M. J. Dudas

Environmental impacts of phosphogypsum

Abstract Phosphogypsum is an acidic by-product from the phosphate fertilizer industry. Large quantities are produced world-wide and it is estimated that by the year 2000 up to 280 million tonnes will be produced annually. Although phosphogypsum is mainly calcium sulfate dihydrate, it contains elevated levels of impurities which originate primarily from the source phosphate rock used in P fertilizer production. The main environmental concerns associated with phosphogypsum are: (i) movement of fluoride, sulfate, total dissolved solids, certain trace elements, and radionuclides from the U-238 decay series below phosphogypsum stacks into groundwater supplies; (ii) radon-222 exhalation which may pose a health risk to workers on the site or people living close to stacks; (iii) acidity; and (iv) radon-222 exhalation from soil into residential homes when agricultural land previously treated with phosphogypsum is converted to residential usage. In order to fully understand the environmental impact of phosphogypsum, it is necessary to understand the geochemical and hydrological processes that control the composition of phosphogypsum leachates and the attenuation of environmentally sensitive chemical species when these leachates enter soil environments. This article will review the chemistry of phosphogypsum and the environmental concerns associated with phosphogypsum that is stockpiled in waste repositories or is used as an agricultural soil amendment.

Cross-Correlation of Polarity Curves To Predict Partition Coefficients of Nonionic Organic Contaminants

Accurate sorption coefficients are important for models to predict fate and movement of organic chemicals in soils or sediments. We report here on a new method for predicting partition coefficients (K d ) of nonionic chemicals onto soils and geological materials. It includes properties of sorbents and of sorbates, thereby yielding more accurate organic carbon-normalized partition coefficients (K oc ) than a single value derived from an octanol-water partition coefficient (K ow ). A regression of log K oc , on log K ow and polarity index (PI: [(O+N)/C]) was established using benzene, toluene, and o-xylene as sorbates and using model organic polymers as sorbents

Sorption of phenol by selected biopolymers : isotherms, energetics, and polarity

The behavior of phenol in the terrestrial environment is strongly regulated by its reaction with soil components. We report here on the uptake of phenol by soil minerals (goethite, kaolinite, and montmorillonite) and by organics that may occur naturally in or be added to soil (two lignins, chitin, cellulose, collagen, and activated carbon). Our objectives were to determine the energetics and capacity for their uptake of phenol using batch equilibration, calorimetry, and CPMAS 13 C NMR and to evaluate the relation of organic carbon referenced sorption coefficient (K oc ) with the polarity of biopolymers. The biopolymers sorbed 2-45-fold more phenol than did the minerals

Variation of 1-naphthol sorption with organic matter fractionation: the role of physical conformation

Abstract Nonionic, hydrophobic organic contaminant sorption to natural organic matter is of continual concern due to the strong association between the contaminant and organic matter. To better understand the underlying mechanisms in sorption interactions, humic acids (HAs) and humin from soils and geologic samples with varying diagenetic properties were isolated and used in sorption studies with 1-naphthol. The organic carbon-normalized sorption coefficient (Koc) values for the fractions differed from those of the source samples and, in most instances, increased. The Black, Brown, and Peat HA 1-naphthol Koc values are statistically similar (p

Sorption of α-naphthol onto organic sorbents varying in polarity and aromaticity

Abstract Relation of α-naphthol uptake with polarity and aromaticity of selected organic matter surrogates was investigated using batch equilibration. The organic sorbents used were: lignin (organosolv), lignin (alkali), collagen, chitin, cellulose, and a collagen-tannic acid mixture (CTM). These materials represent model input sources for soil organic matter. Aromaticity of the sorbents was calculated from CPMAS 13C NMR spectrum data. Concentration of α-naphthol in solution was measured using liquid scientillation counting and elemental composition of the sorbents was determined with an elemental analyzer. Hysteresis between adsorption and desorption was not observed. We infer that uptake of α-naphthol is through partitioning as indicated by linear isotherms. Kd values for lignins were 12 to 24 fold higher than that for chitin and 130 to 260 fold higher than that for cellulose. The carbon referenced sorption coefficient (KOC) decreased from ∼436 to ∼3 mL/g with increasing polarity index [ ( O+N ) C ] and decreasing aromaticity of organic sorbents. We concluded that the quality of organic sorbents significantly influences partitioning of hydrophobic organic chemicals in aqueous systems. If this conclusion holds for all hydrophobic organic pollutants, then KOC of such xenobiotics cannot be accurately predicted from its KOW or solubility without some description of organic sorbent quality such as polarity and degree of aromaticity.

Transformations of elemental mercury to inorganic and organic forms in mercury and hydrocarbon co-contaminated soils

There are many industrial sites, such as gas processing plants, that are contaminated with both mercury and hydrocarbons. These sites tend to be localized but can have very high concentrations of mercury in the soil and heterogeneous distribution of hydrocarbons. The original form of mercury in many cases was elemental mercury from broken manometers. Over time the mercury has become redistributed within soil and has undergone chemical transformations into new forms. The forms of mercury will govern the chemical behavior and the availability of the mercury to biological receptors. The availability of the mercury is important as it will govern the risk associated with the contaminated soil and will also determine the effectiveness of any attempts at remediation. In the present study a chemical extraction protocol was used to determine the forms of mercury in soil originally contaminated by spillage of elemental mercury and petroleum hydrocarbons. Chemical extractions have been used in the past to determine the forms of mercury in uncontaminated soils and several researchers have used them to study contaminated soils. However, to date, no researchers have studied the forms of mercury in soils following years of weathering of elemental mercury after a spill. This study shows that decades after the original spill the elemental mercury has transformed and is dominantly (up to 85%) associated with soil organic matter, and to a lesser extent the mineral fraction of soil.

Heterogeneous distribution of trace elements and fluorine in phosphogypsum by-product

Phosphogypsum (PG), a by-product from phosphate fertilizer production, is composed mainly of gypsum (CaSO 4 .2H 2 O) but also contains minor quantities of trace elements (TE), rare earth elements (REE) and F. Some elements may be elevated in quantities to be of environmental concern. This study determined the distribution of TE, REE and F among three size fractions ( 53 μm) in PGs derived from three different phosphate rock sources. Fine fraction PG (<20 μm) composed of <10% of total PG mass but was highly enriched in TE, REE and F compared to unfractionated PG. For PG derived from Idaho rock, Se in the fine fraction was enriched 830 times over soil and 415 times over shale while Cd was enriched in the fine fraction 70-fold over shale and soil. Fluorine was elevated 37 times in the fine fraction compared to shale. The same trends were observed for PG derived from Togo and Florida rocks. Elevated elemental concentrations in fine particles and particle sorting during PG deposition may contribute to chemical heterogeneity of PG repositories, and make elements more susceptible to mobilization processes, such as leaching and erosion. Removal of fines will improve the utilization of PG in other industries, such as for use as an amendment to agricultural soils

Microbial reduction of amended sulfate in anaerobic mature fine tailings from oil sand

,!Bitumen extraction from oil sands has resulted in large tailings ponds, containing suspended material that requires over one hundred years to densify. The mature fine tailings (MFT) have become anaerobic and bubbles of gas are observed on the pond surface. Gypsum has been proposed as an additive to increase the rate of MFT consolidation. In a laboratory study, MFT was amended with sulfate and monitored. Pore water sulfate concentrations declined and bicarbonate concentration increased. Nitrate was depleted within 36 d and the levels of soluble iron remained below 0.8 mg L-1. Thiosulfate and sulfide were detected only near the end of the experiment. Acid volatile sulfides (AVS) increased until day 39, and then reached a plateau. Methane was not detected throughout the incubation in samples amended with sulfate. The increase in AVS supports sulfide incorporation into the solid phase, however, the plateau after 39 d suggests a secondary fate of reduced sulfide

Leaching behaviour of selected trace elements in chemically weathered alkaline fly ash

Abstract In a laboratory study alkaline fly ash was leached in a series of lysimeters with dilute H 2 SO 4 . The weathered residues retrieved after leaching were analyzed for major constituents by atomic absorption spectrophotometry and trace elements by instrumental neutron activation analysis. The characteristics of the weathered residues ranged from highly leached acidified material, from which many constituent elements had been mobilized, to minimally leached alkaline material containing accumulations of newly formed secondary minerals. The leaching behavior of constituent trace elements was related to the chemical environment of the leachates and partitioning among two previously identified major phases within parent ash particles and with newly formed secondary minerals. Elements such as Rb, Cs, Pb, Ta, Ti, and Hf were enriched in the highly leached portion of the residue sequence, suggesting association with the resistant internal Si-rich glass matrix of ash particles. Between 50 and 80% of the total Mn, Sb, Th, Cr, Zn, Co, Sc, and rare earth elements was also retained in the highly leached ash residues. About 50% of the total Sr, V, and U, and more than 80% of the total As and B was dissolved from the ash under acidic conditions. With the exception of B, all elements that were mobilized from the acidified ash residues were also attenuated in the alkaline residues in association with one or more of the newly formed secondary minerals.

Heterogeneous distribution of radionuclides, barium and strontium in phosphogypsum by-product

Phosphogypsum (PG) is a high volume by-product of the phosphate fertilizer industry which is composed mainly of CaSO4·2H2O. Impurities in PG include F, trace elements and naturally-occurring radionuclides. Radium-226 content is sufficiently high in some PGs that it has limited PG usage in building materials and as an amendment to agricultural soils. Radium likely exists in PG as a sulfate solid solution with Ba, and possibly Sr. This study determined the distribution of 226Ra, Ba, Sr, U, Th and 210Pb among three size fractions (fine: 53 μm) in PGs derived from three different phosphate rock sources. All chemical species, except for Sr in PG derived from Idaho rock, were enriched in the <20-μm fraction relative to the other size fractions and relative to unfractionated PG. On average, fine fraction 226Ra and 210Pb contents were enriched approximately sixfold over unfractionated PG. 226Ra was enriched in the fine fraction to a greater degree than were Ba or Sr, indicating that Ra behavior in PG is distinct from these other elements. The combination of (i) particle sorting during PG deposition, and (ii) non-uniform distribution of radionuclides in PG, may contribute to radionuclide heterogeneity at PG repositories.