Chapter 2 Cation exchange capacity

Results

One objective of this study is to evaluate the cation exchange capacity (CEC) and the relative proportion of major exchangeable cations in Long Island glacial sediments. "The ion exchange capacity of a soil (or sediment) is the number of moles of sorbed ion charge that can be desorbed from unit mass, under given conditions of temperature, pressure, soil solution composition (including pH), and soil solution mass-ratio" (Sposito, 1994). Cation exchange capacity is a function of grain size, amount of organic matter, amount of coatings on the grains and mineralogy of the sorbing material. Different materials have different CEC (Table 1). Organic matter has the highest CEC (200-400 meq/100g). Iron compounds (goethite and hematite) also have high CEC up to 100 meq/100g. The CEC of clay minerals varies in a wide range: from kaolinite (CEC 3-15 meq/100g) to smectite (80-150 meq/100g).

Link to tables for chapter 2 data. This is a large file about 200 Kb.

The CEC of Long Island sediments in this study varies from 0.85 meq/100g to 21.4 meq/100g. Most of the grains consist of quartz. The quartz grains often have a coated surface consisting of clay, organic matter and iron oxides and hydroxides. The weight fraction of coating material relative to the whole sample varies from 0.8-13%.

Measured cation exchange capacity (CEC) values are shown in Table 3 and Figures 1 and 2. Grain size is an important factor in determining CEC. Sample 10, which is fine sand, has the highest CEC (21.3meq/100g). While sample 16, which is coarse sand to gravel, has the lowest CEC (0.85 meq/100g). This most likely is because fine sand has a larger surface area than does coarse material and also because fine sand includes a more significant fraction of clay minerals, which have quite high CEC (Table 1).


Fig. 1 Cation exchange capacity (CEC) of the core from Cathedral Pine County Park 


Fig. 2 Cation exchange capacity of the sediments from Port Jefferson (sample 1), South Setauket (samples 6 and 7), soil near Fox Pond and the beach cliff at David Weld Preserve 


 

For sample B, a sandy soil, CEC was measured on the total sample, on the coatings and on the naked grains (Table 2). Naked grains are grains that have had the coating removed by sonification. The coatings make up 13 weight percent of this sample. The CEC of the whole sample is 11.7 meq/100g. The CEC of the coatings is 78.3 meq/100g. The CEC of the naked grains is 0.29 meq/100g. A mass-balance calculation shows that the coatings contribute 87% of the CEC. We may conclude that coatings affect the exchange capacity significantly.

For both the core from Cathedral Pine County Park and the soil profile near Fox Pond CEC decreases along with organic matter concentration with depth.

The CEC for the samples from the core have the highest values at a depth of 0-30cm (18.3 meq/100g). This upper part of the soil horizon is enriched in organic material and clay. With increasing depth in the core CEC continually decreases. The sample from a depth of 150-180cm (sample 15) has a CEC of 1.32 meq/100g. This variation reflects both a change in grain size (from silt to coarse sand-gravel) and a decreasing fraction of organic matter with depth.

The soil near Fox Pond is sandy. The highest CEC was observed in the organic enriched A1 horizon (14.7 meq/100g). The lowest CEC was observed in the eluvial A2 horizon (9.20meq/100g). An intermediate CEC value was observed in the illuvial B horizon (horizon of oxides accumulation) (11.7 meq/100g). The illuvial B horizon is enriched with Fe and Al oxides and clay material. The A2 horizon may be depleted in cations because soil solutions with low pH were derived from acid rain or the decomposition of organic matter resulting in the formation of humic, fulvic and other organic acids. The acid-leached cations may then have accumulated in the illuvial B horizon.

The soil from the Cathedral Pines County Park is silty and is more enriched with organic matter compared to the Fox Pond soil. The CEC of the A1 organic horizon is 18.3 meq/100g. The eluvial depleted horizon is not as obvious in Cathedral Pine soil as it is in the Fox Pond soil.

The proportion of exchangeable cations as a function of depth is shown for the core from Cathedral Pines County Park in Table 4 and in Figure 1. The upper (0-30cm) organic soil horizon (sample 11) has the highest fraction of exchangeable Ca (8% of total CEC). At a depth of 90-120cm (sample14), the Ca dropped to less than detection (<0.002meq/100g).

The relative proportion of organic matter decreases with depth. Likens (1998) has shown that in New Hampshire the exchangeable Ca concentration in soils is directly related to the organic matter content of soil.

The concentration of exchangeable Al changes significantly with depth (Table 4, Figure 1). The Al in the sediments from 0-120cm makes up to 75 to 82% of the total CEC (Table 4). At depths greater than 150 cm the exchangeable Al rapidly decreases to less than detection (<0.0015 meq/100g).

High proportions of exchangeable Al were also observed in the upper 2 meters of the soil sediments from South Setauket and Port Jefferson (Table 4, Figure 2).

The percentage of exchangeable Na increases with depth from 7-10% at the top to 55-78% at the bottom of the sequence (Table 4, Figure 1).

The exchangeable Fe ion concentration is low (in most cases less than detection) along the profile (Table 4, Figure 1). This may be because most of the iron exists in immobile (Fe3+) form.

Actual and potential acidity

Actual and potential acidity were measured on the core from the Cathedral Pines County Park and for samples from Port Jefferson and South Setauket (Table 5).

Actual acidity is a measure of the hydronium ion concentration (pH) in a soil solution or in a soil-water suspension. Al and Fe occurring as amphoteric hydroxides react with water to release H30+ (Stumm and Morgan, 1996).

Al(H20)63+ + H20 = Al(H20)50H2++ H30+ (eq. 1)

Potential acidity is a measure of a sediments ability to produce exchangeable hydronium (H30+) ion by cation exchange with salt solutions, in this case 0.1M BaCl2, (Kovda and Rosanov, 1988).

Hydronium ion concentration in the soil for actual acidity was calculated based on the pH of the aqueous solution resulting from the mixing of deionized water with soil.

The hydronium ion concentration for the potential acidity was calculated based on the pH of a mixture of 0.1 molar BaCl2 and soil (Table 6). In the upper part of the sequence (0-120 cm) the concentration of H+ in the BaCl2 extraction (potential acidity) is higher than that in the H20 extraction (actual acidity). At greater depths (120-480 cm) the concentration of hydronium ion based on actual acidity (water extraction) is greater than that based on potential acidity (BaCl2 extraction) (Table 6).

In the upper 120cm of the profile Al ions make up to 82% of the exchangeable cations and the concentration of hydronium ions based on potential acidity is always higher than that for actual acidity. When the fraction of exchangeable Al drops to less than detection at 180cm the concentration of hydronium ion related to water extraction (actual acidity) exceeds that related to the salt extraction (potential acidity).

Therefore, correlation of hydronium concentration in the salt extraction (potential acidity) with distribution of exchangeable Al was observed. What causes the excess of hydronium ion in salt (BaCl2) extraction compared with water extraction in the upper 150 cm of the core? First of all, distilled water does not have cations, which will displace exchangeable cations including hydronium ion.

Ba2+ ion has a high sorption affinity (Stumm and Morgan, 1996). Possibly, the BaCl2 extraction removes mainly the exchangeable hydronium ion but another part originated from Al hydroxide dissociation. The exchangeable Al may form hydrated cations Al(H20)63+. As a result of the interaction with water these hydrated cations produce hydronium ions in the solutions (Stumm and Morgan, 1996; Kovda and Rosanov, 1988), (see equation 1).

Generally, the concentration of hydronium ion is higher in the upper part of the core compared to the lower part for both actual and potential acidity. The upper part of profile is enriched with organic matter. Organic acids and humic and fulvic acids may be responsible for higher H+ concentration in the top part of the core compared to the bottom (Kovda, Rosanov, 1988).

Discussion

In the core from the Cathedral Pine County Park Ca made up 8% of exchangeable ions in the upper 30 cm, at grater depth the proportion of Ca decreased significantly. Which factors control the Ca ion distribution along the profile?

Ca is a plant nutrient and its occurrence is directly related to the distribution of organic matter and root penetration (20% of Ca input is by root discharge) (Likens, 1998). Most Ca ion in Long Island sediments comes from dry precipitation (Xin, 1993, Lawrence 1998). Weathering of parent material is not a significant source for Ca in Long Island soils. Long Island sediments are typically sandy, consist mostly of quartz and contain insufficient amount of primary minerals, which can release Ca ion during weathering processes.

Also the soil depletion of Ca could be the result of leaching due to input of acid rain (Likens et al, 1998). The chemistry of bulk precipitation and stream water has been measured continuously at Hubbard Brook Experimental Forest (HBEF) (Likens, 1996). Likens, (1996, 1998) and Finzi, (1998) have shown that the large decline of Ca in both precipitation streamwater, and the depletion in the Ca in the soil since 1963. According to Likens the net biomass storage of Ca in the HBEF decreased from 202 to 54 mol/ha-yr in 1964-69 and 1987-92 respectively. Probably, the low fraction of Ca in soil complex of Long Island sediments reflects both the decline of Ca in dry precipitation (the main source of base cations on Long Island) and leaching Ca away from the soil profile due to effect of acid rain (Likens, 1996,1998).

Why is Al ion the main exchangeable cation (up to 82% of total CEC) within 120 cm of the surface? What is the explanation for such high proportion of Al in an upper part of the sequence? According to Matzner (1998) even in the soil with low carbon content Al bounded with organic matter is the major source of Al in soil solution. This may explain high concentration of Al in the upper part of the sequence. The upper part is enriched with organic matter, with depth concentration of organic matter decreases the exchangeable Al concentration does the same. Another possibility is that Al enrichment combined with Ca depletion is an effect of acid rain. Acid rain has an average pH of 4.05-4.3 in Long Island (Likens, 1996). When rainwater with such low pH drains into the soil it dissolves minerals, such as, gibbsite, which allows free Al ions appear in the soil solution (Stumm and Morgan, 1996). The base cations in soil exchangeable complex are replaced by the positively charged hydrogen and Al ions from the soil solution. As long as the soil has an abundant supply of base cations this buffering system, known as Cation Exchange Capacity, protects the soil from acid rain. The natural reserve of cations becomes depleted if soil is exposed to acid rain for too long time. As result soil becomes enriched with Al and H ions. Another possibility is, that organic acids formed during leaf and needle decomposition, may create acidic conditions in the soil profile. This leads to the mineral dissolution and the release of mobile Al. Possibly, both processes explain high fraction of Al ion in exchangeable complex of Long Island sediments but we do not have enough information to distinguish between them. Other studies also show high Al fraction in soil exchangeable complex in different region of northeastern USA (Larsen, 1998; Likens 1996, 1998).

Some of Long Island sediments have high CEC (up to 21.3 meq/100g). Results, reported by Lani (1996), show even higher CEC for some Long Island sediments. The fine-coarse sand from upper glacial has CEC up to 60 meq/100g and fine-medium sand from Magothy formation has CEC 70 meq/100g. But sediments are sandy and mostly composed of quartz. What is the major source of exchangeable cations? This study indicates that coatings on the sand surface mainly responsible for the cation exchange. Coatings made up to 13 % of sediments. The coatings consist of illite, kaolinite and chlorite with minor amount of organic material (TOC is up to 0.7 %) and iron compounds. Combination of these three components hardly can show such high CEC. Illite, kaolinite and chlorite do not have high CEC and only organic matter and iron compounds have high CEC (Table 1), but their fraction in the coatings is not large enough to give such high CEC for sand. To answer on this question the wider variety of the sediments need to be examined.

We identified that CEC of Long Island sediments varies from 0.85 to 21.3 meq/100g. Coatings are a major sorbent for exchangeable cations. The soil complex in the top part enriched with Al ion (up to 82%) and contains minor amount of Ca (up to 8%). With depth amount Al and Ca drops. Concentration of H ion in both water and BaCl2 extractions are higher in the upper part of sequence. Concentration of hydrogen ion in BaCl2 extraction correlates with exchangeable Al concentration along the core.

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