Suduan Gao

Simultaneous Sorption of Cd, Cu, Ni, Zn, Pb, and Cr on Soils Treated with Sewage Sludge Supernatant

Disposal of sewage sludge creates the potential for heavy metal accumulation in theenvironment. This study assessed nine soils currently used as Dedicated Land Disposal units(DLDs) for treatment and disposal of municipal sewage sludge in the vicinity of Sacramento,California. Adsorption characteristics of these soils for Cd, Cu, Ni, Zn, Pb, and Cr were studiedby simultaneously mixing these elements in the range of 0-50 µmol L-1 with sludgesupernatant and reacting with the soil using a soil:supernatant ratio of 1:30, pH = 4.5 or 6.5, andconstant ionic strength (0.01 M Na-acetate). The concentration of metals in the supernatant wasdetermined after a 24 hr equilibration period. Adsorption isotherms showed that metal sorptionwas linearly related to its concentration in the supernatant solution. The distribution coefficientKd (Kd = concentration on solid phase/concentration in solution phase) was computed as theslope of the sorption isotherm. The distribution coefficients were significantly correlated to soilorganic matter content for Ni, Cu, Cd, and Pb at pH 4.5 and for Ni, Cu, Zn, and Cd at pH 6.5.There was also a correlation between Kd and soil specific surface area but no relationship to othersoil properties such as CEC, clay content, and noncrystalline Fe and Al materials. Therefore, soilorganic carbon and surface area appear to be the most important soil properties influencing metaladsorption through formation of organo-metal complexes. The Kd values for all elements werehigher at pH 6.5 than at 4.5. Selectivity between metals resulted in the following metal affinitiesbased on their Kd values: Pb>Cu>Zn>Ni>Cd≈Cr at pH 4.5 andPb>Cu≈Zn>Cd>Ni>Cr at pH 6.5.

Production of dissolved organic carbon (DOC) and trihalomethane (THM) precursor from peat soils.

Water passing through the Sacramento-San Joaquin Delta contains elevated concentrations of dissolved organic carbon (DOC) and trihalomethane (THM) precursor relative to upstream waters from the Sacramento River and the San Joaquin River. Drainage from agricultural peat soils has been identified as one of the major sources of DOC and THM precursor. A series of controlled laboratory experiments were conducted to evaluate abiotic and biotic effects on the quantity and the nature of DOC and THM precursors produced from oxidized surface and reduced subsurface soils in the Delta. For abiotic effects, DOC was extracted from both soils with synthetic solutions containing a range of salinity (0-4 dS/m) and sodicity (0 to infinity ). The results showed that an increase in salinity significantly decreased the concentration of DOC in the soil-water from both soils but increased its aromaticity, as indicated by specific ultraviolet absorbance at 254 nm (SUVA). For biotic effects, peat soils were incubated over a range of temperatures (10 degrees C, 20 degrees C and 30 degrees C) and soil moisture contents (0.3-10 g water/g soil). After 8 weeks of incubation, only extracted DOC from flooded conditions and flooded and non-flooded cycles showed an increase in DOC. These findings indicate that neither salinity nor sodicity is the major factor for DOC production, but both can affect the solubility and mobility of DOC in the Delta soils. We believe wetting processes in oxidized peat soils produce significant amounts of DOC found in agricultural drainage discharged into the Delta waters.

Arsenic distribution, speciation and solubility in shallow groundwater of Owens Dry Lake, California

Generation of dust particles from the Owens Lake playa creates a severe air pollution hazard in the western United States. Much of the dust produced from the dry lakebed is derived from salts formed by evaporation of saline groundwater that often contains high concentrations of dissolved arsenic (As). The objectives of this research were to study the spatial distribution of dissolved arsenic in the shallow groundwater, and to examine factors affecting arsenic solubility and speciation. Evapoconcentration, redox potential, pH, and mineral solubility were examined as factors regulating arsenic biogeochemistry. Dissolved arsenic concentrations ranged from 0.1 to 96 mg L 1 and showed a general increase from the shoreline to the center of the lakebed. Arsenic concentrations were strongly correlated to electrical conductivity (EC) and D suggesting that evapoconcentration is an important process regulating total As concentrations. Arsenite (As(III)) was the dominant form of inorganic arsenic at Eh values less than about 170 mV while arsenate (As(V)) was predominant at higher Eh values. Organic arsenic was negligible (0.21%) in all shallow groundwater samples. Dissolved arsenic concentrations do not appear to be strongly regulated by solid-phase reactions. Solid-phase arsenic concentrations generally ranged between 4.0 and 42.6 mg kg 1 and a maximum concentration range (20 to 40 mg kg 1 ) was reached as solution concentration increased up to 80 mg L 1 , indicating minimal sorption and/or precipitation of arsenic. Chemical equilibrium modeling indicated that orpiment (As2S3) was the only solid phase with a positive saturation index (indicating over-saturation), but only at high arsenic and sulfide concentrations. The findings of this research are important for assessing the potential environmental impacts of elevated arsenic concentrations on dust mitigation efforts taking place at Owens Dry Lake. Copyright

Characterizing redox status of paddy soils with incorporated rice straw

Redox status is one of the more difficult soil quality criteria to characterize especially in paddy soils. The soil system chosen in this study was an ongoing paddy field trial on alternative practices of incorporating rice straw instead of traditional burning of straw. Upon flooding, rapid changes in redox potential (Eh) occur in paddy soil due to the decomposition of soil organic matter (SOM), including rice straw. This investigation evaluated three methods of characterizing redox status: (i) Eh, a conventional method using Pt black electrode; (ii) terminal electron-accepting processes (TEAPs), a method of diagnosing microbially mediated electron acceptor (oxidized species) consumption, intermediate product concentration of H2 and accumulation of final products (reduced species); and (iii) oxidative capacity (OXC), a comprehensive analysis of the algebraic sum of oxidized and reduced species into a single descriptive parameter. There was a need to develop a sampling technique for anaerobic soil pore water in rice paddy (without exposure to the atmosphere) to measure all the redox parameters required by these methods. The setup comprised of a capped piezometer (sampling well) back-filled with sand and sealed with bentonite, and vacuum extraction of pore water containing dissolved constituents. The pore water containing dissolved gases was collected into previously evacuated 400-ml two-port PVC bags from which gaseous phase sub-samples were withdrawn with a syringe into 10-ml vacutainers previously flushed with N2 gas for H2 and CH4 determinations in the laboratory. Unstable water quality parameters such as DO, Eh, pH and S2− were measured on site using fresh samples, while other parameters such as NO3−, SO42−, EC, Fe(II) and Fe(III), and Mn(II) were analyzed in the laboratory. The above three methods of evaluating redox status were applied to monitoring the paddy pore water over 3 years of field observations (1997–1999). Eh is a simple measure, but it gives at best only qualitative assessment of redox status because this electrode may not respond to many of the important redox couples. The adaptation of TEAPs to assess non-equilibrium redox conditions in flow pathways in large-scale ground water systems to small-scale rice rootzone was not entirely successful because of differences in the two systems and the significant overlapping among electron acceptors in paddy soils. OXC, a non-equilibrium capacity-type redox parameter, clearly identified geochemical redox classes of oxic, post-oxic, sulfidic and methanic conditions in the paddy pore waters during the course of the rice-growing season. Based on data observed for 3 years, it is concluded that straw incorporation does enhance more reducing conditions development compared to that without straw incorporation. We conclude that OXC provides a better characterization of redox status in paddy soils.

Selenium Removal from Irrigation Drainage Water Flowing Through Constructed Wetland Cells with Special Attention to Accumulation in Sediments

A flow-through experimental wetland system has been under investigation since 1996 to remove selenium (Se) fromagricultural drainage water in the Tulare Lake Drainage Districtat Corcoran, California, U.S.A. The system consists of ten cellswhich have dimensions of 15 × 76 m continuously flooded andvarious substrates planted. The objectives of this article are topresent the overall performance in Se removal after establishingthe wetland for three years, and to examine factors affecting Seremoval with special attention to accumulation in the sediments.In 1999, The wetland cells reduced Se from inflow water by 32 to65% in concentration and 43 to 89% in mass. Vegetationplays an important role in Se removal as non-vegetated cellshowed the least removal of Se. The inflow drainage water wasdominated by selenate (Se(VI), 91%) with smaller percentages ofselenite (Se(IV), 7%) and organic Se (org-Se(II-), 2%). Theoutflow water from the cells contained an average of 47% Se(VI),32% Se(IV) and 21% org-Se indicating reduction processesoccurring in the wetland cells. The surface sediment appears as alarge sink of Se removal. The highest Se concentration was foundin fallen litter, followed by the fine organic detrital layer onthe sediment surface. The sediment Se concentration dramaticallydecreased with increasing sediment depth. The mass distribution of Se, however, was sediment (0-20 cm) > fine detrital matter >fallen litter. Fractionation of surface sediment (0-5 cm) reveals that elemental Se was the largest fraction (ave. 47%) followedby organic matter-associated Se (34%). Soluble, adsorbed, and carbonate-associated Se accounted for 1.2, 3.1 and 2.5% ofthe total sediment Se, respectively. The major Se sink mechanism in the cells is the reduction of selenate to elemental Se andimmobilization into the organic phase of the sediments.