Quantify the Amount of Metals in Asphalt Binders with EDXRF

Table of Contents

Introduction
Background
Typical Asphalt Compositions
Liquid Asphalt Components
Elemental Composition Ranges Determined by Research
Quantifying the Metal Content to Meet Environmental Guidelines
     Strieter Non-Polars
     Strieter Mid-Polars
     Strieter Polars
Using XRF to Quantify the Metal Content
EDXRF to Characterize Binders
Validation Results on NIST Standards
Summary
Conclusion

Introduction

The threat of safe drinking water is one of the greatest pollution problems in the world today. Less than 1% of the water on earth is clean and suitable as drinking water. A mixture of surface water reservoirs and groundwater aquifers are the only sources of drinking water for humans. Therefore, these sources should be protected from pollutants in order to ensure the safety of this water.

The discharge of under treated or untreated storm water runoff, more importantly runoff from asphalt roadways, is a major source of pollution to drinking water supplies.(1) Surface deposits of toxic elements from road traffic are also present, in addition to leachable material in the asphalt. In this article, these sources are reviewed and a scheme is proposed for measuring and quantifying the amounts of metals in their source materials used to bind asphalt employed in making roads.

Background

Public health can be directly affected by heavy metals as they can enter the body through dust and soil, breathing or dermal contact. The characteristic elements Cu, Zn, Pb and Cd found in the roadside soils, produced from traffic activity, can reach the human body via the food chain and therefore be very toxic to people. In agricultural areas, a major role in human exposure to heavy metals is played by the intake of heavy metals through the soil-crop system. Health problems that adversely affect the reproductive, renal, cardiovascular and blood forming systems are generally due to the heavy metals with high concentrations in the environment.

Behavioral abnormality, attention deficit and reduced intelligence as well as contribution to cardiovascular disease in adults are some of the consequences of heavy metal pollution. Although some trace metals (such as Cu and Zn) are harmless in small amounts, the others (mainly Pb, As, Hg and Cd), even at very low concentrations, are toxic and are potential initiators, promoters or cofactors in many diseases, including increased cancer risk. The heavy metals’ irreversible mobilization makes their elimination from the soils a challenging process.

The mechanisms of heavy metal emission from vehicles comprises of road abrasion, brake wear, tire wear, engine oil consumption and fuel consumption. The largest emission for Cd is based on engine oil consumption. Tire wear is the most important contributor for Zn emission, while brake wear is the major source of emissions for Pb and Cu. Pb may still be found in exhaust gas and can come from used metal alloys in the engine, with the use of unleaded gasoline that causes a subsequent reduction in fuel emissions of Pb.

Different heavy metal species such as Pb, Cd, Zn and Cu are present in bitumen and mineral filler materials in asphalt road surfaces. Either through atmospheric precipitation or road runoff, heavy metals can be transported into the roadside soils. Public health concerns of contamination of aquifers are assesed as being a severe threat.(2) Breakdown products from asphalt emulsifiers can be a part of roadway runoff, as a significant metal concentration is present in the polar fraction of asphalt binders as measured on the Schieff scale (3). Environmental protection agencies like the state DOT (Department Of Transportation) and Federal Highway Administration have to largely focus on this area, to promote pavement longevity with safety concerns for road construction materials.

Asphalt makers seek alternative material resources and use recycled materials due to economical constraints and handle the waste generated by aging road surfaces and worn out motor and vegetable oils are natural emulsifiers for recycled asphalt paving and they are readily available at low cost. The deleterious elements that are found in these materials are major concerns and the need is to blend the oils correctly to reduce the concentration of metals and other organic components deemed to be public health concerns.

Typical Asphalt Compositions

Table 1. Elemental analysis of asphalts from different crude sources.

Crude sources C
(wt%)
H
(wt%)
N
(wt%)
S
(wt%)
O
(wt%)
V
(mg/kg)
Ni
(mg/kg)
Mexican blend 83.77 9.91 0.28 5.25 0.77 180 22
Arkansas-Louisiana 85.78 10.19 0.26 3.41 0.36 7 4
Boscan 82.90 10.45 0.78 5.43 0.29 1380 109
California 86.77 10.94 1.10 0.99 0.20 4 6

Naturally occurring in crude oil sources, both Nickel and Vanadium are also intrinsically found in asphalt as acquired from refineries.

Liquid Asphalt Components

In recent years, the price of petroleum-based asphalt has increased to a great extent. Therefore, people have begun to search for alternative binders to petroleum asphalt that can be employed in pavement construction (Aziz et al. 2015; Huang et al. 2012). Bio asphalt derived from waste cooking oil, waste engine oil (Jia et al. 2014Jia et al. , 2015) and biochar derived from bio-oil used as a bio modifier for asphalt cement (Zhao et al. 2014a,b) are some of the examples of asphalt alternatives (Wen et al. 2013). The effectiveness of waste cooking vegetable oil as a rejuvenator in restoring the required properties of aged asphalt binder from RAP (Recycled Asphalt Paving) has been evaluated by Sun, Xu, You, Suo, Yao and Ji in this article.

The waste engine oil also had a positive effect, particularly for the stiffness of the aged binder in RAP. After the inclusion of waste engine oil, the optimum content of asphalt binder reduced (Jia et al. 2014 Jia et al., 2015). Earlier, investigations and research primarily focused on the grades and physical properties of the recovered asphalt binder with different contents of waste cooking oils and few studies have been found to evaluate the low-temperature properties and fatigue resistance of these recovered asphalt binders. (3)

Elemental Composition Ranges Determined by Research

A discussion for a need for improvement in the Superpave (M 320) protocols to enhance prediction of cracking performance by using the extended Bending Beam Rheometer (BBR) and Double Edged Notched Tension Test (DENT) techniques was carried out by Hesp et al. from the University of Ontario in 2009. Investigations were carried out for the seven pavement sections, with seven different asphalt binders, placed in 2003 and they all fulfilled the present M 320 low temperature requirement of -34 °C. To confirm the validity of the proposed DENT and extended BBR test methods, chemical analysis results from retrieved asphalt binders were used, as the sections had differing low temperature cracking performance.

The presence of Zinc (Zn) was identified by the X-ray fluorescence (XRF) spectrum in three of the failed sections. Since Zinc is typically there in huge quantities after the refining of waste engine oil and never found in either straight asphalt cement or the aggregate, it may be a fingerprint element for waste engine oil residues. Zinc-Sulphate or Zinc-Carbonate, however, is sometimes employed to scavenge hydrogen sulfide in asphalt binders.

It stands to reason that other engine oil additives would also be present in the asphalt from waste engine oil and it is likely to see concentrations of Calcium (Ca), Phosphorus (P) and probably Molybdenum (Mo) metals in these asphalt samples.

Historical testing has shown the presence of following metals in bitumen in mg/kg. The four samples represent bitumen samples taken from Nigerian Bitumen outcrops (5).

Table 2. Naturally occurring metal levels in Crude Oil.

Sample Fe Pb Cu Cd Ni Mn V TTM V/Ni V/V+Ni Fe/V
VAB 38.00 12.00 3.00 4.00 42.00 6.00 10.00 115.00 0.24 0.20 3.80
OI 283.00 11.00 4.00 8.00 20.00 4.00 50.00 480.00 7.50 0.88 1.89
IL 1537.00 27.00 10.00 7.00 62.00 3.00 100.00 746.00 1.61 0.62 15.37
LD 553.00 11.00 5.00 15.00 9.00 5.00 150.00 748.00 16.67 0.90 3.69

The following metal levels are found in Recycled Motor Oils

Table 3. Levels of elements of interest in REOB materials.

Elements Low Value High Value
S 0.1% 5.00%
P 0.005% 2.00%
Ca 0.005% 2.00%
V 0.0010% 0.100%
Fe 0.005% 0.200%
Cu 0.005% 0.050%
Zn 0.005% 1.00%
Mo 0.005% 0.050%

Quantifying the Metal Content to Meet Environmental Guidelines

Figure 1. Source location of seven samples taken from re-refinery. The numbers in the red boxes refer to REOB/VTAE processed samples. Identifiers in the blue boxes refer to lube oil processes, two of which involve hydrotreating to convert PACs to naphthenic compounds.

Copper (Cu), Zinc (Zn) and Chromium (Cr) are responsible for the toxicity of metals present in VTAE (Vacuum Tower Asphalt Extenders)/REOB (Recycled Engine Oil Bottoms). Through the processing of the used engine oil, some wear metals are eliminated before vacuum tower.

Despite being present in the blended asphalt binder, additive metals in the REOB/VTAE were not found to be leachable.

Table 4. Results for leachable amounts of toxic metals found in REOB/VTAE.

Element CAS No. PG 58-28 Neat mg/L PG 58-28 with REOB/VTAE mg/L Detection Limit mg/L Regulatory Limit mg/L
Arsenic 7440-38-2 BDL BDL 0.05 5
Barium 7440-39-3 0.13 0.11 0.05 100
Cadmium 7440-43-9 BDL BDL 0.025 1
Chromium 7440-47-3 BDL BDL 0.05 5
Lead 7439-92-1 BDL BDL 0.05 5
Mercury 7439-97-6 BDL BDL 0.002 0.2
Selenium 7782-49-2 BDL BDL 0.05 1
Silver 7440-22-4 BDL BDL 0.05 5

BDL = Below Detection Limit

Table 4 clearly shows that none of these materials leached out any of these metals in environmentally significant quantities employing standard leaching tests, but other metals generally found in recycled waste oil may be more labile.

To understand the composition of REOB/VTAE, the major components within the material have to be separated. HRG separated the Polar, Mid-Polars and Non-Polars fractions of REOB/VTAE products, by employing a fractionation procedure exclusively developed by Strieter. The results are shown in Table 5. Based on GPC analysis, the Mid-Polar and Non-Polar fractions are of same apparent molecular size. It was found that the REOB/VTAE was nearly 10.4% Polars, 33.3% Mid-Polars and 56.3% Non-Polars.

Table 5. Elemental Analysis of REOB/VTAE Fractions.

Courtesy of REOB/VTAE Report Asphalt Institute

Since not all of the material could be completely extracted from the media for analysis, the Mid Polar does not add to 100%. ED-XRF was used to carry out further analysis of these three fractions. Table 5 displays the estimated results.

Table 6. Elemental distribution in various polarity fractions as extracted using the Strieter method.

Source A Source B
m/m% Polars Mid-Polars Non-Polars Polars Mid-Polars Non-Polars
Element
Ca 1.760 0.001 0.127
Zn 1.380 0.114
V 0.494 0.003 0.553
Px 0.296
Fe 0.344 0.119
Ni 0.206 0.002 0.238
K 0.148
Si 0.068 0.011 0.002 0.040 0.713
Mg 0.076 0.024 0.013 0.360 0.028
Al 0.042 0.002 0.001 0.007
Mo 0.048
Cu 0.051
Ti 0.021
Pb 0.020
Sn 0.009 0.001
Cr 0.007
Na 0.080 0.040 0.080 0.038

Courtesy of the REOB/VTAE Report – Asphalt Insitute

Strieter Non-Polars

The Non-Polar fraction was 55% of the REOB/VTAE product and was paraffinic base oil with a molecular weight as determined by GPC of 925 daltons. The FTIR data shows this fraction to be primarily aliphatic hydrocarbons.

Strieter Mid-Polars

GPC determined the molecular weight of this fraction to be 1125 daltons. The primary components identified in the semi-volatile and volatile analyzes were organic acids, alcohols, aldehydes and aliphatic hydrocarbons. 40PAC at 123 mg/kg was detected as the quantitative results for the PAC analysis. The RI @ 25 °C is approximately 1.52. Esters were shown as the major functional group in the resin fraction by FTIR. Amides and acids are weak if present. There would be about 2% long chain fatty acids in the Mid Polar fraction, when the acid is calculated as stearic acid, based on a total acid number of 4.4 mg KOH/g. Some of the esters are expected from synthetic oils.

Strieter Polars

The Polars are in the 7500 Dalton range, based on GPC analysis. Zn, Fe, Ca, Ni, and V reside in this fraction. A majority of the Polar fraction are metal complexes from wear metals and additive packages.

Using XRF to Quantify the Metal Content

In 2010, discussion of  the discovery of waste engine oil residues in pavements across Ontario, Canada, was carried out by Shurvell and Hesp. It was concluded that, through X-ray fluorescence (XRF) analysis, recovered asphalts from a large majority of poorly performing contracts test positive for zinc. Xenemetrix X-Cite multi-element spectroscopic bench top analyzer was used to collect the XRF spectra. Since phosphates and zinc are universal additives in engine oils, it was inferred that the use of waste oil residues in asphalt must be extensive with characteristic modification levels in the 5 to 20% range.

Zinc Sulfate or Zinc Carbonate is however, sometimes added to scavenge hydrogen sulfide (H2S) produced both during the routine production of asphalt and also during the production of polymer-modified asphalt cements where a sulfur grafting reaction takes place. The Authors discovered that 12 of the 15 poorly performing contracts tested positive for the presence of considerable quantities of Zinc and hence were possibly contaminated with waste engine oil residues. An extra 2 of the 15 poorly performing contracts showed traces of Manganese and Zinc in the asphalt cement while no sign of zinc was observed in any of the 11 advanced performing contracts.

In the determination of performance, the sum of all cracking was included and the type of cracking was not deciphered. (6) Using the Fundamental Parameter program, this data was generated on a EDXRF system. The data is in m/m%. The metal content remains stable throughout the process to rejuvenate the motor oil and the metals originate chiefly from additives (Ca, P, Zn, Ba and Mo with other metals coming from wear metal debris during the use of the motor oil (Cu, Fe, Cr, Mn, Pb).

The sources refer to the diagram in Figure 2 below.

Figure 2. Calibration coefficients and resultant calibration lines.

EDXRF to Characterize Binders

As the metal values are in the 10s – 1000s of ppms and are readily monitored, the bench top EDXRF can be employed to characterize binders before blending them into asphalt cements. By using the internal ratio technique available on most EDXRF systems, Bitumen and Asphalt can also be characterized. Sample preparation comprises of transferring a constant mass to the cup to keep the mass of sample constant – the volume occupied would be a function of the density of the material.

Xenemetrix X-Cite Benchtop collected the data produced in this study with the help of Analytix software package which has both fundamental and empirical parameter processes for evaluating data. For using the de Jongh algorithm, matrix effects were compensated and the data acquisition was obtained in three segments, offering optimal excitation conditions for the measured metals. To determine calibration curves, synthetic crude oil standards with multi-element compositions that differed independently were employed and NIST reference standards were used to validate the technique.

Table 7. XRF measurements quantified using Xenemetrix X-Cite.

Source 1 2 3 4
Ca 0.81 1.19 1.17 1.16
P 0.40 0.47 0.51 0.44
Zn 0.39 0.63 0.54 0.45
S 0.51 0.21 0.24 1.26
Fe 0.11 0.13 0.28 0.14
Mg 0.08 0.09 0.10 0.07
Al 0.03 0.02 0.08 0.05
Mo 0.02 0.04 0.04 0.01
Na 0.25 0.19 0.21 0.19
Si 0.04 0.04 0.10 0.09
Cu 0.02 0.02 0.03 0.02
Pb <0.01 <0.01 <0.01 <0.01
Ba <0.01 <0.01 <0.01 <0.01
Ti <0.01 <0.01 <0.01 <0.01
Mn <0.01 <0.01 <0.01 <0.01

Figure 3. Xenemetrix Analytix software.

Table 8. Typical ranges of elements of interest to asphalt producers.

Elements Low Value High Value
S 0.1% 5.00%
P 0.005% 2.00%
Ca 0.005% 2.00%
V 0.0010% 0.100%
Fe 0.005% 0.200%
Cu 0.005% 0.050%
Zn 0.005% 1.00%
Mo 0.005% 0.050%

Validation Results on NIST Standards

To validate the method, two NIST standards were used.

Table 9. Comparison of Certified and measured values for NIST certified reference materials 1085-c and 1848.

SRM 1085-c Metals in Lubricating Oil Measured on X-cite SRM 1848 Lube Oil Additives Measured on the X-cite
Element mg/kg mg/kg except * m/m%
Aluminum 292
Aresenic
Barium 306
Boron 304 0.137*
Cadmium 301
Calcium 299 298 0.359* 0.355*
Chlorine (120) 927
Chromium 302
Cobalt
Copper 298 299
Hydrogen 12.3*
Iron 301 298
Lead 303
Magnesium 300 0.821*
Manganese 299
Molybdenum 305 301
Nickel 306
Nitrogen 0.57*
Phosphorus 304 299 0.788* 0.783*
Photassium 295
Silicon 293 50
Silver 298
Sodium 300
Sulfur 2.3270*
Tin 298
Titanium 300
Vanadium 285
Zinc 285 286 0.873* 0.860*

Summary

Using a very simple EDXRF technique, the monitoring of metals in asphalt additives and eventually in roadway runoff can be monitored. This EDXRF technique is robust as well as precise if correctly calibrated.

Because of a characterized EDXRF, it has become the ideal instrument for routine laboratory measurements; for multi-element capability with precise excitation conditions, the interelement effects are correctly assessed and compensated.

The carbon content of solid asphalt, which is aggregated and compacted into a briquette under substantial hydraulic pressure, is quantified by the EDXRF systems.

Conclusion

For characterizing the metal content in asphalt and asphalt related materials, XRF is a very cost effective method. Comparatively simple sample preparation steps and advanced matrix correction methods can lead to highly precise and accurate results. For any large scale XRF monitoring program, cross checking by ICP would be an ideal practice. Exceptionally well characterized samples that are products of round robin analysis that run through the Federal Highways Department can serve as check standards in XRF protocols.

References

1. The Effect of Asphalt Pavement on Stormwater Contamination - Andrew F. Nemeth Devon A. Ward Walter G.Woodington Worcester Polytechnic Institute May , 2010.

2. Int. J. Environ. Res. Public Health 2012, 9, 1715-1731; doi:10.3390/ijerph9051715.

3. The State of Knowledge Series – The use of REOB /VTAE in Asphalt – Asphalt Institute April 2016.

4. International Conference on Environmental Forensics 2015 (iENFORCE2015) , Concentration of heavy metals in virgin, used, recovered and waste oil: a spectroscopic study Munirah Abdul Zalia*,Wan Kamaruzaman Wan Ahmada, Ananthy Retnama, Ng Catrinab.

5. Journal of Egytian Petroleum , Characterization of bitumen samples from four deposits in southwest, Nigeria using trace metals M.C. Onojake *, Leo C. Osuji, C.O. Ndubuka August 2016.

6. X-ray Fluorescence Detection of Waste Engine Oil Residue In Asphalt and Its Effect on Cracking In Service” Simon A. M. Hesp and Herbert F. Shurvell International Journal of Pavement Engineering, Vol. 11, No. 6, Dec. 2010.

7. The Analysis of Asphalt Binders for Recycled Engine Oil Bottoms by X-Ray Fluorescence Spectroscopy” Terence S. Arnold CChem. and Anant Shastry Ph.D. Transportation ResearchBoard Compendium of Papers 2015.

8. Ryan Barborak, P.E., Quantifying Re-Refined Engine Oil Bottoms (REOB) In Asphalt – TxDOT’s Approach, Southeast Asphalt User Producer Group Meeting, Nov. 2015.

This information has been sourced, reviewed and adapted from materials provided by Xenemetrix Ltd.

For more information on this source, please visit Xenemetrix Ltd.

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