The Effect of pH and Chloride Ion Concentration on the Mobilities of Various Cations in Soil Melody Brown, Mary Sutherlancl, and Stephen Lehame Division of Geographical and Environmental Studies. Thames Polytechnic, Bexley Road, London, SE9 2PQ, England In recent vears much environmental concern has been focused on the contamination of soils by toxic metals. Metal contamination of soils may arise in a number of ways. For instance, synthetic fertilizers and sewage sludges may contain varying quantities of metals. The application of these materials to soils inevitably gives rise to contamination. Soils also become contaminated by atmospheric pollutants. For instance. lead oarticulates in exhaust emissions are deposited on roadside soils and vegetation. Coal combustion too eives rise to atmosoheric emissions of metals in flv ash thaiare subsequently deposited in the surrounding area. Once in the soil it is of interest to know how easilv the dt-purited metali are retained by or leached from the-soils and what lactvrs will influence their availability to growing vcytwtion. Hnrriwn nnd Laxcn ( 1 ) point out that lead uptake by vegetation from soils is highly variable and is govemrd I n the md~ilitvoi the cntim in the soil. 'The md~ilitv of a cation in a soil may be thought of as being some function of its solution concentration as it is affected by the movement of water through the soil profile (2). l h u s any factor that is likely to increase solubility is also likely to affect cation movement. Furthermore, any factors that increase the mobilitv of cations mav ~ossiblvenhance cation availability to &owing vegetation-and increase the likelihood of leaching to local rivers. This in turn mav lead to a varietv of polluti& problems. A number of workers (3-5) have investigated the effects of several factors on cation mobility. In all cases their work tends to show that low pH values, low soil organic matter content, and the presence of ligands like chloride ions may increase the mobility of a cation through a soil. A measure of the mobilities of cations in soils may be ascertained quite easily by the use of soil TLC (3, 4). This technique basically consists of usiugaTLC plate coated with the soil of interest. Solutions of the cations are then applied to the plate, which is then placed in a tank containing water or a solution of suitable pH or ligand concentration. The plate is then developed in the usual way. High cation mobility is demonstrated by a high Rf value. The object of this note is to outline a student experiment, using this technique, which demonstrates the effect of pH and chloride ion concentration on the mobility of several cations in soils. Experimental For this seriesof experiments aslit loam soil was used. The soil sample was taken from the too 25 cm of a broadleaf woodland near Westerham in ~ e n t . . The sample was dried, ground in a mortar and pestle, and finally passed through a 100-mesh sieve. A slurry (105 g soil and 150 g water) was coated on glass plates (20 cm X 20 cm) to a thickness of 0.5 mm using a TLC applicator. The plates were then air dried at room temperature. Two lines were scribed a t 3 cm and 13 cm above the base of the plate. Solutions (0.1 M) of lead, copper, zinc, and cobalt nitrates and cadmium and nickel sulfates were prepared using GPRgrade materials and deionized water. These solutions were spotted on the 3-cm line using a drawn capillary tube. The plates were then placed in closed tanks and developed by 448
Journal of Chemical Education
ascending chromatography using either deionized water or suitably emended solutions. Nitric acid was used to adjust the pH, and sodium chloride was used to produce the chloride ion solutions. Once the solvent front reached the 13-cm line the plates were removed. The cations were located with suitahle locat- - -~~~ ~ ing agents (see Table 1) followed by exposure to ammonia fumes. In this work as in previously reported work ( 4 ) the spots showed excessive tailing. Hence the frontal Rr value, (frontal distance moved bv the cation)/(distance moved by the solvent), was measured. ~
~
~
Results and Discussion The results in Table 2 show fairly clearly that as the pH is I t ~ r t r e dthere is an increase in N i values for all the cations except lead. This increase would lead torheexpectation that in soil systems increases in the acidity of percolating waters would lead to increases in cation mobility. This is in accord with the work of Tyler (5). The reason for this increased mobility is possibly due to increased competition between hvdroeen ions and the metal cations for cation exchange " sites on clay/humus materials. The preferential absorption of hydrogen ions (3) means that the metal cations if soluble in the developing solution are likely to be more mobile. The low RFvalues for lead under both sets of conditions demonstrates that lead is fairly immobile, and this possibly explains its very long residence time in soils. I t is possible that this immobility is explained by the formation of insoluble humic acidfiead complexes (5).
Table 1. Detalls oi Locating Agents Used Cations indicated Pb Cu Cd
Spat
Locating agent (concentrations)
color red blue
orange Rubeanic acid in ethanol (0.1 gin 100 mL) fluorescem yellow
Zn
Oxine in 80120 ethanollwater (0.5 gin 100 mL)
Table 2. The Effect of pH on Catlon Moblllty (Values Based on Replicate Readings) R, value Cation
OH= 7
DH = 4.7
Table 3.
The Effect of Chloride Ion Concentration on Cdtf Ion Moblllty Concentration (M)
R,
0.00 0.08
0.25 0.30 0.38 0.47
0.08
0.10
demonstrates that as the &loride ion con~ ~3 b l ~ centration increases there is an increase in the mobility of ions. This increase in the mobility has been attrib-
uted to the formation of stable chloro-complexes (6), which significantly increase the solubility (7)of cadmium ions. The resultant increased affinity of the complex ions for the mobility phase must explain the increased mobility. It has also been pointed out (6) that the increasing ionic strength of the eluant also plays a role in this increased mobility. Literature Cited 1. Hnrriron.R.M.:Laren.D.P.H.L~odPaliu~inn:Coures.Eff~ds.ndConfr"l:Chapman Hall: I.ondon. 1981:p60. 2. Hodgann, J. F.Adu. zn Apron. 1963.15.119. % Khan. S.: Nandan, D.: Khsn, N.N. Enuiron. Pollui. Ser. B. 1982.4,119. 4. Sinehal, J. P.: Sinph,N.; Singh.R. P . J . l n d . Chsm.Snc. 1977,54,457. s. G. 1978.9.137, 6. D o n m H . ~ . s ~ i i s ~ i .nm. s ~ Jc. .1976.4%882. 7. Hahne, H.C. H.;Kmofje, W. J.Enoiron. Quol. 1973.2.444.
.,I,,,
Volume 64
Number 5
May 1987
449
