Chip Appel

Determination of Surface Heterogeneity and Enthalpies of Ca-K And Pb-K Systems in Tropical Soil Using Flow Adsorption Calorimetry

Chip Appel et al. — Tue, 12 Apr 2011 08:55:47 PDT

Selectivities of K/Ca and K/Pb Exchange in Two Tropical Soils

C. S. Appel et al. — Fri, 25 Feb 2011 13:50:55 PST

Measurement of cation selectivity in soils provides important information about the affinity and binding strength of a particular cation on soil surfaces. Gaines-Thomas (KGT) selectivity coefficients were determined for a variety of K/Ca and K/Pb ratios on an Oxisol and Ultisol soil from Puerto Rico. The calculated KGT values indicated a preference for K+ over Ca2+ or Pb2+. The selectivity for Pb2+ was significantly greater than that for Ca2+ due to the larger hydrated charge density of Pb2+ relative to that of Ca2+. The patterns of selectivity were independent of metal type. The selectivity of the Oxisol for Ca2+ or Pb2+ exhibited no trend/did not change with changes in divalent metal surface coverage indicating exchange sites had similar affinities for Ca2+ and Pb2+. The Ultisol displayed a decrease in selectivity for Ca2+ and Pb2+ with increasing surface coverage of these ions. This was attributed to the presence of smectite in the Ultisol which was able to partially collapse when K+ saturated. Some of the Pb sorption in the soils was due to chemisorption. The Oxisol chemisorbed ~ 3000 ppm Pb while that value for the Ultisol was ~ 1900 ppm. The differences were due to the greater quantities of Fe/Al oxides and organic matter in the Oxisol relative to the Ultisol. SEM-EDX spectroscopy detected discrete Pb-C phases in both soils. The C was from organic matter as PbCO3 was not stable under experimental conditions. It was possible Pb was associated with organic sulfhydral groups. The selectivity exhibited by soil systems for various nutrient and heavy metals is important in elucidating how available these metals will be for plant/animal uptake as well as their mobility and stability in the soil environment.

Understanding the Mechanisms of Lead, Copper, and Zinc Retention by Phosphate Rock

R. X. Cao et al. — Fri, 25 Feb 2011 13:50:54 PST

The solid-liquid interface reaction between phosphate rock (PR) and metals (Pb, Cu, and Zn)was studied. Phosphate rock has the highest affinity for Pb, followed by Cu and Zn, with sorption capacities of 131, 114, and 83.2 mmol kg-1 PR, respectively. In the Pb-Cu-Zn ternary system, competitive metal sorption occurred with sorption capacity reduction of 15.2%, 48.3%, and 75.6% for Pb, Cu, and Zn, respectively. A fractional factorial design showed the interfering effect in the order of Pb>Cu>Zn. Desorption of Cu and Zn was sensitive to pH change, increasing with pH decline, whereas Pb desorption was decreased with a strongly acidic TCLP extracting solution (pH=2.93). The greater stability of Pb retention by PR can be attributed to the fact that P-induced formation of fluoropyromorphite [Pb10(PO4)6F2] (i.e. irreversibly chemisorbed) was primarily responsible for Pb immobilization (up to 78.3%), which was confirmed by XRD and SEM. The other 21.7% of Pb retained by PR was via surface adsorption or complexation, compared to 74.5% for Cu and 96.7% for Zn. Solution pH reduction during metal retention and flow calorimetry analysis both supported the hypothesis of retention of Pb, Cu, and Zn by surface adsorption or complexation. Flow calorimetry indicated that Pb and Cu adsorption onto PR was exothermic, while Zn sorption was endothermic. Our research further demonstrated that PR could be used effectively to remove Pb from wastewater or remediate Pb-contaminated soil and sediments.

Effects of Composted and Raw Manure and Ammonium Sulfate Applications On the Chemical Properties of the Soils at Chorro Creek Ranch, San Luis Obispo, CA.

Renee Nall, et al. — Fri, 25 Feb 2011 13:50:53 PST

The majority of agricultural systems, especially large-scale, require inputs of nutrients. In addition, many large scale dairies produce vast quantities of waste that can be potentially recycled as nutrients. This study was conducted to determine the differences in properties of soils amended with manure, compost, and fertilizer, and to assess implications of using animal waste products for forage crop fertilization. Forage crops, wheat, barley, and oat mix have been grown on the Chorro Creek Ranch part of Cal Poly farmland, for 15 years. Ammonium sulfate fertilizer has been applied at a rate of 200 lbs/acre/year for the past 4 years. In 2008 two parcels were altered from the norm, and Cal Poly compost and dairy cattle were applied as nutrient sources instead of ammonium sulfate fertilizer. Manure was broadcast at a rate of 4000 lbs/acre and compost was broadcast at a rate of 400 lbs/acre. Nitrogen, phosphorus, potassium soil EC, pH, CEC and % OM were investigated on all 3 parcels. The organic amendments increased the EC and the pH. The ammonium sulfate decreased the EC, pH, nitrate and P, and increased the CEC. Though applied at the highest rate of 4000 lbs/acre, the manure was not significantly different from the control. Cal Poly has a large dairy unit and has had a history of waste management problems. We believe applications of manure on forage crops to be a good solution to this problem.

Analysis of Chromium and Nickel in Soil and Plant Tissue Collected from the Experimental Vineyard at California Polytechnic State University, San Luis Obispo, CA

Ian Leslie et al. — Fri, 25 Feb 2011 13:50:52 PST

Serpentinitic soils often contain high concentrations of heavy metals, including chromium (Cr) and nickel (Ni). Soils derived from serpentine parent material are used for agricultural purposes throughout San Luis Obispo County, CA. This study was undertaken to determine the amount of total and bioaccessible Cr and Ni in soil and vine tissue from a vineyard planted on soils derived from serpentine geology. Eighteen soil and plant samples were collected from different slope position and vine types (syrah and chardonnay). Soil samples were collected from about 0 to 30 cm at the base of the vines and 15 to 20 leaf with attached petiole samples were collected from one vine at each sample location. Soil and plant samples were extracted with 1:1 HNO₃ for total Cr and Ni concentrations using U.S. EPA method 3050a. Bioaccessible Cr and Ni concentrations from soil samples were determined by extracting samples twice with neutral 1 N NH₄C₂H₃O₂. Metal concentrations were determined using flame atomic absorption spectroscopy.
Mean total Cr and Ni concentrations from all soil samples were 49.26 and 42.43 mg kg^-1, respectively. Total mean Cr concentrations in vine tissue samples were below the method detection limit (MDL) for Cr, and total mean Ni concentrations in vine tissue were 2.75 mg kg^-1. Bioaccessible Cr and Ni concentrations from soil samples were below MDLs for each metal. The undetectable amounts of bioaccessible Cr and Ni indicate that they are most likely strongly sorbed to soil particles. The low heavy metal bioaccesiblility in this vineyard soil indicates that Cr and Ni poise little risk to vine health for toxicity considerations. Future research will evaluate Cr and Ni concentrations in other local agricultural soils derived from serpentinite.

Evaluation of Extent and Bioavailability of Chromium Contamination Near an Abandoned Strip Mine

C. Appel et al. — Fri, 25 Feb 2011 13:50:52 PST

Chromium is a potentially toxic element to plants and animals commonly associated with serpentine and anthropogenic sources. An abandoned Cr strip-mine at US National Guard Camp SLO, CA containing serpentinitic parent material was tested for total and bioavailable Cr in the soil and overlying vegetation via US EPA Methods 3050a (Total Cr) and 1311 (Total Characteristic Leaching Procedure, TCLP) (US EPA, 1995), respectively. The analysis of total and bioavailable Cr was used to assess the risk of nearby surface and groundwater contamination and to evaluate the site’s need for remediation. In addition, trends in soil Cr levels with respect to slope position and sampling transect were identified. Total and most bioavailable Cr concentrations in plants were below the method detection limits (TotalMDL = 0.12 mg L-1; TCLPMDL = 0.04 mg L-1), suggesting the Cr was immobile and not bioavailable. The average total Cr level was 403.5 +/- 177.6 mg kg-1, significantly below the background total Cr level of 829.4 +/- 213.6 mg kg-1. Based on Chromium’s immobility, and its low levels in the site relative to background concentrations, remediation practices were not recommended. No significant trends were observed in total soil Cr levels along any transect or between sampled slope positions. The information gained from this was used by Camp SLO officers to assess the threat of Cr mobility and bioavailability in the soils at this site, as well as in nearby waterways and ground water.

Sequential Extraction of Copper and Zinc from Two Californian Soils

Katharine Carr et al. — Fri, 25 Feb 2011 13:50:51 PST

In order to understand how to remediate soils contaminated with heavy metals, it is important to first understand their sorption mechanisms. The objective of this study was to evaluate the bioavailability of elevated levels of soil copper (Cu) and zinc (Zn) via sequential extraction. Elevated levels of Cu and Zn were added separately and concurrently to a Mollisol and an Entisol from the central coast of California. The exchangeable amount of Cu and Zn was then extracted from the sorbed amount via 0.1 M NaNO₃. The residual fraction was then extracted with 0.1 M sodium citrate (NaC₆O₇H₇) to determine if citrate removed any additional metals. Higher amounts of Cu (> 80 %) was sorbed than Zn (>64 %) for both soils. Cu was shown to inhibit Zn sorption to the Entisol, but not in the Mollisol, when the metals were added concurrently. Zn was more exchangeable in the Mollisol but less exchangeable than Cu in the Entisol. Citrate extracted significantly more Cu and Zn from the Mollisol than the Entisol. Both metals had a low bioavailability. Exchangeable and extractable Cu and Zn were 12-16 % and 3 – 6 % of the originally sorbed amount for the Mollisol and the Entisol. Citrate was able to desorb non-exchangeable Cu and Zn from both a Mollisol and an Entisol, thus, was an important extractant used in assessing the bioavailability of these metals.

Total and Bioavailable Chromium Along a Toposequence in San Luis Obispo, CA.

Britani Harris et al. — Fri, 25 Feb 2011 13:50:50 PST

The presence of large quantities of Chromium metal in soil and plants is of major concern due to its toxicity to humans. Total (USEPA 3050a) and bioavailable (USEPA 1311) levels of chromium were measured along a serpentinic Central Coast toposequence. Soil from the surface and subsurface, as well as the above ground plants of the toposequence were determined and compared to several soil chemical and physical properties using a least squares regression. The binding environments and oxidation states were found through XANES and EXAFS x-ray spectroscopies. The total and bioavailable chromium concentrations of the soil averaged 1457.1 and 1.6 mg Cr/kg respectively, and the average total plant chromium was 115.2 mg Cr/kg. The difference between chromium in the soil surface and subsurface was not statistically significant. The concentrations of total soil and plant chromium were highest in the backslope positions with a value of 2528.8 mg Cr/kg in the soil and 288.5 mg Cr/kg in the above ground plants. Though the backslope positions had slightly elevated levels of Cr, slope position did not have a large influence on soil and plant total chromium levels. The soil chemical and physical properties did not show a significant linear relationship with chromium levels. The dominant form of
chromium found in both the bulk-soil and plant samples was Cr³⁺, this agrees with the low amount of bioavailable chromium found at the site. Sepentinitic parent materials naturally lead to high chromium levels in soils and plants, so the ability to understand how chromium interacts with soil properties is crucial in the effective landscape management of
areas rich in this metal.

Establishment of Native Vegetation for Erosion Control on the Cal Poly, San Luis Obispo, Campus.

C Appel et al. — Fri, 25 Feb 2011 13:50:49 PST

When applied to a bare slope, vegetation and mulch will prevent further erosion and enhance the aesthetic appeal of the area, ultimately improving the quality of the soil. If nothing is done to this barren slope, its condition will continue to deteriorate. Core samples were taken at six different sites located on a highly degraded and barren slope on the Cal Poly campus. Bulk density, porosity, soil temperature, and nitrogen and phosphorous levels were measured prior to vegetating the slope. Thirty-three groundcover rose bushes were planted and drip irrigation was installed. Mulch was laid down and two un-official coarse-sand walking trails were constructed to direct foot traffic. After a period of three months, core samples were taken again at each of the six sites, and the measurements were repeated. We expected the condition of the slope to be much improved, with minimal erosion and increased aesthetic value. The bulk density decreased and porosity increased. Decomposition of the mulch and vegetative growth added organic matter to the soil. The benefits of such improvement to the soil will be enhanced water quality of the runoff that flows into the sewer drains and an educational opportunity for all who participate in the project and observe the site as they walk to class.

Cal Poly's ESWCC Teaches the Next Generation of Soil Scientists

M. Perry et al. — Fri, 25 Feb 2011 13:50:49 PST

Since 1992, members of Cal Poly San Luis Obispo’s Earth, Soil, and Water Conservation Club (ESWCC), sponsored by the MESA Agricultural Initiative, have volunteered their time and expertise to educate middle-school students in soil science. ESWCC members assemble soil testing kits containing three soils, pH testing materials, and instructions for analyzing soil pore space, water infiltration rates, and soil texture by “feel”. The kits help over 200 California teachers convey the importance of soil science through simple physical and chemical procedures that provide a deeper understanding of soil. ESWCC members train teachers to run classroom experiments and to relate soil properties to resource management. MESA students attend local and regional soil judging contests administered by the ESWCC. Contestants solve problems and measure soil pH, pore space, and soil texture for two unknown samples. Cooperation between the ESWCC and the MESA Ag Initiative benefits both Cal Poly and California middle-school students. Club members gain valuable experience teaching younger students. Middle-schoolers exposed to soil science gain valuable skills, are rewarded for their achievement, have positive interactions with college students, and discover opportunities afforded by attending college. Anyone interested should visit www.calpoly.edu/~agmesa/ for ideas about developing a similar program.

Determination of Total and Bioavailable Soil Lead from a Shooting Range in Central California.

Phil Roberts et al. — Fri, 25 Feb 2011 13:50:48 PST

Lead can pose a significant risk to environmental quality at and around shooting ranges due to its use in bullets and shot. The concentrations of Pb in soils, plants and surficial waters from a shooting range were determined in this study. Soil and plant samples were analyzed for total Pb (US EPA method 3050a) to determine the extent of Pb contamination. The toxicity characteristic leach procedure (TCLP; US EPA method 1311) was followed to ascertain bioavailable Pb. Soil samples ranged from 14.71 to 6346.15 mg Pb kg-1 soil with an average value of 1157.43 (±2000.57) mg Pb kg-1 soil across the shooting range. Plant samples ranged from 632.76 to 2896.00 mg Pb kg-1 plant with an average value of 1410.31 (±1287.11) mg Pb kg-1 plant, demonstrating significant Pb uptake. Bioavailable Pb was highest in the berm at 2038.00 mg Pb kg-1 soil. Sampling at depth showed Pb concentrations of 72.92 mg Pb kg-1 soil. When compared to surface samples (897.96 mg Pb kg-1), this shows some Pb is leaching through the profile. High Pb concentrations were detected in soil samples collected from the drainage (457.84 mg Pb kg-1), while low Pb levels were detected in the stormwater retention pond and sediments (0.11 mg Pb L-1 and 39.36 mg Pb kg-1 respectively). This indicates Pb is being transported through erosion of soil colloids. Elevated Pb levels from soil sampled in the drainage indicate most Pb present is attached to soil colloids and not free (Pb+2) to leach or runoff. Higher concentrations of Pb were detected in plant samples than extracted by TCLP bioavailable Pb estimations. This could present a problem for any pastoral activities and should come under further scrutiny.

Measuring Surface Chemical Properties of Soil Using Flow Calorimetry

R. D. Rhue et al. — Mon, 14 Feb 2011 14:47:30 PST

Flow calorimetry, which is ideally suited for measuring reactions occurring at the liquid/solid interface, has been used to study the surface chemistry of many types of solids, but little use of it has been made in the study of surface reactions of soils. The purpose of this study was to demonstrate the application of flow calorimetry to the study of two fundamental soil chemical processes, namely cation exchange and phosphate sorption. Surface horizon samples of a Typic Acrorthox and a Typic Tropohumult from Puerto Rico, a strong acid cation exchange resin (Dowex 50W-8), and an amorphous Al(OH)₃ were used. Heats for K/Ca exchange on the Dowex resin and the Oxisol, and K/Na exchange on the Ultisol, were consistent with literature values that were obtained using conventional batch calorimetry or derived from the temperature dependence of the exchange constant. Although peak areas associated with a given pair of exchange reactions were equal, peak shapes were generally not equivalent, indicating differences in the rate at which the two reactions occurred. For example, Ca displacing exchangeable K occurred more rapidly than the reverse reaction on the Dowex resin. The reaction of phosphate with the Ultisol and amorphous Al(OH)₃ was exothermic. Exposure of the soil to several cycles of phosphate was sufficient to saturate the sorption sites, as evidenced by the loss of a detectable heat signal. However, phosphate reactive sites were regenerated by flushing the column with a salt solution at pH 10. Precipitation of Al-phosphate was shown to be endothermic, confirming that precipitation was not the primary mechanism for phosphate sorption in this study. The results of this study show that flow calorimetry can provide valuable information about surface chemical reactions in soils that cannot be obtained readily by other methods.

Selectivities of Potassium-Calcium and Potassium-Lead Exchange in Two Tropical Soils

Chip Appel et al. — Mon, 14 Feb 2011 14:47:29 PST

Measurement of cation selectivity in soils provides important information about the affinity and binding strength of a particular cation on soil surfaces. Gaines-Thomas (K_GT) selectivity coefficients were determined for a variety of K/Ca and K/Pb ratios on an Oxisol and Ultisol soil from Puerto Rico. The calculated K_GT values indicated a preference for K⁺ over Ca²⁺ or Pb²⁺. The selectivity for Pb²⁺ was significantly greater than that for Ca²⁺ due to Pb²⁺'s larger hydrated charge density relative to that of Ca²⁺. The patterns of selectivity were independent of metal type. The selectivity of the Oxisol for Ca²⁺ or Pb²⁺ exhibited no trend and changed little with changes in divalent metal surface coverage. The Ultisol displayed a decrease in selectivity for Ca2+ and Pb2+ with increasing surface coverage of these ions. This was attributed to the presence of smectite in the Ultisol, which was able to partially collapse when K+ saturated. Some of the Pb sorption in the soils was due to chemisorption. The Oxisol chemisorbed 3000 mg Pb kg^-1 while that value for the Ultisol was ≈1900 mg kg^-1. The differences were due to the greater quantities of Fe/Al oxides and organic matter in the Oxisol relative to the Ultisol. Scanning electron microscopy-energy dispersive X-ray (SEM-EDX) spectroscopy detected discrete Pb-C phase in both soils. The C was from organic matter. Under experimental conditions, any Pb-carbonate phase would not have been stable. It was possible Pb was associated with organic sulfhydral groups. The selectivity exhibited by soil systems for various nutrient and heavy metals is important in elucidating how available these metals will be for plant/animal uptake as well as their mobility and stability in the soil environment.

Heats of K/Ca And K/Pb Exchange in Two Tropical Soils as Measured By Flow Calorimetry

Chip Appel et al. — Mon, 14 Feb 2011 14:47:28 PST

Flow calorimetry can provide useful information about surface chemical reactions in soils that cannot be obtained readily by other methods. When flow calorimetry is conducted over a range of surface coverages, different sorption heats can be calculated to yield information about how binding energies vary with coverage, i.e., surface heterogeneity. The purpose of this study was to determine heats of exchange for K/Ca and K/Pb systems using flow calorimetry and to evaluate the degree of surface heterogeneity with respect to cation exchange. Surface horizon samples from a Typic Acrorthox and Typic Tropohumult from Puerto Rico were used. Lead was adsorbed specifically in both soils, but no adsorption heat was detected for this reaction in either soil. However, heats associated with reversible cation exchange between K and Pb were observed. Heats for K/Ca exchange were greater than those generated for K/Pb exchange in both soils. Heats of exchange were greater in the Ultisol than in the Oxisol. The differential heats of exchange were independent of exchange composition for both K/Pb and K/Ca exchange in the Oxisol, indicating that all cation exchange sites were similar energetically. In the Ultisol, the differential heats of exchange increased as exchangeable K decreased, indicating that the exchange sites were not similar energetically. These differences were attributed to the presence of smectite in the Ultisol, which was able, in part, to collapse when saturated with K.

Concentration, pH, and Surface Charge Effects on Cadmium and Lead Sorption in Three Tropical Soils

Chip Appel et al. — Tue, 08 Feb 2011 15:27:26 PST

Reactions of heavy metals with soil are important in determining metal fates in the environment. Sorption characteristics of two heavy metals, Cd and Pb, in three tropical soils (Mollisol, Oxisol, and Ultisol) from Puerto Rico were assessed at varying metal concentrations (0 to 1.2 mM) and pH values (approximately 2 to 7). All soils sorbed more Pb than Cd. Sorption maxima were obtained for each metal for the Oxisol and Ultisol soils, but not the Mollisol. Sorption appeared to depend more on soil mineralogy than organic matter content. Sorption isotherms were linear within the sorption envelope with similar slopes for each soil-metal curve, when plotting metal sorption as a function of pH. Cadmium and Pb isotherms yielded average slopes of approximately 36+/-1 and 28+/-1 units (percent increase in metal sorption per 1-unit increase in pH), respectively. Metal sorption depended more on metal type than soil composition. Cadmium sorption displayed a greater pH dependence than Pb. Cadmium sorption was less than or equal to the amount of negative surface charge except at pH values greater than the point of zero net charge (PZNC). This suggests that Cd was probably sorbed via electrostatic surface reactions and/or possible inner-sphere complexation at pH > 3.7. However, the amount of Pb sorbed by the Oxisol was greater than the amount of negative surface charge, suggesting that Pb participates in inner-sphere surface reactions. Lead was sorbed more strongly than Cd in our soils and poses less of a threat to underlying ground water systems due to its lower mobility and availability.

Mechanisms of Lead, Copper, and Zinc Retention by Phosphate Rock

Xinde Cao et al. — Tue, 08 Feb 2011 15:27:25 PST

The solid–liquid interface reaction between phosphate rock (PR) and metals (Pb, Cu, and Zn) was studied. Phosphate rock has the highest affinity for Pb, followed by Cu and Zn, with sorption capacities of 138, 114, and 83.2 mmol/kg PR, respectively. In the Pb–Cu–Zn ternary system, competitive metal sorption occurred with sorption capacity reduction of 15.2%, 48.3%, and 75.6% for Pb, Cu, and Zn, respectively compared to the mono-metal systems. A fractional factorial design showed the interfering effect in the order of Pb > Cu > Zn. Desorption of Cu and Zn was sensitive to pH change, increasing with pH decline, whereas Pb desorption was decreased with a strongly acidic TCLP extracting solution (pH=2.93). The greatest stability of Pb retention by PR can be attributed to the formation of insoluble fluoropyromorphite [Pb10(PO4)6F2], which was primarily responsible for Pb immobilization (up to 78.3%), with less contribution from the surface adsorption or complexation (21.7%), compared to 74.5% for Cu and 95.7% for Zn. Solution pH reduction during metal retention and flow calorimetry analysis both supported the hypothesis of retention of Pb, Cu, and Zn by surface adsorption or complexation. Flow calorimetry indicated that Pb and Cu adsorption onto PR was exothermic, while Zn sorption was endothermic. Our research demonstrated that PR can effectively remove Pb from solutions, even in the presence of other heavy metals (e.g. Cu, Zn).

‘‘Capsule’’: Phosphate-induced formation of fluoropyromorphite is primarily responsible for Pb immobilization by phosphate rock, whereas Cu and Zn retention is mainly attributable to the surface adsorption or complexation.

Point Of Zero Charge Determination in Soils and Minerals via Traditional Methods and Detection of Electroacoustic Mobility

Chip Appel et al. — Tue, 08 Feb 2011 15:27:24 PST

Points of zero charge were determined on two highly weathered surface soils from Puerto Rico, an Oxisol and Ultisol, as well as mineral-standard kaolinite and synthetic goethite using three methods: (1) potentiometric titration measuring the adsorption of H+ and OH− on amphoteric surfaces in solutions of varying ionic strength (I) (point of zero salt effect), (2) direct assessment of surface charges via non-specific ion adsorption as a function of pH and I (point of zero net charge), and (3) electroacoustic mobility of reversible particles as it varies with pH and I (isoelectric point). The first two methods yielded points of zero charge for kaolinite (2.7–3.2) and synthetic goethite (7.4–8.2) comparable to those reported previously, indicating the reliability of these analyses. The soil values ranged from 3.9 to 4.4 for the Oxisol and 2.3 to 3.7 for the Ultisol. Electroacoustic mobility, as measured by the AcoustoSizer™, is a parameter that has yet to be thoroughly tested for mineral or soil systems as a viable alternative to PZC assessment. The points of zero charge from electroacoustic mobility of kaolinite (3.8–4.1) and synthetic goethite (8.1–8.2) were similar to values obtained by electrophoretic mobility. Furthermore, the values found for the Oxisol (3.4–3.5) and Ultisol (2.6–2.7) were in the range expected for these soils.

Sequential Sorption of Lead and Cadmium in Three Tropical Soils

Chip Appel et al. — Tue, 08 Feb 2011 15:27:22 PST

It is important to examine mechanisms of Pb and Cd sorption in soils to understand their bioavailability. The ability of three tropical soils to retain Pb, Cd, and Ca was evaluated. The objectives of this study were to (1) determine the extent to which soil sorption sites are metal specific, (2) investigate the nature of reactions between metals and soil surfaces, and (3) identify how metals compete for sorption sites when they are introduced to soils sequentially or concurrently. Lead was shown to be much less exchangeable than Cd and inhibited Cd sorption. Cadmium had little effect on Pb sorption, though both Ca and Cd inhibited the adsorption of Pb at exchange sites. Lead appears to more readily undergo inner-sphere surface complexation with soil surface functional groups than either Cd or Ca. Thus, regardless of when Pb is introduced to a soil, it should be less labile than Cd.

The sorption and desorption of Pb and Cd in tropical soils is strongly influenced by the order in which these metals enter the soil environment.