The Uniqueness of Microconstituent Fates in Biosolids-amended Systems
Abstract:All chemicals added to soils are subject to the same reactions/processes, including solid phase retention/release, degradation, bioaccumulation, volatilization, runoff, and leaching. The reactions/processes of organics have been studied for decades and the corresponding risk to human and environmental health assessed/estimated. Examples of organic chemicals so studied include pesticides, priority pollutants, and others with chemical and physical properties similar to many of today's “emerging chemicals of concern”, also know as “microconstituents. ” Advantage can be taken of this extensive database and computer models developed from the data and theory to predict microconstituent fate and risk in biosolids-amended soils. There are potential unique characteristics of biosolids and biosolids-amended soil, however, which must be considered before such predictions should be regarded as reliable. My purpose is to identify some of the uniquenesses and to demonstrate how they affect, or can be expected to affect, microconstituent fates and risk assessments. Emphasis is given to two antimicrobial microconstituents, triclosan (TCS) and triclocarban (TCC) common in biosolids, but the lessons are universal.
Decades of research have identified some generalized hazardous/risky traits of organic contaminants, including high detection frequency/concentration/toxicity, high solubility, especially low (or high) log Kow value, long half-life, high Henry's Constant, and large bioaccumulation factors. A sub-group of microconstituents that are persistent, bioaccumulative, and toxic [e.g., polychlorinated biphenyls (PCBs), dioxins, polyaromatic hydrocarbons (PAHs)] has attained much notoriety. The chemicals share similar properties: long half-lives (persistent), high log Kow values (bioaccumulative, in some biota), and high toxicity (to some species). The high log Kow values, however, also portend that solid phase retention is great and that release is small, that leaching through soils and subsequent groundwater contamination is small, that water solubility is very low, and that availability to organisms dependent on water solubility (plant uptake, degradation) is small. Thus, long-term persistence need not mean long-term risk to environmental or human health, as the lability (availability to plants and other organisms) of chemicals with high log Kow values tends to be low and to decrease further with time. The assessment of chemical fate and risk is complex in even “simple“ soil systems, and could be expected to be even more complex in biosolids-amended systems. Fortunately, much research has been conducted on the concentration and behavior of the chemicals in biosolids and biosolids-amended soils. Modern biosolids have consistently low in PCB, dioxin, and PAH concentrations (sub-ppm), and typical biosolids application rates of 5 to 10 Mt/ha mixed in soils to a 15 cm depth reduce the chemical concentration in the amended soil 100 to 200-fold below that in the biosolids. The low biosolids concentrations, low soil concentrations, strong soil retention, limited soil mobility and very limited lability of the chemicals means that risk of biosolids-borne PCBs, dioxins, and PAHs are minimal to humans and the environment.
Abundance data for various microconstituents in biosolids are beginning to be accumulated (e.g., EPA's recent Targeted National Sewage Sludge Survey – TNSSS). Much less is known about microconstituent behavior in biosolids or amended soils, and even basic chemical/physical data for some microconstituents are scant or questionable. Data gaps are frequently filled with model-generated values, such as generated from QSAR, PBT Profiler, or ECOSAR models. The models are largely invalidated, especially for biosolids systems, and should be regarded as only first approximations of chemical properties and behavior in real-world systems. Examples include a QSAR-predicted log Kow value of TCC of 4.9 vs. a measured value of 3.5 and predicted water solubility of 0.65 to 1.55 mg/L vs. a measured value of 0.045 mg/L. Both log Kow and water solubility values are fundamental characteristics used in most fate and transport models, and errors in the assumed values are not inconsequential. Familiar empirical relationships used to predict one chemical property (e.g., water solubility) from another (e.g., log Kow) also appear inaccurate for some microconstituents; thus, the expected increased water solubility of TCC if log Kow is 3.5 rather than 4.9 is contradicted by measured data.
Some data exist pertaining to microconstituent behavior in soils, often for unamended soils or soils spiked with the chemicals at high concentrations, or with radio-labeled (14C) forms of the chemicals, to simplify analytical challenges. Guidelines exist (EPA OPPTS) that direct how organic contaminant behavior in soil/plant/microbial systems should be conducted, but potential biosolids effects on contaminant interactions have been largely ignored. Data collected using biosolids-borne microconstituents, or biosolids spiked with radioisotopes, suggest much different chemical behavior in biosolids-amended systems.
The persistences of TCC and TCS have been predicted (PBT Profiler) to be similar and to be greater under anaerobic conditions (540 d) than aerobic conditions (120 d). The similar persistence results were predicted because estimated log Kow values (4.9 for TCC and 4.8 for TCS) are similar, but actual log Kow measurements differ (3.5 for TCC and 4.8 for TCS). Measured persistence values under aerobic conditions vary from a few weeks to several years, and can vary with the presence of biosolids. Thus, one of the most important parameters used to predict chemical fate and transport (persistence or half-life) appears to be poorly predicted by models and to vary with biosolids presence.
Solid phase (soil) retention/release of microconstituents is usually characterized by distribution coefficients (Kd) and is the second most important parameter to predict chemical fate and transport. OPPTS guidelines exist for Kd determination for activated sludge and for soils/sediments. The guidelines also address chemical release (desorption), an important parameter of chemical retention that some researchers neglect because they expect (and models assume) the retention reaction to be reversible. Retention of many microconstituents (especially, non-polar chemicals) is expected to increase with increasing organic carbon (OC) content of the solid phase, and leads to normalization of Kd values across solids using solid OC values to arrive at Koc values expected to be reasonably constant for all solids. Values of Koc can also be estimated from empirical relationships between (similar) chemical Koc and Kow values, but measured values are preferable. Measures of retention/release of TCC and TCS by biosolids suggest extensive binding, and release that is slow, strongly hysteretic, and incomplete. Normalization of soil and biosolids Kd values by OC contents of the solids removes much, but not all, of the variation in retention by the solids. Retention and hysteresis in desorption of TCS is greater than for TCC. Simplified approaches to predicting TCC/TCS retention/release are inappropriate in biosolids-amended soils.
The study of microconstituent behavior in the environment is complicated by numerous challenges, primarily analytical issues (low chemical concentrations and sophisticated instrumentation requirements), but also non-uniform distribution and unknown chemical impacts. The situation becomes even more challenging in biosolids-amended systems that not only introduce another solid phase (besides soil) that can interact with the chemicals, but usually worsens heterogeneity and analytical problems. Further, the newness of issues surrounding the chemicals tends to promote “every man for himself” analytical and research approaches. The generated databases are, therefore, difficult to interpret and to build upon. The OPPTS guidelines, developed for pesticides, offer opportunities for databases to be harmonized and consistently developed. The OPPTS guidelines, however, neglect possible biosolids impacts on chemical behavior and data suggest that the impacts are not inconsequential. Researchers familiar with biosolids-amended systems and biosolids impacts on chemical behavior should join with toxicologists, modelers, etc. to ensure that meaningful and transferrable databases are accumulated. Awareness of past research and lessons learned with similar biosolids-borne trace organics, and with typical land application of biosolids practices is essential for the most useful data generation and interpretation.
Document Type: Research Article
Publication date: January 1, 2009
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