Predicted Speciation and Mineral Solid Phases of some Heavy Metals in Sludge - amended Soil

Abstract

Speciation of heavy metals exerts important controls on chemical behavior, solubility, toxicity, adsorption and even physical properties of solutions (Ure and Davidson 2002). In the present study, speciation and mineral-solid phase of Cd, Ni, and Pb in solution of soil prolonged irrigated with sewage effluent were predicted using GEOCHEM model (Sposito and Mattigod (1980) and constructed stability diagrams (Sposito ,1989 andLindsay ,1979).The present data indicate that total contents of all tested metals have increased dramatically as a result of sludge amending, particularly, for Cd. In virgin soil solution, the results of speciation indicated that, free species M 2+ was the prevalent form (60.35%) followed by carbonate species MCO 3 aq (20.12%), MSO 4 (5.93%), MHCO 3 + (5.56%), MOH + (4.21%) and MCL + (4.21%), while in sludge -soil solution the sequence was; M-DOC (41.92%), followed by MHPO 4 species (26.74%), M 2+ ( 20.07%) , MHCO3 + (4.22%), MSO 4 aq (3.49%) and MCO 3 aq (2.02%). It is clear from the results that amending soil with sludge decreases active portion (M +2 ) contribution to total speciation of the tested metals. In virgin soil, the majority of both Cd and Ni was (M +2 ) species. It amounted (79.03%) and (83.8%) , respectively . While MCO 3 aq was the prominent for pb (55.86%). In sludgesoil, Pb was almost exclusively in organically complexed forms (88.54%), while the majority species of Ni was M +2 (38.56%) and for Cd, CdHPO 4aq (58.72%). The data also showed that in sludge-soil the activities (Ml -1 ) of both Pb and Ni decreased by 26% and 72%, respectively, compared to their corresponding values in virgin soil. Meanwhile, the activity of Cd increased. These findings suggesting that amending soil with sludge probably inhibit activities of some metals, while promote activity of others. The data showed that the values of activities of Cd in virgin soil was closed to the solubility of CdCO 3 (log CO 2 =-2.52) and CdSO 4 -2Cd (OH) 2 at (log So 4 =-2) , while in sludge soil ,the value of Cd +2 was under saturated with respect the constructed Cd-minerals. This suggested that other possible solid phase of cadmium may be controlling its activity in that soil .Also , the data suggested that activity of Ni +2 in virgin soil is governed by Ni 3 (PO 4 ) 2 in equilibrium by gypsite. While in sludge-soil, value of Ni +2 was controlled by Ni 3 (PO 4 ) 2 in equilibrium with Caphosphate mineral at (log CO 2 = -4.52). Also, the results indicated that Pb 3 (CO 3 ) 2 (OH) 2 was the possible solid phase which may be controlled the activity of Pb +2 , in both virgin and sludge soil.

Determining Cadmium Activities and Predicting Solid- Phases Which Control Its solubility in the Soil irrigated with Sewage Water, in Egypt

“…The influences of soil contamination on the activities of Cd 2+ in soils were also investigated in the present study,as shown from the Table (3), pCd 2+ values were 9.08 and 8.09 in the 0-yrs and 100-yrs soil samples, respectively, indicating that the continual use of sewage effluent water in irrigation increased Cd 2+ activity by 9.94 folds than the control soil sample.These results are consistent with results reported by [11] who computed the activity of some metals (M/L) in soil consecutive irrigated with sewage effluent water for more than 50-years concentration. (M/L) as follows, Cd 2+ (1.61E-07), Ni 2+ ( 5.96E-07), and Pb 2+ (1.29E-08),they also added that continual use of sewage effluent in irrigation decreased the activity of both of Pb and Cd than the 0-yrs by ( 26% and 72%, respectively ), meanwhile Cd 2+ activity increased by about 8 times due to sewage effluent application.…”

“…This suggested that other possible solid phase of cadmium may be controlling its activity in that soil. They added that No known Cd mineral has yet been identified whose solubility, reflecting low activity of Cd 2+ .Meanwhile , [11] The data show that the solubility of Cd 2+ in virgin soil was closed to the solubility of CdCO3 (log CO2=-2.52) and CdSO4-2Cd(OH)2 at (log SO4=-2) , while in sludge soil ,the value of Cd 2+ was under saturated with respect the constructed Cd minerals. Cd 3 (PO 4 ) 2(logSO4=4.5)…”

“…Each method has its advantages and limitations and generally based on a more profound analytical back ground (e.g., pH, concentration of competing ions, concentration of complexing ligands in solution, and the soil colloid) and characteristics of soil biota, for instance, the ion selective electrometer (ISE) method, [8], voltammeter and polarography methods [9], and Donnan membrane Egyptian Journal of Chemistry http://ejchem.journals.ekb.eg/ technique(DMT), [7]. Also, the competitive chelation method which proposed by [10] was used widely for determining activity of some metals [11]. They reported that this method was good sensitive and convenient for measuring metal activities in neutral to slightly alkaline soils, it's and provides estimates of ion.Stability diagrams are used as a convenient technique for illustrating how the solubility of metal compounds varies with soil pH and with metal concentration (or activity), [12,13,14].…”

Solid Phases of Zn in Some Contaminated Soils in Sohag, Egypt.

“…It provides little indication of metal's bioavailability, mobility, and reactivity in soils, particularly soils treated with wastewater, sludge, and other organic fertilizers [3,4]. Therefore, it has been established that in most circumstances, the concentration of free ions (or activity) is the key factor in determining metal bioavailability and toxicity [1,[5][6][7][8].…”

“…2023, 8(2), 233-239 234 (or activity). The diagrams also allow some prediction of which solid phase regulates metal activity in the soil solution [6,7,17].…”