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Perspective Chapter: Rapid Measurement of Potentially Toxic Elements (PTEs) in Petroleum Hydrocarbons Polluted Soils by X-Ray Fluorescence (XRF) Spectroscopy
Written By
Reward Kokah Douglas
Submitted: 01 October 2022Reviewed: 09 November 2022Published: 11 December 2022
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Potentially toxic elements (PTEs) contamination in soils threats human wellbeing and ecological health because of their toxicity and bioaccumulation. This research presents a portable Olympus Delta Premium 6000 Series XRF Analyser (Olympus, USA) as a rapid measurement tool (RMT) for PTEs: Cr, Cu, Fe, Pb, Mn, and Zn in contaminated soils in the Niger Delta, Nigeria. A total of 45 crude oil-contaminated soils were collected from three genuinely oil spill sites. The range of measured PTEs concentrations (mg/kg) in the study sites are as follows: Site 1: chromium (Cr) 54–75, copper (Cu) 5.4–16.6, iron (Fe) 14,841–23,404, lead (Pb) 13.5–21.4, manganese (Mn) 158–555, and zinc (Zn) 32.6–47.2; Site 2: (35–66), (5–16.1), (10166–20,967), (12–17.8), (209–440), (17.6–33.6); and Site 3: (32–115), (6.5–20.8), (7538–22,800), (12–135), (98–338), (19.9–177). The trend of PTEs across the three sites follows the same order: Fe > Mn > Cr > Zn > Pb > Cu. The average concentration values of PTEs in all the 3 sites were higher than background concentration values. Thus, crude oil spill spiked the PTEs concentrations. XRF spectroscopy is recommended as a cost-effective and RMT for PTEs in soils.
Department of Chemical Engineering, Niger Delta University, Wilberforce Island, Bayelsa, Nigeria
*Address all correspondence to: douglasreward@ndu.edu.ng
1. Introduction
Soil is a great reservoir for contaminants as well as a natural buffer for transportation of chemical materials and elements in the environment. There has been an increasing concern in many countries of the world about the levels of potentially toxic elements (PTEs) in the soil environment [1, 2, 3]. There are over 40 chemical elements in the soil [4]; out of which, 21 elements are commonly considered as PTEs, which are zinc, Zn; vanadium, V; uranium, U; tungsten, W; tin, Sn; thallium, TI; silver, Ag; selenium, Se; molybdenum, Mo; mercury, Hg; manganese, Mn; lead, Pb; gold, Au; copper, Cu; cobalt, Co; chromium, Cr; cadmium, Cd; barium, Ba; arsenic, As, and antimony, Sb [5]. Among these, Pb, Cr, As, Zn, Cd, Cu, Hg, and Ni are most commonly found at contaminated sites [6].
Petroleum hydrocarbons contain PTEs such as cobalt (Co), copper (Cu), lead (Pb), iron (Fe), magnesium (Mg), manganese (Mn), zinc (Zn), cadmium (Cd), chromium (Cr), nickel (Ni), arsenic (As), titanium (Ti), silver (Ag) among others. PTEs contamination in soil has attracted significant ecological concerns because of their toxic, bioaccumulative, and persistence nature in the existing environment. Unlike most pollutants (e.g. petroleum hydrocarbons), PTEs cannot be degraded and have long-lasting effects in soil as a result of strong adsorption of many metal ions on humic and clay colloids in soils [7]. PTEs have been reported to have physiological effects on living organisms as they are not degradable [8]. Vehicle emissions, metal plating/finishing operations, disposal of industrial waste, fertilizer applications, and fly ash from incineration/combustion processes, among others are also sources of PTEs in soils [9]. It is pertinent to also mention that mining, smelting, chemical production, and factory emissions release large quantities of Cd and Pb into soils, causing significant soil pollution [10, 11].
In Nigeria, the Niger Delta region is the heart of the oil and gas Industry (OGI) and has contributed enormously to the growth and development of the country. However, since the beginning of the establishment of the OGI in the region, several oil spill incidents have been reported; and, to date, it has been estimated that 13 million tons of hydrocarbons have been spilled in the region due to pipeline fatigue, well blowout, pipeline vandalism, and sabotage [12, 13]. Additionally, Ite et al. [14] reported that the number of contaminated sites in the Niger Delta region is in excess of 2000. Furthermore, the United Nation Environment Programme (UNEP) reported in 2011 that in Ogoniland alone (a small part of the Niger Delta), over 69 sites were heavily contaminated with crude oil (concentration exceeding 139,000 mg/kg) affecting soil, air, and water quality criteria and posing a serious human health threat. This, in turn, impacts the quality of water resources, directly affecting the health of local communities, which are drinking contaminated water [15, 16, 17]. Therefore, there is urgent research need to assess and quantify PTEs in polluted soils using simple, rapid, inexpensive, and accurate analytical methods to help appraise the environmental risk of PTEs to ensure food security, environment safety, and public health safety in the Niger Delta region of Nigeria, and anywhere in the globe faced with such challenges.
Numerous analytical techniques are in use for PTEs detection and quantitative measurement in soils, including atomic absorption spectrophotometry (AAS), inductively coupled plasma-mass spectroscopy (ICP-MS), inductively coupled plasma atomic emission spectroscopy (ICP-AES), atomic fluorescence spectrophotometry (AFS), X-ray fluorescence (XRF) spectroscopy, and laser-induced breakdown (LIB) spectroscopy. Also included are optical techniques [18], electrochemical [19, 20], and voltammetry [21].
It is pertinent to mention that generally laboratory methods (in this case, laboratory methods are referred to the “chemical methods”) are known for quantitative measurement with good detection limits and have been commonly used for the detection of various PTEs in soils, with high sensitivity, selectivity, and accuracy [18, 21]. However, chemical methods require comparatively costly instrumentation, relatively lengthy measurement protocols, and specialized operators needed to achieve the correct measurements [21]. Given the relative disadvantage and cost of the laboratory methods mentioned above, there is a need to assess and use analytical devices that can offer rapid, inexpensive measurements, and requires little or no sample preparation. It is worth mentioning that to date, there is no published work yet comparing the measurement accuracy between the candidate chemical methods including AAS, ICP-MS, ICP-AES, etc., and analytical techniques that do not involve wet chemistry methods on measurement accuracy of PTEs in soil and in sediment samples. In the absence of such a crucial study that is key for method selection for environmental analyses, and knowing that the first step toward decision making on the selection of the best analytical techniques for the measurement of contaminants in environmental sample(s) is driven by time, cost, and the measurement accuracy [22]; methods that can offer timely and cost-effective analysis of environmental contaminants can be applied. As a result, this study aims at using a portable X-ray fluorescence (PXRF) spectroscopy as a cost-effective and rapid measurement tool (RMT) for PTEs in petroleum hydrocarbon-contaminated soils collected from crude oil spill sites in the Niger Delta region of Nigeria.
The study area located in Bayelsa and Rivers State, Niger Delta, Southern Nigeria has a tropical rain forest climate characterized by two seasons: The rainy season lasts for about 7 months between April and October with an overriding dry period in August (known as August break); and the dry season lasts for about 5 months, between November and March. The temperature varies between 25 and 35°C. The regional geology of the Niger Delta is relatively simple, consisting of Benin, Agbada (the kitchen of kerogen), and Akata formations, overlain by various types of quaternary deposits [23, 24]. Soils of the area studied were classified according to the United State Department of Agriculture (USDA) [25] soil taxonomy into two orders, that is, inceptisols and entisols, which include four subgroups of typic dystrudepts, aeric endoaquepts, typic udipsammerts, and typic psammaqnents [26]. A total of 45 representative spot sample points were collected from three oil-contaminated sites (Site 1 = Ikarama: 15 samples; Site 2 = Kalabar: 15 samples; and Site 3 = Joinkrama: 15 samples) in August 2015. The three sites were selected for sampling due to their similar exploration activities and oil spill history. The soil samples (approx. 5 kg) were collected from visible “hot-spots” in the top 15 cm soil layer using a shovel. Figure 1 shows the sampling location map. Soil samples were kept in airtight centrifuge tubes and stored at 4°C using ice block to avoid hydrocarbon volatilization and preserve field-moist status until transported to Cranfield University in the United Kingdom. The samples were then stored in a freezer at –20°C prior to PTEs analysis by XRF spectroscopy.
Figure 1.
Soil sampling locations for the three petroleum hydrocarbon-released sites in the Niger Delta region of Nigeria. Source: [27].
2.2 Sample preparation and XRF analysis of PTEs in soils
The concentrations of PTEs: Cr, Cu, Fe, Pb, Mn, and Zn in petroleum hydrocarbon-contaminated soils were determined using a portable Olympus Delta Premium 6000 Series XRF Analyser (Olympus, USA). In diffuse reflectance mode, the Delta XRF analyzer is three beams configured, where each beam was programmed to scan soil samples for 30 seconds. Prior to soil scanning, the instrument’s setting and operational conditions were done in accordance with the manufacturer’s specifications, and the analyzer was calibrated with alloy 316 stainless steel coupon. Fresh soil samples were thoroughly mixed and scanned using single open-ended and snap-post venting (30.7 mm O.D x 23.1 mm High) sample cups, sealed by Prolene Thin-Film (Diam. 63.5 mm) (Chemplex, USA). Each soil sample was analyzed for Cr, Cu, Fe, Pb, Mn, and Zn concentrations.
Table 1 reports on the 6 PTEs (Cr, Cu, Fe, Pb, Mn, and Zn) concentrations in 45 genuinely crude oil-contaminated soil samples collected from three petroleum hydrocarbon-contaminated sites in the Niger Delta, Nigeria. Summary statistics of the 6 PTEs concentrations for all the three study sites range as follows: Cr ranged from 54 to 75, Cu (5.4–16.6), Fe (14841–23,404), Pb (13.5–21.4), Mn (158–555), Zn (32.6–47.2) in Site 1; Cr (35–66), Cu (5–16.1), Fe (10166–20,967), Pb (12–17.8), Mn (209–440), Zn (17.6–33.6) in Site 2; in Site 3, Cr (32–115), Cu (6.5–20.8), Fe (7538–22,800), Pb (12–135), Mn (98–338), Zn (19.9–177). As it can be seen, the trend of heavy metal contamination in the three sites is in the order: Fe > Mn > Cr > Zn > Pb > Cu. Fe contamination level was found to be the highest in site 1. Results (mean values) were compared with other studies [7, 28] conducted in the Niger Delta, Nigeria, the case study area of the current research. Except Fe that had no records at the moment to be compared with, most of the metal values were higher than those reported by [7, 28]. Also, the mean concentrations of Mn, Cr, Cu, Zn, and Pb were observed to be higher than the background concentrations reported by [29]. These results are presented in Table 2. Results show that crude oil spill is the source of PTEs pollution in the study sites.
Site 1 (number of samples = 15)
Fe
22,368
23,040
20,393
21,392
23,344
23,073
23,404
21,912
23,251
17,951
18,201
18,555
19,146
19,495
14,841
Mn
312
401
445
463
555
506
517
488
511
210
193
179
158
222
171
Cr
62
66
55
61
66
63
65
59
54
66
72
67
59
59
75
Zn
44.2
32.6
33.9
37.3
45.8
34.4
38.4
32.6
34.1
46.9
47.2
43.3
39.6
44.6
46.6
Pb
18.5
14.9
17.5
21.4
20.9
16.2
16.6
17.3
13.5
18.6
18.5
17.9
15.8
14.3
16.8
Cu
16.6
7.6
11.7
12.4
15.6
11.5
13.3
11.6
12.8
8.6
11.7
8.9
10.4
5.4
8.5
Site 2 (number of samples = 15)
Fe
11,721
15,595
10,924
13,899
10,166
14,278
20,967
14,017
12,097
13,812
10,313
16,361
13,577
19,285
13,699
Mn
216
358
259
292
209
372
372
343
210
286
278
440
325
415
300
Cr
40
66
36
43
41
48
53
40
39
43
35
49
40
48
38
Zn
21
27.8
19.8
33.6
17.6
22.7
31.3
24.4
30.6
30.6
22.3
30.4
19.4
30.9
28
Pb
15.7
15.9
16.7
15
14.9
17.8
16
12.9
12
15.3
15.8
16.6
16.1
17
14.1
Cu
8.3
11.1
7.3
10.6
6.2
7.8
16.1
7.1
6.8
5
7.8
10.5
nd
9.5
6.6
Site 3 (number of samples = 15)
Fe
13,538
17,296
15,194
19,087
22,213
21,148
13,042
22,800
13,848
7538
19,418
19,667
15,182
20,222
13,730
Mn
234
270
146
255
251
284
191
338
209
98
236
263
212
276
205
Cr
37
73
32
65
115
49
34
62
52
47
49
58
61
61
39
Zn
26.9
29.3
41.7
78.4
177
41.1
28.9
60.6
43.3
19.9
61.7
47.7
63.7
68.9
64.9
Pb
16.9
12.7
35.8
80.4
135
14.5
12
29.6
47.2
21.4
24
28.6
34.9
56.5
69.9
Cu
7.6
11.5
13.9
20.1
20.8
12.2
7.6
12.1
6.6
6.5
16.2
14.9
13.4
13.3
9
Table 1.
Potentially toxic elements concentrations (mg/kg) in soils collected from three oil spill sites in the Niger Delta, Nigeria.
Fe = iron, Mn = manganese, Cr = chromium, Zn = zinc, Pb = lead, Cu = copper, and nd = not detected. Source: [27].
Metal
Site 1
Site 2
Site 3
A
B
C
Mean
Min
Max
Mean
Min
Max
Mean
Min
Max
Fe
20691.07
14,841
23,404
14047.40
10,166
20,967
16928.20
7538
22,800
NR
61,625
NR
Mn
355.4
158
555
311.7
209
440
231.2
98
338
26.1
199.6
201.8
Cr
63.3
54
75
43.9
35
66
55.6
32
115
2
17.8
28.8
Zn
40.1
32.6
47.2
26
17.6
33.6
56.9
19.9
177
23
38.1
29.3
Pb
17.3
13.5
21.4
15.5
12
17.8
41.3
12
135
11
7.4
25
Cu
11.1
5.4
16.6
8.1
5
16.1
12.4
6.5
20.8
4
7.8
11.2
Table 2.
Descriptive statistics of PTEs concentrations (mg/kg) in 45 crude oil-contaminated soils collected from three spill sites in the Niger Delta, Nigeria.
Results were compared with those of previous studies (A, B, C) in the same region. A = background level of PTEs in soils [29]; B = mean concentration of PTEs in oil field soils [28]; C = mean concentration of PTEs in crude oil impacted soils [7] and NR = not reported.
This chapter presents a portable XRF Analyser (Olympus, USA) as a rapid measurement tool (RMT) for PTEs (Cr, Cu, Fe, Pb, Mn, and Zn) in crude oil-contaminated soils. A total of 45 field samples collected from three genuinely PHC released sites in the Niger Delta, Nigeria were analyzed. The trend of PTEs contamination across the three sites follows the same order: Fe > Mn > Cr > Zn > Pb > Cu. While the mean values of some PTEs obtained in this study (sites 1 and 2) ranked higher than those reported in previous studies; the mean values of all the PTEs in Site 3 were higher than all the previously published results in the region. Consequently, results conclude that crude oil spills on land sites contributed to the higher concentrations of the PTEs relative to the natural background values. Since PTEs are bioaccumulants, they may pose a threat to environment and human well-being. Thus, there is research need to assess the site-specific risks of PTEs contamination in both the areas where soils are potentially polluted, and sites that have recorded series of oil spill incidents in the Niger Delta, Nigeria. Furthermore, the following recommendations are made:
Research on the comparison between well-implemented techniques involving chemical methods and analytical techniques that do not involve chemical methods in the measurement accuracy of PTEs in soil and in sediment samples should be carried out.
In the interim, if time is not a crucial factor and accuracy is more appealing, techniques for soil PTEs involving chemical methods are the most appropriate option, as accuracy is higher than analytical techniques that do not use chemical methods.
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Written By
Reward Kokah Douglas
Submitted: 01 October 2022Reviewed: 09 November 2022Published: 11 December 2022
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