2%). The sample pH was adjus ted

using proper buer reagents of sodium acetate (CAS

No.: 6131-90-4, CH3COO-Na/CH3COOH) for pH

3 to 7.5. Pure GO (CAS No.: 1034343, CxHyOz)

prepared from Sigma Aldrich, Germany. Sodium

sulphite (CAS No.: 7757-83-7, Na2S) was purchased

from Merck, Germany.

2.3. Synthesis of sulfur-doped graphene oxide

The pure GO was prepared from Sigma, Germany.

Firs t, 5.0 mg of pure GO was dispersed in 10 mL

of DW and was sonicated for 30 min as suspension

material before adding 1.5 mL of Na2S (2%). The

mixture was shaken for 85 minutes at 95 °C, and

S-doped reduced graphene oxide was produced

(SDGO). Then, the SDGO adsorbent was cooled and

separated (25oC) by centrifuging. The nal product

was washed with DW many times and separated by

the Whatman lter. After drying for 2 hours in the

oven (70OC), the adsorbent is used for further work

(Fig.1).

Fig. 1. Synthesis of sulfur-doped graphene oxide by Na2S

Anal. Methods Environ. Chem. J. 6 (4) (2023) 52-64

Sludge drying package and heavy metals analysis in sludge Mos tafa Hassani et al

2.4. Characterization

The eld emission scanning electron microscopy

(FE-SEM), XRD, and FTIR analysis characterized

the S-doped graphene. FE-SEM images were

obtained with a JEOL JSM-IT500 device. X-ray

diraction (XRD, Hitachi, Japan, EA8000A model,

λ=1.5 Å) was used to analyze the crys talline

nanos tructures. Fourier Transform Infrared (FT-IR)

spectra in the 400–4000 cm-1 range were achieved

by the IR-Anity-Shimadzu (Japan).

2.5. Description of the pilot

The designed drying sludge bed has an approximate

volume of 0.6 cubic meters and sand layers with

the size of sand from bottom to top (8 cm with 60

mm grain size, 6 cm with 18 to 25-grain size, 6 0

to 12 cm and 10 cm will be coarse grain sand). The

material of the pilot body is s tainless carbon with a

thickness of 3 mm (Fig.2).

2.6. General procedure

To disinfect the sludge, it is enough to heat it twice

at 60OC [21]. F-AAS and HG-AAS validated the

heavy metal analysis with a polarography device.

Firs t, 10 mL of DW was mixed with 1.0 g of dried

sludge with pure nitric acid (2% HNO3), and then

the solid phase was ltered with the Whatman lter

(WF). The heavy metals (M: As, Cd, Cu, Pb, Hg,

Mo, Ni, Co, Se, Zn) in 20 mL of the remaining

solution of sludge and was tewater (µg L-1) were

extracted based on 30 mg of SD-rGO as solid-phase

microextraction procedure (SPME) at pH 6.5 and

then heavy metals back-extracted from adsorbent

with eluent (1.0 mL, HNO3, 0.5 M) before being

determined by the ame and hydride generation

atomic absorption spectrometry (F-AAS; HG-AAS)

after dilution up to 2.0 mL with DW. The results

showed heavy metal concentrations were like the

VT analysis after sample treatment. For the VT,

10 g of each drying sludge or sludge sample was

prepared with 60 mL of 3.0 M of HNO3 for 2 hours

of s tirring. Finally, the samples were vacuum ltered

(200 nm, Watman lter) and put into Cole-Parmer

Essentials Plus Class A (100 mL, Canada), which

was kept at 0oC before determining heavy metals.

An aliquot of 0.1 mL of each sample was diluted

up to 200 mL with DW for the VT measurements.

The dilution solution was put into the voltammetry

cell before being degassed by Ar/N2 for 25 minutes.

Then, heavy metal ions (M: Cd, Cu, Pb, Ni, Co,

Fig.2. Image of the pilot design of the sludge dryer bed of the current project

Zn) were deposited for 2.5 minutes at −1.2 V under

s tirring. (Fig.3) After 0.5 minutes, as equilibration

time, the potential was scanned from −1.2 to 0.2 V

by the anodic s tripping voltammetry (ASV) mode (5

and 30 mV in square wave). Before peak integration,

the VT software acquired baseline subtraction.

2.6.1.Measurement of sludge heavy metals with

polarography

Voltammetry is an electrochemical method that, by

measuring the amount of current in terms of potential

changes in a three-electrode set, provides the

possibility of qualitative and quantitative analysis

of heavy metals in water and was tewater samples.

This method is called polarography in special cases

where the working electrode and a mercury drop

electrode are used. This method allows qualitative

and quantitative analysis of metals such as zinc,

lead, tin, iron, nickel, cobalt, chromium, cadmium,

etc. with high reproducibility. After equilibration

time, the potential was scanned from −1.2 to 0.2 V

by the anodic s tripping voltammetry (ASV) mode.

2.6.2.Measurement of sludge heavy metals with AAS

All sample solutions were treated using SD-rGO

adsorbent as an SPME procedure at pH 6.5. Then,

after the back-extraction of heavy metals from

SD-rGO adsorbent and dilution with DW, the

concentrations of heavy metals were determined

by F-AAS/HG-AAS.

3. Results and Discussion

3.1. FTIR spectra of SDGO

the FT-IR spectra of SD-rGO are shown in

Figure 4. Based on the FT-IR spectrum, the peaks

at 3458 cm−1 related to s tretching vibrations of

Fig.3. Procedure for extraction and determination of heavy metals by polarography and F-AAS

Anal. Methods Environ. Chem. J. 6 (4) (2023) 52-64

-OH groups, 1628 cm−1 observed the carbonyl

group(C=O), 1588 cm−1 peak showed carbon

binding (C=C) and 1402 cm−1 are attributed to

O=C–C bonding groups. Further, the peaks at 1201

cm−1, 892 cm−1 and 575 cm−1 are related to the

vibrations of C–S–C, C–S and C=S, respectively.

The sulphur ions in Na2S (negative charge) are

absorbed with the carbonyl group (C=O) by the

nucleophilic subs titution mechanism.

3.2. XRS analysis

The XRD analysis of the pris tine GO sheets and the

SD-rGO sheets (2% sulphur doping) were obtained

in Figure 5. Due to the XRD patterns, 30 mg of GO

Fig. 4. FTIR spectra of SD-rGO adsorbent

Fig.5. XRD Analysis of SD-rGO adsorbent

Sludge drying package and heavy metals analysis in sludge Mos tafa Hassani et al

sheets have a sharp diraction peak at 2θ =10.24°

with an interlayer spacing of ~ 8.66 Å, related to

the oxygen functional groups between graphene

sheets. The SD-rGO adsorbent peaked at 2θ =

24.21° associated with an interlayer spacing of ~

3.55 Å. Thus, the XRD results showed the s tructural

res toration of the graphitic during the hydrothermal

reaction.

3.3. SEM of SD-rGO adsorbent

The SEM of GO and SD-rGO is shown in Figure

6. The size of GO and SD-rGO was between 35-

100 nm. In images, the SD-rGO is in the nanometer

range, and functionalization of S- did not result in

aggregation of rGO. The images showed that the

functionalization sulphur on rGO does not change

the general s tructure and morphology of rGO.

3.4. Sludge volume measurement

To determine the amount of sludge volume, the SVI

criterion with the name of the sludge volume index

will be used. In this method, a 100 mL cylinder

of the sample is poured, and after 30 minutes, its

sedimentation rate is measured. It is ltered by

lter paper and dried in an oven at a temperature

of 105oC, and the weight of the dry sludge is

put into the following formula and calculated as

the SVI [32]. (SVI: Sludge rate per 30-minute

sedimentation rate)

3.5. Pilot control of sludge bed dryer

At this s tage, rs t, the cons tructed sludge bed will

be loaded with 200 litres of excess sludge from

the sedimentation pond, whose volume index has

already been measured, and the volume index of

the output sludge will also be measured. Then, the

pond is exposed to the ambient air for 120 hours to

dry the sludge. The amount of heavy metals arsenic,

cadmium, copper, lead, mercury, molybdenum,

nickel, cobalt, and selenium is measured on it by a

polarography and AAS device.

3.6. Optimization of pH, amount of adsorbent

and sample volume for extraction in sludge

For ecient extraction of heavy metals, the

amount of SD-rGO adsorbent between 5-50 mg

has been examined in sludge samples by heavy

metal concentration between 0.05-10 µg L-1 for

Hg/As and 10-1000 µg L-1 for Cd/Cu/Pb/ Mo/

Ni/Se/Zn. The results showed that the maximum

extraction of heavy metal ions in sludge samples

Fig.6. SEM of GO (Right) and SD-rGO (left)

Anal. Methods Environ. Chem. J. 6 (4) (2023) 52-64

was achieved at 30 mg of SD-rGO adsorbent at

optimized conditions (Fig.7). The pH is the main

factor for extracting heavy metals by SD-rGO

adsorbent. Therefore, the various pH levels from

2 to 12 were s tudied in sludge samples through the

buer solutions. Due to the result, the bes t pH for

Cd/Cu/Pb/Ni/Co/Zn concentration was obtained at

6.5 in sludge samples (Fig.8).

Fig.7. The eect of the amount of SD-rGO adsorbent on the extraction of heavy metals

Fig. 8. The eect of pH on the extraction of heavy metals by the SD-rGO adsorbent

Sludge drying package and heavy metals analysis in sludge Mos tafa Hassani et al

The extraction recoveries were decreased at less

than a pH of 5 and more than 7. Also, the bes t

volume for extraction was obtained at 20 mL. The

various eluents such as HNO3, HCl, and H2SO4

were used for back extraction of heavy metal ions

from SD-rGO adsorbent. At low pH, the covalence

bonding in S-Metal was broken, and metals were

released in eluents. So, the procedure used the

eluents (HNO3, HCl, and H2SO4) with dierent

volumes and concentrations (0.2-0.8 mol L-1, 0.2-2

mL). The results showed the HNO3 (1 mL, 0.5 M)

had maximum recovery.

3.7. Analysis in real samples

Table 1 shows the amount of SVI of the input and

output of the sludge dryer. According to this table,

the eciency of removing SVI from the bed input

has been 96% [33,34]. Table 2 and Figure 9 relate

to the analysis of heavy metals in the output bed

sludge by the polarography (Zn, Cd, Pb, Ni, Co,

Table 1. SVI of inlet and outlet was tewater sluge dryer

typeSVI

sewage inlet bed

sewage outlet bed

1900

Table .2 Concentration of heavy metals in dried sludge by polarography

and F-AAS (mg kg-1)

Heavy Metals Polarography F-AAS HG-AAS

As ----- ----- 0.048

Cd 0.041 0.039 -----

Cu 36.33 35.72 -----

Pb 0.096 0.101 -----

Hg ----- ----- 0.801

Mo ----- ----- 0.053

Ni 6.27 6.39 -----

Se ---- 0.028 -----

Zn 8.69 8.92 -----

Co 5.23 5.18 -----

Table 3a. The amount of diges tive and fecal coliform in dried sludge

Types of pathogens

Concentration

(MPN/100 mL)

Fecal coliforms191

Total coliforms383

MPN: Fecal coliform of organisms per 100 mL of sample water.

Maximum Acceptable Concentration for Drinking Water = no detected coliforms in

100 mL water.

Table 3b. S tandard concentration of pathogens in biological solids [35].

Types of pathogens

Concentration

(MPN/100 mL)

Fecal coliforms191

Total coliforms400

MPN: Fecal coliform of organisms per 100 mL of sample water.

Maximum Acceptable Concentration for Drinking Water = no detected coliforms in

100 mL water.

Anal. Methods Environ. Chem. J. 6 (4) (2023) 52-64

Table 4. S tandard concentration and loading rate of biological solids for use on land [35].

Heavy

metals

Max.

Concentration

(mg kg-1)

Concentration

(mg kg-1)

Annual

loading rate

(mg kg-1)

Cumulative

loading rate

(mg kg-1)

Ni/Co

4300

840

420

100

7500

1500

300

420

2800

1.9

0.85

1500

300

420

Fig. 9. Determination of heavy metals by the polarography (ASV mode)

Cu) and F-AAS. Also, the amount of diges tive and

fecal coliform in dried sludge and the s tandard

concentration of pathogens in biological solids

[35] are shown in Tables 3a and b. The s tandard

concentration and loading rate of biological

solids for use on land as sludge environmental

s tandards are shown in Table 4 [35]. According

to the results of Table 1, the dry sludge bed made

in this research has a sludge removal eciency

of %96. The comparison of Tables 2 and 4 shows

that the amount of heavy metals in the resulting

sludge is by the environmental s tandards for use

on the ground, and in this sense, there is no threat

to the environment. On the other hand, comparing

the results of microbiological analyses (Table 3a

and 3b) with microbiological s tandards conrms

the microbial s tandard of sludge. Therefore, the

surplus sludge from this sys tem can be used as

compos t in the green space with current conditions

and disinfection at 60 degrees Celsius.

Sludge drying package and heavy metals analysis in sludge Mos tafa Hassani et al

4. Conclusion

Managing and controlling excess sewage sludge

will be an important approach to preserving the

environment and the health of living organisms

in applications such as the cement indus try and

agriculture. Therefore, its purication is done

by the methods of concentration, diges tion, and

dehydration, and its monitoring will prevent the

entry of dangerous chemicals, such as heavy

metals, into the environment. Therefore, in this

research, a metal drying sludge package with an

approximate capacity of 0.7 cubic meters with a

bed of sand (8 cm with 60 mm grain size, 6 cm with

18 to 25-grain size, 6.0 to 12 cm and 10 cm will be

coarse grain sand) was designed and built, which

could reach 96% Physically remove excess sewage

sludge. Considering the importance of the amount

of heavy metals in the sludge, the concentration of

these metals in the sludge obtained from the dryer

sludge package was measured by polarography

(ASV; ranges from 0.041 to 36.33 mg kg-1) and

atomic absorption methods (F-AAS, HG-AAS;

ranges from 0.028 to 35.72 mg kg-1). The results

showed that the concentration of heavy metals

in the resulting sludge is equal to s tandards and

indicates the high-eciency removal by packages

(n=10, AAS: RSD<3% and CV: RSD<7%). Among

the advantages of this package, we can mention the

economical and changeable adsorbent bed so that

in future research, we can use the method of sludge

pretreatment with nanoabsorbents and change the

bed of the dry sludge package to a large extent. So,

the dierent amounts of heavy metals in the sludge

are removed based on the changeable adsorbent

bed. The package absorbed and removed the water

coming out of the bed. As a result, this subs trate

can be widely used in the sewage indus try due to its

small volume and high sludge removal eciency.

5. Acknowledgments

We are grateful for the unques tionable eorts of Mr.

Farid Nikpour, Behnam Khodabakhshi, Ruhollah

Asadi, Sajjad Kiyani, Yunes Koravand, and

Habibullah Tahmasebi in designing and building

the pilot package.

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