| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
a USDA-ARS, Grazinglands Research Lab., 7207 W. Cheyenne St., El Reno, OK 73036
b Dep. of Chemistry, Purdue Univ., West Lafayette, IN 47907
c USDA-ARS, National Soil Erosion Research Lab., Purdue Univ., West Lafayette, IN 47907
* Corresponding author (jzhang{at}grl.ars.usda.gov)
 |
ABSTRACT |
|---|
 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
|---|
Abbreviations: ARE, average relative error • D50, particle size at which 50% of particles by weight are finer than that value • DW, deionized water • ICP-MS, inductively coupled plasma-mass spectrometry • REE, rare earth element
|
INTRODUCTION |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
|---|
To obtain spatially-distributed data, various types of tracers have been developed and used. Most of them involve atomic bomb fallout radionuclide 137Cs (Brown et al., 1981; Ritchie and McHenry, 1990; Martz and De Jong, 1991; Walling and He, 1999), naturally occurring radionuclides such as 210Pb, 7Be, and 234Th (Wallbrink and Murray, 1993), deliberately introduced radionuclides such as 56Fe and 60Co (Wooldridge, 1965; Toth and Alderfer, 1960), noble metals-labeled natural particles (Olmez et al, 1994), exotic particles including glass beads, fluorescent dye coated particles (Young and Holt, 1968), magnetic plastic beads (Ventura et al., 2001), steel nuts (Lindstrom et al., 1992), and rare earth element oxides (Riebe, 1995; Plante et al., 1999; Tian et al., 1994; Institute of Soil and Water Conservation, 1997). Although these tracers have proven useful, each has limitations.
Bomb-produced 137Cs (half-life of 30 yr) is one of the most widely used tracers for estimating long term soil erosion rates (normally across 30 yr) and erosion patterns across a landscape. However, this method is hampered by inconsistencies and uncertainties in providing quantitative estimates due to various assumptions stemming from indeterminate input functions and indefinite spatial and depth distributions of the original deposition at the location of interest. Estimates of soil erosion rates associated with a particular level of 137Cs depletion can vary by more than one order of magnitude, using different calibration relationships derived under different assumptions (Walling and Quine, 1990). Moreover, since a uniform spatial distribution of 137Cs is always assumed, this tracer provides little indication regarding sediment sources.
Cosmogenic radionuclides such as 210Pb (half-life of 20.2 yr) and 7Be (half-life of 53 d) are naturally formed in the atmosphere and are brought to the earth's surface as precipitation. Due to differential depth distributions between 210Pb and 7Be, these tracers, when used in conjunction, have a potential for identifying the predominant erosion processes such as interrill or rill erosion (Wallbrink and Murray, 1993). However, because of uncertainties associated with the continuous input functions, as well as their spatial and depth distributions, these tracers, similar to 137Cs, necessitate similar assumptions and therefore suffer from similar drawbacks.
Unlike bomb-produced and naturally occurring radionuclides, radioactive ions can be manipulated and deliberately introduced for studying soil erosion. For example, Wooldridge (1965) sprayed 56Fe solution to the soil surfaces to study soil translocation. Toth and Alderfer (1960) tagged soil aggregates with 60Co solution for studying soil aggregation. A major concern with this type of tracer is the radiological risks to both researchers and the environment. Exotic particles such as fluorescent glass beads and magnetic plastic beads have been used to overcome this concern, but pose new problems. The major criticisms for the exotic particles are that (i) exotic particles differ in size distribution, particle density, shape, surface morphology, and surface chemical properties from ambient soil particles and aggregates; (ii) these particles may not bind with soil particles and soil aggregates, and therefore are transported independently; and (iii) sensitivity of quantitative analysis is normally low, which means that a large quantity of tracers are needed to trace soil and sediment movement in a sizeable area such as in a small watershed.
Labeling natural particles of interest with noble metals (e.g., Au, Ag, In, Ir), which can be activated by neutron irradiation only during measurements, circumvents most of the drawbacks discussed above. Olmez et al. (1994) labeled natural sediment particles with Au and Ag by heating Au- or Ag-treated sediment at a high temperature. The heating allows the labeling elements to diffuse into sediment crystalline lattice. However, this procedure may not be applicable to soils because excessive heating would dramatically alter soil chemical compositions, especially clay and organic matter fractions. Moreover, labeling procedures can be tedious and time consuming if a large quantity of tracers is needed.
Ideal tracers for studying soil erosion and sediment sources should overcome the drawbacks discussed above and possess the following properties: be strongly bound with soil particles or easily incorporated into soil aggregates, have great sensitivity in analyses, be easy and inexpensive to measure, possess low background concentration in soils, not interfere with sediment transport, have low plant uptake, be environmentally benign, and have availability of multiple tracers that are similar in physicochemical properties but are distinct in signature.
The Lanthanide elements, defined as REEs of atomic number 57 through 71 with similar chemical properties, are good candidates for use in sediment source studies. Rare earth oxides, which are insoluble in water, have been tested and used in a few pioneer studies. Tian et al. (1994) tagged soils at different slope positions with different rare earth oxides by directly mixing REE oxide powders with soils to track sediment sources and movement. Their results were satisfactory and the method was promising. Riebe (1995) used europium-labeled glass particles to monitor soil translocation. Europium oxide was fused with glass and other ingredients at 1100°C for 5 h to produce a homogenous material that had a particle density of 2.65 Mg m-3. The material was further ground into sand- and silt-size fractions, and was then mixed with bulk soils. Plante et al. (1999) developed a particulate tracer by incorporating dysprosium oxide into various-size ceramic prills during prill manufacture. The use of ceramic prills was based on the assumption that surface properties of the prills would be similar to those of soil aggregates.
Availability of a sensitive, rapid, and inexpensive analysis method is another concern for selecting ideal tracers. Traditional instrumental methods of analysis include instrumental neutron activation analysis, x-ray fluorescence, atomic absorption spectrometry, and inductively coupled plasma atomic emission spectrometry. Instrumental neutron activation analysis was widely used for REE measurement for many years. However, it is relatively insensitive to the middle mass REE and is expensive to implement due to the requirement of a nuclear reactor. X-ray fluorescence is another widely used technique, but the sensitivity for REE is relatively poor (detection limits >1 µg g-1). Atomic absorption spectrometry has not been widely used for REE measurement because (i) it is less sensitive to the REE than the light elements and (ii) it lacks in multi-element capability. Inductively coupled plasma atomic emission spectrometry is now widely used for measuring REE because of its low detection limits, but it can require extensive sample pretreatments, including ionic separation.
Inductively coupled plasma-mass spectrometry is a relatively new technique that overcomes many of the above limitations. Jarvis (1989) identified the following advantages: exceptional detection limits (in the range of ng kg-1 to µg kg-1), uniform sensitivity across the mass range, simple spectra, wide linear dynamic range (
6 orders), simultaneous measurement of all REE elements, rapid analysis time (minutes per sample), and no need for chemical separation. A disadvantage of ICP-MS is the requirement to introduce the sample as an aqueous solution. Conventional sample dissolution procedures are time-consuming, which can substantially limit sample throughput. If rapid acid-leaching of the sample can be done, the high throughput of ICP-MS would be better utilized.
One advantage of direct mixing of REE oxide powders with soils is to eliminate the need of manufacturing REE-tagged exotic particles. Although Tian et al. (1994) and his colleagues (Institute of Soil and Water Conservation, 1997) have conducted some pioneer research in this aspect, the feasibility of direct use of REE oxide powders as tracers for erosion studies has not yet been fully examined. There is little information in the literature regarding the binding ability between REE oxide powders and soil particles or aggregates, which is the most important criterion in selecting ideal tracers for erosion studies. The objectives of this study were to evaluate the feasibility of direct use of REE oxide powders as tracers for soil erosion studies by examining their binding abilities with soil particles and the mobility in a soil matrix, and also to develop a quick method for sample extraction for ICP-MS analysis.
|
MATERIALS AND METHODS |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
|---|
13% sand, 18% clay, 69% silt, and 1.3% organic carbon. Clay mineralogy consisted in a decreasing order illite, kaolinite, vermiculite, and smectite. Soil pH in water and 0.01 M CaCl2 solution (1:1 solid:water ratio) was 5.7 and 5.24, respectively (Franzmeier et al., 1977). Particle size distribution was measured with a pipette method (Franzmeier et al., 1977). Organic C was measured by a dry combustion method (model CHN-60, Leco Corp., St Joseph, MI).
Rare Earth Oxide Characteristics
Five REE oxides (La2O3, Pr6O11, Nd2O3, Sm2O3, and Gd2O3), selected mainly on the basis of their costs, were imported from the People's Republic of China. Price ranged from 4 to 25 US dollars per kilogram of oxides, depending on element and purity. Selected physical and chemical properties of REE oxides were given in Table 1. The particle density ranged from 6.5 to 7.7 Mg m-3.
|
4 h for the 1-µm size. All measurements were conducted at room temperature (25°C). A logistic function was fitted to cumulative particle size distribution curves. The function takes the form of:
 | (1) |
Sample Preparation and Leaching
Ten kilograms of air-dried Miami soil, which was sieved though a 6-mm sieve, was prewetted with DW to 15% water content. To ensure complete mixing, soil and REE oxides were mixed by serial dilutions. Predetermined amounts of REE were first mixed with 1 kg of Miami soil. Additional blank soil was sequentially introduced and mixed completely. Expected REE concentrations resulted from REE addition were 177, 196, 90, 102, and 102 µg g-1 for La, Pr, Nd, Sm, and Gd, respectively. Following 2 d of incubation at a temperature of 25 °C, 25 mm of the REE-tagged soil was packed over a 150-mm layer of air-dried, untreated Miami soil in a 450 by 350 mm box, separated by a Nylon window screen. The soil was leached progressively with 20 liters of DW (130-mm depth of water) in a period of 40 d. Two liters of DW were slowly poured into the soil box approximately every 4 d. During leaching, the box was covered with a plastic sheet to prevent evaporation, and leachate was collected from the bottom of the soil box.
Following leaching, the tagged soil layer was removed at field water capacity, air-dried, and wet-sieved to determine REE concentration in each size class. The soil below the tagged layer was sampled in 25-mm depth increments, and air-dried, sieved (<2 mm), and analyzed for REE concentration.
Wet-Sieving
A wet-sieving machine similar to Yoder's device (1936) with a displacement of 38 mm and a frequency of 36 cycles min-1 was used to obtain aggregate size distributions. After passing through an 8-mm sieve, 580 g of air-dried, leached REE-soil was prewetted with DW under 0.5 J kg-1 suction on a 0.033 MPa suction plate for >6 h. Following wetting, the tagged soil was sieved in DW for 25 min using a series of five 127-mm sieves (4.75, 2, 1, 0.5, and 0.21 mm). After sieving, the materials <0.21 mm were sequentially sieved through 53-, 38-, 25-, and 10-µm sieves to obtain additional size classes. These materials were oven-dried and analyzed for REE concentration.
Acid-Extraction of Rare Earth Elements
A simple, quick acid leaching procedure, similar to a standard method used by the USEPA for extractions of metals from environmental samples (USEPA, 1995), was proposed. The procedure includes: (i) weigh 2 g of soil sample into a 50-mL Erlenmeyer flask; (ii) add 10 mL concentrated HNO3 (70%, w/w), and reflux at 85 °C in a water bath for 1 h; (iii) after cooling to <70°C, add 10 mL H2O2 (30%) slowly to remove organically-bound REEs, and heat the solution for 2 to 3 min after effervescence subsided; (iv) add 5-mL concentrated HCl (36%, w/w), and reflux at 85°C in water bath for 1 h; (v) after cooling to room temperature, filter through a No. 5 Whatman (Clifton, NJ) filter paper, and elute with 5 mL DW (18 M
cm-1); (vi) filter through a 0.45 µm membrane.
Inductively Coupled Plasma-Mass Spectrometry Sample and Standard Preparations
A 250-mg aliquot of each extract was transferred to a 50-mL polyethylene centrifuge tube and massed exactly. Stock internal standard solution containing In and Tl was added to the tube, and the volume was brought to 50 mL with 18 M cm-1 water. Final internal standard concentration was 10 ng g-1 for all samples and standards. Total acid concentration of final analyzed solution was 1.5%.
External calibration standards were prepared from a certified stock REE solution (SPEX Sample Preparation, Metuchen, NJ; 10 µg g-1) by serial dilutions in concentrations of 0, 10, 20, 50, and 80 ng g-1. Calibration standards were matched in acid concentration (1.5%) and composition (HCl and HNO3) to a typical sample. With these standards, all sample concentrations fell within the calibration range. Three replicates of each dilution were prepared for each sample, and each dilution sample was analyzed once using ICP-MS technology (Table 2).
|
|
RESULTS AND DISCUSSION |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
|---|
99% of REE oxides by weight being smaller than 10 µm. The Pr oxide had the widest and coarsest particle size distribution (Fig. 1A), ranging from 2 to 25 µm, with 90% by weight being larger than 5 µm.
|
86% of the total mass, while the fraction <25 µm accounted for <5%.
|
92% of the total soil mass.
|
30% less than the whole soil. This trend was consistent for all five REEs. We do not have a satisfactory explanation for these results. The REE concentrations increased as particle and microaggregate sizes decreased (<38 µm) and peaked at the 10- to 25-µm fraction, which were 30% greater than the whole soil for all REEs. This indicated that the degree of oxide-particle association apparently increased as the size decreased from 53 to 10 µm. Apparently, for the <10-µm group, concentrations were lower than those of the 10- to 25-µm group. Since 99% of all REEs excluding Pr by weight were smaller than 10 µm, we do not have information on quantity of REE oxides that were actually bound with primary particles or microaggregates in this fraction. However, this fraction accounted for only 0.24% of the total soil mass. If the REE oxides had not bound with soil materials, the REE concentrations in this fraction would have been at least 400 times that of the whole soil. Overall, results demonstrated that REE oxides were uniformly incorporated into soil aggregates that were larger than 53 µm, and the oxides were bound well with silt-size particles or aggregates, except for sizes between 38 and 53 µm. Concentration-depth profiles of REE oxides after leaching with 130 mm of DW are presented in Table 3. The REE-tagged soil was packed in the top 25-mm layer. The REE concentrations below the tagged soil layer (>25 mm in depth) were not significantly different from the background soil values. There was no apparent trend of REE movement within the soil profile. However, average concentrations of the five REEs in the leachate (Table 3) were higher than those in DW, which were <0.005 ng mL-1 for all REEs. This result suggested that REE oxides were slightly soluble at pH 5 to 6 conditions, under which this experiment was conducted. It also implies that REE mobility may increase with an increase in soil acidity. Thus, cumulative downward leaching of REEs could be significant across long periods of acidic conditions. Leaching or redistribution of REEs could cause a problem for tracing soil erosion in soils with very low pH.
|
|
Average percentage of recoveries along with SDs and CVs are shown in Table 5. Recovery rates were satisfactorily high (>91%) for La, Pr, Nd, and Gd, and somewhat low for Sm (83%). Reasons for the low recovery of Sm are unknown at this stage. More importantly, the SDs and CVs of the recovery rates were between 5 to 8% for all REEs except for Nd, whose values were around 10%. This showed that the inter-sample variations of the proposed extraction method were small, and the method was able to produce a coherent recovery rate across samples. Actually, the magnitude of percentage recovery is not a critical factor; as long as the percentage recovery is consistent, actual amounts of REEs in soils or sediments can be accurately estimated by considering the recovery rate for a particular soil in question.
|
Aggregate size distribution of eroded sediment from runoff plots depends on factors such as soil properties (e.g., soil texture and aggregate stability), rainfall and runoff characteristics, topography, surface cover and roughness, time, and predominant erosion processes (Nearing et al., 2001). However, the general consensus is that most sediment from either interrill or rill areas on a hillslope were eroded and transported in aggregates (Alberts et al., 1980; Gabriels and Moldenhauer, 1978; Meyer et al., 1980). Meyer et al. (1980) also reported that particle size distribution of eroded sediment from interrill area resembled those of dispersed surface soil from which sediment was eroded. These results indicated that the proportion of particles that made up soil aggregates was similar to that of ambient soil. Based on these findings, REE oxide powders, which can be easily incorporated into soil by tillage operations such as disks, may be ideal tracers for studying upland erosion and sediment transport dynamics along a hillslope or within a small watershed where sediments are transported mainly in aggregates. Tian et al. (1994) and his coworkers (Institute of Soil and Water Conservation, 1997) have conducted several laboratory and field test experiments using REE oxides to trace soil erosion. Their results indicate that the REE oxide tracing technique is promising and has a great potential of studying soil erosion mechanisms and sediment transport processes.
For soil aggregation and translocation studies, Plante et al. (1999) developed a sphere tracer by tagging ceramic prills with Dy2O3, and Riebe (1995) produced a particle tracer by fusing Eu2O3 with glass. Although satisfactory results were achieved by those researchers, major concerns of using exotic particle tracers for soil erosion and aggregation studies remain as the particle tracers should posses similar particle density, size distribution, and surface reactivity to those of soil aggregates or objects of interests. This work has shown that the direct use of REE oxide powder is feasible and will eliminate the need of artificially reproducing the size and density distribution of the soil. Since REE oxide powder is very fine and only a trace amount is needed for tagging soil aggregates, it will not substantially alter the aggregate density and surface reactivity. Moreover, the aggregate size distributions will be the same as those of the soil matrix and the eroded sediment. This is critical, especially for studying upland soil erosion by water in which sediments are eroded and transported in aggregates and those aggregates are in fact varying dynamically in response to detachment and transport processes, management operations, and other changes in natural conditions.
|
CONCLUSIONS |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
|---|
The REE oxide powders when directly mixed with soil were uniformly incorporated into various-size soil aggregates (>53 µm), and were bound with soil particles or aggregates. Since soil aggregates always change dynamically with time, the direct use of REE oxides has the potential to mimic the real time aggregate size distribution of soil in question. This finding has shown that the direct use of REE oxides is not only feasible, but also superior to REE-tagged particulate tracers. This is because trace amounts of REE oxides introduced would not alter physicochemical properties of soil aggregates such as density, size, shape, and surface reactivity, and it will also eliminate the problems in manufacturing artificial REE-tagged particle tracers.
Lanthanide oxides may be ideal tracers for studying soil erosion and aggregation because of their strong binding ability, low mobility, great sensitivity in analysis, quick and inexpensive determination, low backgrounds in soils, non-hazard to environment, and availability of multiple tracers that are similar in properties but distinct in signature. This work has shown that further tests of these REE tracers at plot and small watershed scales are desirable. Overall results indicate that REE tracers have a great potential for tracing soil erosion and aggregation in nonacidic moderate and heavy texture soils. However, the feasibility for tracing sandy soils needs to be further explored because of low aggregate-forming abilities.
Received for publication November 21, 2000.
|
REFERENCES |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
|---|
This article has been cited by other articles:
|
|
 |
|
  K. S. Black, S. Athey, P. Wilson, and D. Evans The use of particle tracking in sediment transport studies: a review Geological Society, London, Special Publications, January 1, 2007; 274(1): 73 - 91. [Abstract] [PDF]
|
|
|
|
 |
|
 X.-C. Zhang, Z.-B. Li, and W.-F. Ding Validation of WEPP Sediment Feedback Relationships using Spatially Distributed Rill Erosion Data Soil Sci. Soc. Am. J., August 4, 2005; 69(5): 1440 - 1447. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
X. C. Zhang, M. A. Nearing, V. O. Polyakov, and J. M. Friedrich Using Rare-Earth Oxide Tracers for Studying Soil Erosion Dynamics Soil Sci. Soc. Am. J., January 1, 2003; 67(1): 279 - 288. [Abstract] [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
| The SCI Journals | Agronomy Journal | Crop Science | |||
| Journal of Natural Resources and Life Sciences Education |
Vadose Zone Journal | ||||
| Journal of Plant Registrations | Journal of Environmental Quality |
The Plant Genome | |||
