Scholarly article on topic 'Acid leaching of heavy metals from contaminated soil collected from Jeddah, Saudi Arabia: kinetic and thermodynamics studies'

Acid leaching of heavy metals from contaminated soil collected from Jeddah, Saudi Arabia: kinetic and thermodynamics studies Academic research paper on "Chemical sciences"

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Abstract of research paper on Chemical sciences, author of scientific article — Shorouq Al-Ghanmi, Amani F. Al Sulami, Tahani A. El-Zayat, Basma G. Al-Hogbi, Mohamed Abdel Salam

Abstract Urban soils polluted with heavy metals are of increasing concern because it is greatly affecting human health and the ecological systems. Hence, it is mandatory to understand the reasons behind this pollution and remediate the contaminated solid. The removal of heavy metals from contaminated soil samples collected from the vicinity of the sewage lake in Jeddah, Saudi Arabia, was explored. The leaching process was studied kinetically and thermodynamically for better understanding of the remediation process. The results showed that the soil samples were slightly basic in nature, and tend to be more neutral away from the main contaminated sewage lake area. The total metal content in the soil samples was measured using the aqua regia extractions by ICP-OES and the results showed that many of the heavy metals present have significant concentrations above the tolerable limits. In general, the metal concentrations at different sites indicated that the heavy metal pollution is mainly due to the sewage discharge to the lake. The results showed excellent correlation between the concentrations of Co, As, and Hg with the distance from the main contaminated area. The leaching of Co, As, and Hg using 1.0M hydrochloric acid from the soil was studied kinetically at different temperatures and the experimental results were fitted using different kinetics models. The experimental data were best described with two-constant rate and Elovich equation kinetic models. Also, the thermodynamic study showed that the leaching process was spontaneous, endothermic and accompanied with increase in the entropy. In general, the polluted soil could be remediated successfully from the heavy metals using the acid leaching procedure in a short period of time.

Academic research paper on topic "Acid leaching of heavy metals from contaminated soil collected from Jeddah, Saudi Arabia: kinetic and thermodynamics studies"

Author's Accepted Manuscript

Acid leaching of heavy metals from contaminated soil collected from Jeddah, Saudi Arabia: kinetic and thermodynamics studies

Shorouq Al-Ghanmi, Amani F. Al Sulami, Tahani A. El-Zayat, Basma G. Al-Hogbi, Mohamed Abdel Salam

INTERNATIONAL SOIL AND WATER CONSERVATION RESEARCH

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www.elsevier.com/locate/iswcr

PII: S2095-6339(15)30074-5

DOI: http://dx.doi.Org/10.1016/j.iswcr.2015.08.002

Reference: ISWCR27

To appear in:

International Soil and Water Conservation Research

Received date: Revised date: Accepted date:

25 May 2015 3 August 2015 10 August 2015

Cite this article as: Shorouq Al-Ghanmi, Amani F. Al Sulami, Tahani A. El-Zayat, Basma G. Al-Hogbi, Mohamed Abdel Salam, Acid leaching of heavy metals from contaminated soil collected from Jeddah, Saudi Arabia: kinetic and thermodynamics studies, International Soil and Water Conservation Research, http://dx.doi.org/10.1016/j. iswcr.2015.08.002

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Acid leaching of heavy metals from contaminated soil collected from Jeddah, Saudi Arabia: kinetic and thermodynamics studies

3horouq Al-Ghanmi ab, Amani F. Al Sulami a, Tahani A. El-Zayat a, Basma G. Al-Hogbi a, Mohamed Abdel Salam a*

5 Chemistry Department, Faculty of Science, King Abdulaziz University, P.O Box 80200-feddah 21589, Kingdom of Saudi Arabia

7 Center of Excellence in Environmental Studies, King Abdulaziz University, P.O. Box 80216, J eddah 21589, Saudi Arabia

ICorresponding author Tel.+966-541886660; fax+966-2-6952292 13asalam16@hotmail.com

Abstract

15 Urban soils polluted with heavy metals are of increasing concern because it is greatly affecting human health and the ecological systems. Hence, it is mandatory to understand the leasons behind this pollution and remediate the contaminated solid. The removal of heavy metals from contaminated soil samples collected from the vicinity of the sewage lake in leddah, Saudi Arabia, was explored. The leaching process was studied kinetically and 20brmodynamically for better understanding of the remediation process. The results showed 2at the soil samples were slightly basic in nature, and tend to be more neutral away from 22 main contaminated sewage lake area. The total metal content in the soil samples were Measured using the aqua regia extractions by ICP-OES and the results showed that many of

1he heavy metals present have significant concentrations above the tolerable limits. In general, the metal concentrations at different sites indicated that the heavy metal pollution 3s mainly due to the sewage discharge to the lake. The results showed excellent correlation between the concentrations of Co, As, and Hg with the distance from the main 5ontaminated area. The leaching of Co, As, and Hg using 1.0 M hydrochloric acid from the foil was studied kinetically at different temperatures and the experimental results were fitted using different kinetics models. The experimental data were best described with two-Bonstant rate and Elovich equation kinetic models. Also, the thermodynamic study showed t9hat the leaching process was, spontaneous, endothermic and accompanied with increase in t1h0e entropy. In general, the polluted soil could be remediated successfully from the heavy lletals using the acid leaching procedure in a short period of time.

Keywords: Acid leaching; Contaminated soil; Heavy metals; Kinetics; Thermodynamics 1n troduction

16 Since the industrial revolution, anthropogenic activities introduced various hazardous

heavy metals into soil. Heavy metal pollution of soils is an increasingly urgent problem all

1ver the world, resulting from the intensive use of wastewater for irrigation, sewage sludge,

pesticide and emissions from vehicle exhausts, mining, and smelting (Shi et al. 2009).

generally, heavy metals are persistent, and undegradable unlike other organic pollutants

gMam et al. 2012). Normally, soil works as a natural adsorbent which immobilizes heavy

ggetals and decreases their bioavailability through different mechanisms (precipitation,

gdsorption process and redox reactions), but when the concentrations of heavy metals

gxceed the soil capacity, the heavy metals become mobilized, resulting in serious

1ontamination of agricultural products or ground water. Contamination of the soil by heavy m etals has a great impact on the soil parameters, and it may negatively affect the soil Sharacteristics and limit their productive and environmental functions. For example, heavy metals decreases the microbial activity of the soil microorganisms. These microorganisms usually contribute significantly to the degradation and mineralization of organic matter and fonsequently to the recycling of nutrients in the soil (Castaldi et al. 2004). Also, the immobilization of the heavy metals within different organic and inorganic colloids present Bi the soil may decrease their availability as nutrients for other living organisms including 9lants (Nannipieri et al. 1997). Furthermore, the presence of heavy metals in the soil may Head to a great change in the physiological and biochemical processes in plants, which cause 11owth reduction, and accordingly reduces crop yield (Chibuike and Obiora, 2014). Hence, 12mediation of polluted soils from heavy metal contamination is crucial (Rosestolato et al. 1015; Szendrak et al. 2011). Soil washing using chemical reagents is considered as one of He few permanent treatment to remove heavy metals from polluted soils (Dermont et al. 1008). Chemical reagents such as acids/bases, and chelating agents are usually employed to 1emove the heavy metals from the soils into an aqueous solution. Hydrochloric acid, EDTA, 1nd subcritical water are the most used reagents used for soil washing and removal of 18btals from contaminated soil due to their high metal removal efficiency (Fedje et al. 2013; Wasay et al. 2001; Isoyama and Wada 2007; Rao et al. 2007; Udovic and Lestan 2007; gObglar and Lestan 2013). Many of the research studies mentioned the higher efficiency of goil washing using HCl compared with other leaching agents including EDTA, H2SO4 and 22NO3 (Moutsatsou et al. 2006; Moon et al. 2012). Generally, soil washing with acids such

23 HCl relies on ion exchange and dissolution of soil components/discrete metal compounds

24 extract metals, whereas chelating agents such as EDTA solubilize metals through

gomplexation. One of the important factors which affects the soil's washing efficiency is

26e extraction time with the chemical reagent as the rate of metal extraction is a factor of

lime. However, despite the fact that many studies focus on the soil washing using different 2hemical reagents, the number of researches focused on the leaching process kinetically and ihermodynamically are still scarce in literature. Kinetic studies are crucial to understand the 4actors and means of transport for metals from the soil to the aqueous phase. Thermodynamic calculation of the washing process is required to understand the mechanism of dissolution and mobilization and their spontaneity by calculating the different thermodynamic parameters.

There has been a considerable increase in the population of Jeddah, Saudi Arabia, during 9ie last few decades. Most of the collected treated sewage in Jeddah, usually treated in 10veral sewage treatment plants, are usually discharged into the Red Sea and/or a sewage lake. This sewage lake was established in 1992, and stretched over an area

of 2.6 km2. It

became the main sewage downstream, which makes the lake a direct source of pollution in 13 city, especially to residential areas around the lake. The hypotheses of this research is Hat the sewage discharge is the main source of contamination of the soils around the 16wage lake, and this contamination reached its maximum in the vicinity of the lake and decreased as we go further from the lake. Also, the acid leaching could be an effective jbFocedure for the environmental remediation of the polluted soil from heavy metals.

18 The objectives of this research was to investigate the heavy metals contamination and lemediation of three soil samples collected from different areas in the vicinity of the sewage 2Cke in Jeddah, Saudi Arabia, in addition to studying the correlation between the heavy 21etal contamination and the distance from the main contamination site. This research also 22cused on the remediation of the contaminated soils using the acid leaching procedure using 1.0 M hydrochloric acid and explore the effect of leaching time, and temperature kihetically and thermodynamically in order to achieve a better understanding of the metal 2aching and mobilization process from the contaminated soil samples.

M1 aterial and Methods

C hemicals

All chemicals used in this study were obtained from Sigma-Aldrich (analytical grade), and 4ll solutions were prepared using deionized water. 5

S oil samples collection and preparation

7 As it is presented in Fig. 1, soil samples were collected with a shovel forced into the 8oil; 10 cm depth approximately, from three different areas around the sewage lake; soil 9amples from area (A) (representing the area around lake drainage which is usually 1ontaminated by the sewage water), soil samples from the National Park area (B) (located Hutheast of the Lake (2.7 km from area A) and is irrigated by the recycled water from the 1ake), and finally, soil samples from area (C) (representing the residential neighborhood 1djacent to the lake (13.0 km from area A) in the northwest and its level is below the lake 1evel). The collected soil samples were air-dried at room temperature (20-30 °C) for seven 1ays, then were passed through a No. 14 stainless steel sieve to obtain particle size of 1.40 16m, where rocks and other large material unable to pass through the sieve were taken 1w ay, and then the soil samples were thoroughly mixed to ensure uniformity. The soil 18mples were then dried at 80 °C until a constant weight was achieved, and finally were directly cooled down in desiccators to room temperature before being stored in self-sealing gtastic bags for subsequent experiments. 213. Soil characterization

gg Soil pH was determined in a soil:distilled water ratios of 1:2.5 suspensions by a gombined glass-reference electrode and Mettler Delta 3510 pH meter. Soil conductivity was determined in a 1:2.5 soil: water suspensions by accumet conductivity metter. The

luspensions were prepared by weighing 10 g sample of each soil into 120 ml screw cap glass bottles, 25 ml water was added to each bottle and the pH and conductivity were m easured.

2.4. Total metal determination

The total metal contents in the soil samples were determined using the aqua regia 6xtractions. Samples were digested at room temperature with 37% HCl / 70% HNO3 (3:1) mixture (10.0 ml per 0.5 g of [sample) for 12 h. After this, the suspension was digested at 830 °C, for 3 h under reflux conditions. The suspension was then filtered and diluted to 900 ml with 0.50 M HNO3 for analysis. The total metal concentrations were determined ussing inductively coupled plasma-optical emission spectrometry (ICP-OES Optima 4100 11V), Perkin Elmer, USA. Calibration curves were constructed for the target heavy metals using ICP-OES, the measurements were verified using initial calibration verification (ICV) dilution contain the target heavy metals. The ICV values were within 95 - 105% of the ixpected value, and the limits of detection were calculated; based on 3 x standard deviation 15 the blank, and were 2.9, 27.0, and 30.0 mg/kg for Co, As, and Hg; respectively. The lachate samples were then analyzed using the calibration of internal standard addition, based on four measurements and the reported value was the average ± their standard deviation.

295. Acid leaching kinetic experiment

2G0 g of soil sample were added to 0.250 L of the leaching reagent; 1.0 M HCl, and continuously stirred at 500 rpm and at different temperatures; 298±1 K, 308±1 K, and 22 3±1 K, separately. At defined points in time, a part of the soil leachate solution was withdrawn using a glass pipette, and filtered immediately through a filter paper. The filtrate 24as kept at 5 C in the dark for metal analysis. The percentage metal leached from the soil

a1nd the amount of metal leached per unit mass of soil at any time; qt, were calculated using E quation (1) and (2), respectively.

3% Metal leached =—^X 100

CSmS (1)

q = CV (2)

Where Cl and CS are the concentrations of metal in supernatant (in mg/L) and soil (mg/kg), fespectively; Vl is the volume of supernatant (in L) and mS is the dry mass of the soil (in kg). The leaching experimental data was analyzed using different kinetic models that Brequently used for the description of soil leaching.

3. Results and Discussion

3o1. Soil Characterization

11 Soil mineral characterization was performed using XRD, and the results showed that Soil collected from site A composed of quartz (SiO2), albite (NaAlSi3O8), halite (NaCl), 1ylvite (KCl), and synthetic iron (Fe), soil collected from site B composed of quartz (SiO2), 1lbite (NaAlSi3O8), synthetic magnesium calcite, dolomite (MgCO3)2, and synthetic hematite (Fe2O3), whereas soil collected from site C composed of quartz (SiO2), albite 1N aAlSi3O8), halite (NaCl), and sylvite (KCl). Generally, most contaminants in soil exist as 17tionic species of metals. These metals cations can complex with inorganic soil 1onstituents, e.g., carbonates, sulfates, hydroxides, sulfides, to form either precipitates or positively charged complexes. Both complexation and precipitation reactions are pH gependant. Therefore, although these metals can form complexes with a net negative gharge, under most environmentally relevant scenarios (pH = 4 to 8.5), these metals either grecipitate or exist as cations (US-EPA 2003). The pH values were 8.74, 8.56, and 7.80 for

loil samples collected from sites A, B, and C, respectively. This indicates the slight basic 2ature of the soil sample in general and the tendency to be more neutral away from the m ain contaminated sewage lake site A.

The total metal contents in the soil samples were measured using the aqua regia extractions by ICP-OES and the results are tabulated in Table 1. It is clear from the table that only As, C o, Cr, Ni, Pb, V, Hg and Zn presents with significant concentrations in the soil collected from the three sites A, B and C. The total metal concentrations were significantly above the 8olerable limits recommended by World Health Organization (WHO) and European Union 9EU) (Lee et al. 2009). In general, the concentrations of the metals were in the order of Site A > Site B >> Site C; indicating that the metal pollution is mainly due to the sewage discharge to the lake at site A. Studying the correlation between the different sites and the metal concentrations revealed the existence of strong correlation between Co, As, and Hg ind the soil samples collected from Site A, B, and C, as it is presented in Fig. 2. The lorrelation coefficients (R ); which indicated the true correlation between X and Y within lie general population, were 0.9922, 0.9287, and 0.9964, for the Co, As, and Hg, iespectively. This indicates the strong relation between the metal concentration and the distance from the main discharge area; site A. The concentration of the total metal decreased away from the main contamination site. Accordingly, this study focus on dnvironmental remediation of the soil collected from sites A, B, and C, from their contamination with Co, As, and Hg using the chemical leaching treatment. 3i2. Kinetics study

22 The influence of time on the leaching of Co, As, and Hg from Site C soil was studied using 1.0 M HCl at different temperature and the results were presented in Fig. 3. It is clear 2i4)m the figure that leaching percentage of the target metal enhanced significantly with the teaching time till equilibrium was attained. Leaching of Co from the soil increased with

1ime and start to equilibrate after 5.0 hrs with leaching percentage equals 71.90 % (151.0 m g Co/kg soil), and this percentage reached 77.8 % (163.3 mg Co/kg soil) within 24 hrs at 398 K. Also, leaching of As from the soil increased with time and start to equilibrate with teaching percentage equals 55.70 % (38.7 mg As/kg soil) after 12.0 hrs, and this percentage Beached 59.2 % (41.1 mg As/kg soil) within 24 hrs. The same trend was observed for Hg, teaching percentage increased with time and start to equilibrate after 5.0 hrs with leaching percentage equals 82.1 % (9.52 mg Hg/kg soil), and this percentage reached 83.9 % (9.73 mg Hg/kg soil) within 24 hrs. This could be attributed to the accessibility of the HCl Solution to the metal cations from their complexes with organic matter, or their salts within the soil with time. Also, it was clearly observable that raising the treatment temperature, 11sociated with remarkable increase in the % metal leached from the soil. Raising the ttrfcatment temperature from 298 K, to 308 K, and 323 K increased the leaching percentage torn 77.8 %, to 82.8 %, and 87.1 % for Co, from 59.2 %, to 71.7 %, and 83.7 % for As, Mom 83.9 %, to 87.5 %, and 93.0 % for Hg, respectively. This is mainly due to the 1nhancement of the target metal salts solubility as a result of raising the treatment 1emperature and hence they are more accessible to the HCl solution, which increased the teaching percentage.

18 The leaching behavior of C7o, As, and Hg from Site C soil using HCl acid was

1tudied kinetically to understands the rate processes by which the acid leaching of target

gfibtals occurs from soil. The evaluation of the leaching rate usually generates valuable

gUformation about the interactions between the metal ions, the leaching agent, and the soil.

There are many theoretical models that could be serve this purpose, and the best fit of

gxperimental data to any of these models can be interpreted as providing appropriate

kinetics for the leaching process. The leaching kinetics data of Co, As, and Hg from Site C

g(5il by acid washing at different temperature; Fig. 3, were analyzed using the most common

kinetic models used for the description of metal desorption from soil; zero-order (Dang et

dl. 1994), first-order (Kuo and Lotse 1973), second-order (Dang et al. 1994; Griffin and 2urinak 1973), third-order (Dang et al. 1994), parabolic diffusion (Khater and Zaghloul 3001), two-constant rate (Dang et al. 1994), and simple Elovich (Havlin et al. 1985). K4 inetic models are shown in Table 2. Applying these models to the current leaching experiments data, it was found that only the two-constant rate and Elovich kinetic models 6onverge well with acceptable regression coefficients, as it is presented in Table 3, and Fig. 7 and Fig. 5; respectively. This indicates the suitability of the two-constant rate and E8 lovich kinetic models for the description of the leaching of Co, As, and Hg from 9ontaminated soil C by acid washing. As it is presented in Table 3, there was a good dorrelation between the values obtained from the rate constants for the two-constant rate dquation and the values obtained from simple Elovich equation. The same observations were found when copper was desorbed from selected calcareous soils collected from Iran by di-ethylenetriamin-penta-acetic acid (Reyhanitabar and Karimian 2008; Fasaei et al. d006), and for the cadmium desorption from selected sub-tropical soils (Rashti et al. 2014). These results indicated the dependency of the acid leaching soil remediation procedure on d6e nature of the soil as well as the type of the heavy metals present within the soil.

dtfis noteworthy to mention the presence of an excellent correlation between the calculated Mtial metal desorption rate constants and coefficients obtained from both of two-constant date equation model and Elovich equation model with the temperature for Co, As and Hg leached from soil using HCl as it is shown in Fig. 6, and Fig. 7. It was observed that raising the temperature of the leaching process accompanied by a significant increase in the calculated initial metal desorption rate constants obtained from both models with excellent regression coefficients values. This is another trend which confirm the suitability of the two-constant rate equation model and Elovich equation model for the description of the leeching process. As it is presented in Fig. 7, there was a little variation of the effect of the

temperature with calculated initial metal desorption rate coefficients obtained from the two kinetic model, especially for leaching of Hg, which showed very poor correlation Soefficient with the application of the Elovich model. This may indicate that the two-4onstant rate equation model is the most appropriate model compared with the Elovich equation kinetic model.

3.3.3 Thermodynamic studies

7 The acid leaching of Co, As, and Hg from contaminated soil C was studied thermodynamically to explore the feasibility and spontaneity of the leaching process 9irough the calculation of thermodynamic parameters: Gibbs free energy change (AG), dnthalpy change (AH), and entropy change (AS). The thermodynamic parameters were i1lculated from the variation of the thermodynamic distribution coefficient D with a change Hg temperature and according to equation (3):

where ql and qS are the amount of metal in supernatant and the soil at equilibrium (mg Metal/kg soil), respectively. The AH and AS may be calculated according to the following dquation (Abdel Salam 2013):

The AH and AS values were calculated from the slope and the intercept of the straight line 19 the In D vs 1/T plot presented in Fig. 8; respectively. The enthalpy change value; AH, gfifere found to be positive; +20.9 kJ mole-1, +40.4 kJ mole-1, and +30.3 kJ mole-1, for Co, As, and Hg ; respectively, confirming the endothermic nature of the acid leaching of these 22etals from the contaminated soil as it is presented in Table 4 . The positive values of

17 D =

1ntropy change; AS, - +80.75 J mole-1 K-1, +138.9 kJ mole-1 K-1, and +115.1 kJ mole-1 K-1, gor Co, As, and Hg ; respectively, suggested an increase in randomness due to the leaching 3f the metal from the contaminated soil. The AG value was calculated according to the following equation at different temperature:

AG= AH - TAS (5)

The AG calculated value at 298 K was equal to -3.14 kJ mole-1, -0.947 kJ mole-1, and -3.99 kJ mole-1, for Co, As, and Hg ; respectively, as would be expected for a product-favored a8nd spontaneous reaction, indicating the high affinity of the metal ions towards the acidic teaching solution and the spontaneity of the leaching process. Also, the negative value also 1onfirms the exothermic nature of the leaching of the heavy metal by the 1.0 M HCl, which 1xplains the enhancement of the leaching process by raising the solution temperature. The magnitude of AH suggests a weak type of bonding between heavy metals and the soil, such 13 physical binding, rather than chemical binding (Nollet et al. 2003). These results verified Hat the Co, As, and Hg complexes with the soil were not stable, and most of these heavy 1i5etal ions were mobilized and washed upon chemically treating the soil with 1.0 M HCl 1<6lution. It is noteworthy to mention that soil washing and acid leaching is one of the 1ommon engineering practice that had been adopted (USEPA, 1991; Stegmann et al. 2001, Wise et al. 2000) for the remediation of different soils contaminated with heavy metals.

19 Conclusions

go Different soil samples were collected from the vicinity of the sewage lake to study the heavy metal contamination and remediation. The pH values were in the range of 8.74-7.80. go soil samples, these values decreased further down from the main contaminated site. The g3tal metal contents in the soil samples were measured and the results showed that As, Co, g)t, Ni, Pb, V, Hg and Zn were present with significant concentrations above the tolerable

Mmit. Also, excellent correlation between the concentrations of Co, As, and Hg with the gistance from the main contaminated area was obtained which confirms that the Sontamination was mainly due to the discharge of the untreated sewage to the lake. The teaching of Co, As, and Hg using 1.0 M hydrochloric acid from one of the contaminated 5oil was studied kinetically and thermodynamically at different temperatures, and the e6xperimental data were best described with the two-constant rate and Elovich equation kinetic models. Also, the thermodynamic study showed that the leaching of Co, As, and Hg ttrom the soil were spontaneous at all temperature, endothermic, and accompanied with 9ncrease in the entropies. Finally, it could concluded that the contamination of the soil was mainly due to the discharge of the untreated sewage to the sewage lake and this dontamination became minimum further away from the main contamination site. The dontaminated soil could be remediated by using the acid leaching process effectively and 13ccessfully within short period of time and at ambient conditions. 14

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Table 1. Average concentrations of heavy metals in area A, B, and C using aqua regia digestion in mg/kg.

Element Area (A) Soil Area (B) Soil Area (C) Soil

As 122.6 ± 0.2 100 ± 0.5 69.4 ± 0.4

Co 310 ± 0.8 263.4 ± 1.4 210 ± 0.9

Cr 872 ± 2.5 1128 ± 5.4 680 ± 3.4

Ni 434.6 ± 3.1 870 ± 2.5 394.8 ± 1.8

Pb 7.2 ± 0.2 ND 1.8 ± 0.4

V 152.6 ± 2.5 75.4 ± 3.1 124.8 ± 1.4

Hg 208 ± 2.6 120 ± 2.1 11.6 ± 1.0

Zn 4821 ± 10.2 1108.6 ± 9.5 2339.8 ± 8.7

Table 2. Different kinetic models used to study the leaching of Co, As, and Hg from Contaminated soil C using HCl.

Kinetic Model

Equation

Parameters

Zero order

First order

Second order

Third order

Parabolic Diffusion Two-constant rate

qt=qo-ko . t

ln qt=lnqo-ki . t

1/qt=1/qo-k2 . t

(1/qt)= (1/qo) - ks . t

qt=qo+ kp . t

qt=a . tL

Elovich qt = (1/ft). ln (as . ft) equation + (1/ft). ln t

ko, zero-order rate constant [(mg metal kg-1 hr-1)] k1, first-order rate constant [(hr)]

k2, second-order rate constant

[(mg metal kg-1)-1] k3, third-order rate constant [(mg metal kg-1)-2] kp, diffusion rate constant [(mg metal kg-1)-05] a, initial metal desorption rate constant [(mg metal kg-1hr-1)] and b, desorption rate coefficient

as and ft the initial adsorption rate

[(mg metal kg hr")], and the desorption nt [(mg metal respectively.

coefficient [(mg metal kg-1-)-1].

T able 3. Different kinetic models parameters for the leaching of Co, As, and Hg from soil gollected from Site C at different temperatures.

Two-constant rate equation model

Temperature a [ mg Co kg-1hr-1] b R2

298 K 63.15 0.374 0.970

308 K 74.06 0.367 0.957

323 K 95.73 0.270 0.966

Elovich equation model

Temperature as [ mg Co kg-1hr-1] Ps [(mg Co kg-1-)-1] R2

298 K 543.5 0.040 0.886

308 K 698.7 0.036 0.917

323 K 1208 0.037 0.964

Two-constant rate equation model

Temperature a [ mg As kg-1hr-1] b R2

298 K 10.88 0.503 0.985

308 K 18.95 0.424 0.947

323 K 29.80 0.312 0.924

Two-constant rate equation model Elovich equation model

Temperature as [ mg As kg-1hr-1] Ps [(mg As kg-1-)-1] R2

298 K 83.92 0.164 0.856

308 K 174.3 0.126 0.937

323 K 361.1 0.109 0.953

Two-constant rate equation model

Temperature a [ mg Hg kg-1hr-1] b R2

298 K 5.74 0.249 0.872

308 K 6.52 0.231 0.816

323 K 7.67 0.178 0.805

Elovich equation model

Temperature as [ mg Hg kg-1hr-1] Ps [(mg Hg kg-1-)-1] R2

298 K 103.2 0.711 0.975

308 K 148.3 0.676 0.939

323 K 286.2 0.692 0.928

T able 4: Values of the different thermodynamic parameters for the acid leaching of Co, As, 2nd Hg from soil collected from Site C.

Temperature (K) D AG° aH AS"

(KJ/mol) (KJ/mol) (J/K mol)

298 3.49 -3.14 +20.9 +80.75

308 4.81 -3.95 +20.9 +80.75

323 6.75 -5.16 +20.9 +80.75

298 1.45 -0.947 +40.4 +138.9

308 2.53 -2.34 +40.4 +138.9

323 5.15 -4.42 +40.4 +138.9

298 5.21 -3.99 +30.3 +115.1

308 7.00 -5.15 +30.3 +115.1

323 13.3 -6.87 +30.3 +115.1

Sample C

Lat: 21° 35r 49.49&8FI Long: 39° 15r 35.643"

Sample B Sample A Lat:21°38' 1S.7S3" Lat: 21° 37 54.0012" Long: 39* 23- i5.7308ri Long: 39° 21r 51.0012n

LrKlific^'" Ldfev IttRlPt I - Jfclfr \" -.SKA. .1

m I "jBL "'ttiftsr^i ' ^ Ttrr jM f trie Si

Fig. 1. Locations of the soil sample A, B, and C, around the sewage lake; Jeddah, Saudi A rabia.

L. ■H

« 100 H-"

Ri= 0.9922 ■ ^ V. ♦ As ■ Co

- M AHg

R2= 0.9964 Hi ■ Av S

R2 = 0.92B7 % v. A

Soil A

Soil B

Soil C

Fig. 2. Correlation between the metal concentrations and the soil location collected from 5he sewage lake, Jeddah.

-♦—298 K

-■-308K

10 15 20

Leaching time (hrs)

10 15 20

Leaching time (hrs)

Fig. 3. Effect of the temperature and time on the leaching of Co, As, and Hg from Sontaminated soil by 1.0 M HCl.

♦ 298 K ■ 308 K i 323 K ^ £

♦ 298 K E 308 K A 323 K

A A***/'***

♦ >- ^

-5-4-3-2-101234

Fig. 5. The two-constant equation kinetic model plots for the leaching of Co, As, and Hg from soil using 1.0 M HCl.

■S 150 U

£ 100

♦ 298 K E 308 K 323 K

a Aid A a«"

+v ■ ♦

■50 70

■g 20 n 01

- 10 c

§ 0 E

-20 14

irt 12 CiO

♦ 298 K E 308 K A 323 K

4A a^aa

-r ♦

♦ ♦

♦ 298 K E 308 K h 323 K

A * - ' -JT

■H» i 2

-10 12 3 4

Fig. 4. The Elovich equation kinetic model plots for the leaching of Co, As, and Hg from 3oil using 1.0 M HCl.

<D +->

iu rr1

(fl fO

-n +•>

— ClQ

100 ♦ Co ■ As A Hg Two constant rate equation model R2 = 0.9667 K i* ^

80 ^ ^

40 R2 = 0.9991 ____— m

20 ______ R2 - 1

fU i—.

■s a

1400 ♦ Co ■ As * Hg Elovich equation model

1200 R2 = 0.9667 +

800 J*

600 400 ** R2= 0.9931 - ' '//.-A

200 0 1°........

295 300 305 310 315 320 325

Temperature (K)

Fig. 6. Correlation between the initial metal desorption rate constants obtained from the Bwo-constant rate equation and Elovich equation kinetic models with the temperature for C o, As and Hg leached from soil using 1.0 M HCl.

0) —

Two constant rate equation

0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0

Rï= 0.9998 R2 = 0.9667

R2 = 0.9739

4 Co ■ As a Hg

= 0.1956 Elovich equation model

R2= 0.3933 4 Co 1 As a Hg

■---

R2 = 0.9667 A

-----^---

305 310

Temperature (Kj

Fig. 7. Correlation between the initial metal desorption rate coefficients obtained from the iWo-constant rate equation and Elovich equation kinetic models with the temperature for C o, As and Hg leached from soil using 1.0 M HCl.

3 2.5 2

% ,5 1

0.00305 0.0031 0.00315 0.0032 0.00325 0.0033 0.00335 0.0034

Fig. 8. Plot of In D vs. 1/T for the calculations of thermodynamic parameters for the teaching of Co, As, and Hg from soil by 1.0 M HCl.

R2 = 0.9862 ♦ CO ■ AS A Hg

R2= 0.9667

■- ^

R2 — 0.9994 W

KING ABDULAZIZ UNIVERSITY

Dr. Mohamed Abdel Sal am Ch e mistry De partme nt F acu Ity of Sci e nee King Abdulaziz University P.OBox 80200-Jeddah 21S89

Kingdom of Saudi Arabia Phone: +966 640-0000 x 68951 Mobile: +966 541-8S6660 E-mail: ma5alaml6@hotmail.com

Graphical Abstract

■'fan HrrrjUiCriCi

Locations of the soil sample A, B, and C, around the sewage lake; Jeddah, Saudi Arabia.