The aim of this work was to study the influence of phosphate and citrate, which are common inorganic and organic aniozlS in soils, on the adsorption of acid phosphatasc by kaolin, goethite and the colloids separated fi'onl yellow-brown soil (YBS) and latosol (LS) in central-south China. The YBS colloid has the major clay mineral composition of 1.4 nm mineral, illite and koolinite while the LS colloid mainly contains kaolinite and oxides. Thc adsorption isotherm of acid phosphatase on the examined soil colloids and minerals fitted to the Langmuir model. Tbe amounl of enzyme adsorbed in the absence of ligands was in the order of YBS colloid>LS colloid>kaolin≈gocthite. In the presence of phosphate or citrate, the amounts of the enzyme adsorbed followed the sequence YBS olloid>kaolin>LS colloid>goethite. The presence of ligands also decreased the binding energy between the enzyme and soil colloids or minerals. With the increase of ligand concentration from 10 nltaol L I to 400 m tool h-I differenl bchaviors for the adsorption of enzyme were found in the colloid and mineral systems studied. A sharp decrease in enzyme adsorption was observed on goethite while graduai decreases of enzynle adsorption were recorded in the two soil colloid systenls. However, no any decrease was found for the amount of enzyme adsorbed on kaolin at higher ligand conceiltrations. Wiken phosphate or citrate was introdnced to the system before the addition of enzyme, the ligands usually enhealced the adsorption of enzyfne. The results obtained in Lids study suggested the important role of kaolinite mineral in the adsorption of enzyme molecules in acidic soils in the presence of various ligands.

Experiments were conducted to study the adsorption of Cd on two soil colloids (red soil and yellowbrown soil) and three variable-charge minerals (goethite, noncrystalline Pe oxide and kaolin) in the absence and presence of rhizobia. The tested strain Rhizobium fredii C6, tolerant to 0.8 mmol L-1 Cd, was selected from 30 rhizobial strains. Results showed that the isotherms for the adsorption of Cd by examined soil colloids and minerals in the presence of rhizobia could be described by Langmuir equation. Within the range of the numbers of rhizobial ceils studied, the amount of Cd adsorbed by each system increased with increasing rhizobial cells. Greater increases for the adsorption of Cd were found in red soil and kaolin systems. Rhizobia influence on the adsorption of Cd by examined soil colloids and minerals was different from that on the adsorption of Cu. The presence of rhizobia increased the adsorption affinity of soil colloids and minerals for Cd, particularly for the goethite and kaolin systems. The discrepancies in the influence of rhizobia on the adsorbabliity and affinity of selected soil colloids and minerals for Cd suggested the different interactions of rhizobia with various soll components. It is assumed that bacterial biomass plays an important role in controlling the mobility and bioavallability of Cd in soils with kaolinite and goethite as the major colloidal components, such as in variablecharge soil.

Adsorption of heavy metals and redox reactions of Cr(W) ions on Fe-Mn nodules from five soils of China were investigated by chemical analysis, equilibrium adsorption/redox, and X-ray photoelectron spectroscopy (XPS). Results show that Mn is mainly present as Mn3+and Mn4+ forms in Fe-Mn nodules. The maximum adsorption amounts for different heavy metal ions follow the order Pb2+≈Cu2+>Zn2+>Co2+>Ni2+>Cd2+. The adsorption capacity for heavy metals by Fe-Mn nodules from calciaquert in Shandong province (N5-1) is the highest, while that from hapludalf in Shandong province (N6-1) is the lowest. About 44-100% of the heavy metals adsorbed on Fe-Mn nodules were dissolved in 0.1 mol/L hydroxylamine hydrochloride (HAHC). The maximum amounts of Cr(VI) production by Fe-Mn nodules follow the order of N1-1 (69 mmol/kg)>N4-1 (57 mmol/kg)>2-1 (52 mmol/kg)>N5-1 (44 mmol/kg). Based on the content of MnO2 in Fe-Mn nodules dissolved in HAHC, the amount of Cr(VI) production by Mn oxides in N1-1, N2-1, N4-1, and N5-1 is 326, 624, 726, and 482 mmol/kg (MnO2), respectively. We propose that the amounts of Cr(VI) production through oxidation Cr(III) by Mn oxides are related to the types of Mn oxides in Fe–Mn nodules.

Red soil and cinnamon soil were collected from Chenzhou of Hunan and Gongyi of Henan, respectively. Soils were treated with Cu(NO3)2, Zn(NO3)2 or Cd(NO3)2, respectively, for two weeks. Rhizobium fredii strain HN01 was inoculated into the two soils polluted with three heavy metals. Sequential extraction method was employed to investigate the forms of copper (Cu), zinc (Zn), and cadmium (Ca) in the examined soils with the absence and presence of rhizobia. Results showed that the total amount of solid-bound Zn decreased 10% after the inoculation. The decrease for the amount of Zn associated with carbonate, manganese (Mn) oxides, and organic matter fraction was from 9 to 26%. No significant change was observed for the total amount of Zn combined with solid phase of red soil in the presence of rhizobia. However, the amount of specifically adsorbed and Mn oxides bound Zn decreased, while the amount of exchangeable Zn increased. Inoculation of rhizobia depressed the release of Cu to the soil solution and increased the total amount of Cu associated with solid phase in cinnamon soil. The ncrease for the amount of exchangeable Cu and the Cu in fractions of carbonate, Mn oxides, and organic matter ranged from 20 to 54%. There was no significant change for the level of Cd in the solution in both soils after rhizobia inoculation. The amount of Cd in the fractions of exchangeable and organic increased 22 and 11%, while that in the fractions of specific and Mn oxides decreased 14 and 29%, respectively. The different influence of rhizobia on the distribution of three heavy metals in two soils was mainly ascribed to the growth status and pH changes exerted by the metabolites of rhizobia. These data are helpful for the understanding of the chemical behavior and biogeochemical cycle of heavy metals affected by microorganisms in soil environment, which is fundamental for heavy metal bioremediation.

Adsorption of acid phosphatase on goethite, kaolinite and two colloids from the soils in central and south China in the presence of organic acids and phosphate was studied. With the increase of anion concentration, the ability in decreasing enzyme adsorption followed the sequence: phosphate>tartrate>oxalate>acetate. Acetate showed promotive effect on enzyme adsorption at lower anion concentrations whereas oxalate, tartrate and phosphate compete effectively with enzyme in a broad range of anion concentration. The adsorption isotherms of enzyme in most of the anionic systems studied conformed to the Langmuir equation. Phosphate reduced the affinity of enzyme on goethite more significantly than the other anions. However, tartrate decreased the affinity of enzyme on soil colloids and kaolinite to a greater extent than phosphate, oxalate and acetate. This observation suggested that the impact of anions on enzyme adsorption varies with anionic type and the surface characteristics of soil components. The influence of the addition order of ligand on enzyme adsorption was found greater in tartrate and phosphate systems. In general, simultaneous introduction of ligand and enzyme into the system had the lowest enzyme adsorption, showing more competition between ligand and enzyme molecules in this system. Data from this work indicated that the status and activity of enzyme in certain soil microenvironments especially the rhizosphere where various organic and norganic ligands are active can be altered and may be completely different from the bulk soil.