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SO L U B I L I T Y O F H E A V Y ME T A L S I N A C O N T A MI N A T E D SO I L :E F F E C T S O F R E D O X P O T E N T I A L A N D p H

M. C. CHUAN, G. Y. SHU and J. C. LIU*De par tme nt o f Che mical Engineering, Nation al Taiwan Institute o f Technology, Taipei, Taiwan(Received 15 July, 1994; accepted 9 May, 1995)

Abstract. To assess the mobilities of Pb, Cd, and Zn from a contaminated soil, the effects of redoxpotential and pH value on metal solubilities were investigated. Both redox potential and pH werefound to greatly affect heavy metal solubility n the soil. Results showed that the soil suspension undercontinuous oxygen aeration for 21 days resulted in increases of redox potential from 290 to 440 mVand pH value from 6.9 to 7.0, respectively. Soluble concentrations of Pb, Cd, and Zn varied with time,and were all lower than 1 mg kg -1 . When the soil suspension was aerated with nitrogen, final redoxpotential was - 140 mV, and pH value of 7.1. The soluble metal concentrations were slightly higherthan those aerated with oxygen. The equilibrium solubility experiments were conducted under threedifferent pH values (3.3, 5.0, 8.0) and three redox potential (325, 0, -100 mV). Results showed thatmetals were sparingly soluble under alkaline conditions (pH ---- 8.0). Metal solubilities were higherwhen under slightly acidic conditions (pH = 5.0), and increased drastically when pH was kept at3.3. When solubilities were compared under same pH values, it was observed that metal solubilitiesincreased as redox potential decreased. Generally speaking, acidic and reducing conditions weremost favorable for metal solubilization, and the effect of pH was more significant than that of redoxpotential. It was proposed that heavy metals were mostly adsorbed onto Fe-Mn oxyhydroxides. ThepH-dependent metal adsorption reaction and the dissolution of Fe-Mn oxyhydroxides under reducingconditions was the mechanism controlling the release of heavy metals from soils.Key words: heavy metals, solubility, redox potential, pH, soil

1. IntroductionHeavy metal contamination of agricultural soils has been a widespread problemin Taiwan. I t was est imated that over 50 000 hectares of arable land were seri-ously po lluted with heavy m etals from industrial wastewater. Cases of rice padd yconta min ated with Pb, Cd, and Zn were reported (Shu and Liu, 1994). Rice con-ta in ing high concentra t ions of cadmium had a lso been found on the market . Inorder to assess soi l pol lu tion problems and seek remedia l measures , som e w ork hasbeen con duc ted to d eterm ine the bioavailabil ity, speciation, and transport of heav ymetals in the soil systems. In addit ion, pilot-scale remedial techniques, such as,che m ical stabil ization, soil washing, and bioreme diation have been tested. Th oug hthere has been great concern about g roundw ater pollution, however, l it t le detailedinformation is available on the mobili ty of heavy metals in soil systems. Sincereactions at soil-water interfaces are crit ical in assessing the fate o f heavy metalsin soil systems, i t is important to model each possible step. The major objective

* Author for correspondence.

Water, Air, a nd Soil Pollution 90: 543-556, 1996.(~) 1996 Klu wer Academ ic Publishers. Printed in the Netherlands.

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544 M.C. CHUAN ET AL.of the present study was to examine the influences of pH and redox potential onhea vy meta l solubilities.

Both p H and oxidation-reduction condition have long been r ecogniz ed as crit-ical parameters controlling the fate o f pollutants in the environment. Theoreticalprediction of heav y metal equilibria as affected by pH and redox potential has beenstudied (Som me rs and Lindsay, 1974). In the mea ntime , an experim ental apparatuscontro lling redox potential and pH of soil suspension was deve loped (Patrick, Jr.and Delaune, 1972; Patrick et al. , 1973). The set-up was used to study the spe-ciation and transformation of Se and other trace elements as affected by pH andredo x potential in sediments and soil. Dom inant species and solubility we re foundto vary as pH and redox potential varied (Dev erel and Millard, 1988; Mass che leynet al. , 1990; Masscheleyn et al., 1991 a; A lem i et al., 1991). T he influe nce of redo xpotential and pH on As speciation and solubility in a contaminated soil was alsoreported (Masscheleyn et al., 1991b). Concerning heavy metal mobility in soils,chro miu m redox chemistry in a wetland was studied (Masscheleyn et al., 1992).Anothe r application was to assess the long-term metal mobility from heat process-ing wastes as influenced by pH and redox potential (Forstner et al., 1991). Whendealing with mobilities of Pb, Cd, and Zn, that do not possess a redox chemistryas complex as Cr, Se, and As, som e new m odel might be need ed to assess theirmobility in soil systems. Soil is a dynam ic and complex system, and any changes inintrinsic physico -chem ical properties of heavy m etal would surely change the fateof heav y metals in a soil system. Amo ng all parameters that might affect transportof heavy metals in soil systems, redox potential is also an important parameter,although pH is usually considered to be of prime importance. Man y authors haveindicated and discussed its importance and ranges of possible redox conditions(Alloway, 1990; Farrell et al., 1991; Risser and Bailey, 1992; Heron et al., 1994).In the curre nt study, soil samples from rice paddy con tam inated with Pb, Cd, andZn we re collected and investigated. The effects of pH and redox potential on metalsolubilities we re studied.

2 . Metho ds and Mater ia ls

Soil samples were obtained from Taoyuan County, Taiwan. The site was contam -inated by untreated effluent from a plasticizer manufacturing plant. Soil sampleswer e collected to the depth o f 30 cm. Total concentration of Pb (4N HNO3 diges-tion) w as 1,955 m g k g -1, Cd, 205 mg kg -1, and Zn, 282 mg kg -1. The soilconsists of 54.5% sand, 38.2% silt, and 7.3% clay. Cation excha nge capacity wa s2.98 me q/100 g. The p H o f the soil was 6.38. A detailed description of the sam plingsite and basic prope rties can be found in our previous work (Shu a nd Liu, 1994).

To automatically control pH and redox potential of soil suspensions, a modi-fication of the pH /Eh contro l system (Patrick, Jr. et al., 1972, 1973) was utilized(Figure 1). In order to assess the mobility of heavy metals in soil systems and

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S OLUB I LI TY OF HEAVY M ETALS I N A C ONTAM I NATED S OI L

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