Pyrolysis-induced phosphorus transformations for biosolids from diverse sources.

Biosolids have been long used as a soil amendment to promote nutrient recovery. The readily releasable forms of nutrients present in this biowaste, such as phosphorus (P), along with their over application, can be detrimental to the environment, causing eutrophication. Pyrolysis, the thermal decomposition of organic materials at elevated temperature and low oxygen, seems to be a promising strategy to lower P release from biowastes such as biosolids. We pyrolyzed biosolids from various treatments and locations (Florida and Illinois; Galicia, Spain; and São Paulo, Brazil) to convert to biochar. Our objectives were (a) to use solid-state assessments, such as X-ray diffraction and scanning electron microscopy, and chemical assessments, such as water-soluble P (WSP), pH, Mehlich 3-extractable P (M3-P), total P (TP), and total Kjeldahl nitrogen, to evaluate changes caused by the conversion and (b) to compare P leaching potentials of biosolids and their corresponding biochars on two soils with varying P retention capacities. Pairwise comparisons indicated that biochar conversion significantly increased TP in the final material, but the absolute WSP decreased. However, M3-P remained the same after conversion to biochar. Cumulative P leached as a fraction of TP was greater for biosolids than their corresponding biochars. Two soils with contrasting P retention capacities predictably differed in P leaching behaviors as amended with biosolids and biochars. Differences suggest that future research could evaluate the efficacy of using mixtures of biosolids and biochar for a given soil to maintain soil fertility while reducing environmental P loss risk.

Valorization of farm pond biomass as fertilizer for reducing basin-scale phosphorus losses.

Long-term fertilizer phosphorus (P) inputs are causing phosphorous saturation of agricultural soils globally. The saturation is spreading to the edge-of-the-farm stormwater detention systems (SDSs) from where the legacy P is potentially being released to downstream surface waters. We use site-specific and literature data for P-saturated SDSs, to develop and evaluate the biogeochemical and economic feasibility of a P recycling program that targets both low (LIC, sugarcane) and high intensity cropping (HIC, fresh-produce) systems within a watershed. The focus is to close the P cycle loop to rejuvenate P sink function of SDSs. It involves harvesting and composting the SDS's biomass and it's on-farm use as an organic fertilizer for crops. Results showed that harvesting-composting can conservatively increase the P retention from 50% to 77% for HIC and almost complete treatment for LIC. Beyond potentially increasing yield and improving soil health, compost use can further increase in-field retention of P (and water). Additional costs incurred in harvesting and composting can be offset by the economic value of compost and the reduction in State's expenditure on regional P treatment systems. Treatment costs were $26/kg of P for HIC and $42/kg for LIC, 10 times less than the current state expenditure of $355-$909/kg P using constructed wetlands. We propose an incentivized, payment for services (PS) program, where producers are paid for P recycling. The PS program considers the intensity of cropping systems and their location along the drainage network from headwaters to the outlet, to achieve basin-scale P load reduction. The LIC SDSs recover regional P by passing the public water through them while recycling is implemented at the HIC. The estimated basin-scale P retention with harvest-compost approach was 854 metric tons, 5 times the P that entered the Everglades Protection Area in 2018, at 88%-93% less cost than the State treatment systems.

Arsenic release from Floridan Aquifer rock during incubations simulating aquifer storage and recovery operations.

While aquifer storage and recovery (ASR) is becoming widely accepted as a way to address water supply shortages, there are concerns that it may lead to release of harmful trace elements such as arsenic (As). Thus, mechanisms of As release from limestone during ASR operations were investigated using 110-day laboratory incubations of core material collected from the Floridan Aquifer, with treatment additions of labile or refractory dissolved organic matter (DOM) or microbes. During the first experimental phase, core materials were equilibrated with native groundwater lacking in DO to simulate initial non-perturbed anaerobic aquifer conditions. Then, ASR was simulated by replacing the native groundwater in the incubations vessels with DO-rich ASR source water, with DOM or microbes added to some treatments. Finally, the vessels were opened to the atmosphere to mimic oxidizing conditions during later stages of ASR. Arsenic was released from aquifer materials, mainly during transitional periods at the beginning of each incubation stage. Most As released was during the initial anaerobic experimental phase via reductive dissolution of Fe oxides in the core materials, some or all of which may have formed during the core storage or sample preparation period. Oxidation of As-bearing Fe sulfides released smaller amounts of As during the start of later aerobic experimental phases. Additions of labile DOM fueled microbially-mediated reactions that mobilized As, while the addition of refractory DOM did not, probably due to mineral sorption of DOM that made it unavailable for microbial utilization or metal chelation. The results suggest that oscillations of groundwater redox conditions, such as might be expected to occur during an ASR operation, are the underlying cause of enhanced As release in these systems. Further, ASR operations using DOM-rich surface waters may not necessarily lead to additional As releases.

Physicochemical and sorptive properties of biochars derived from woody and herbaceous biomass.

It is unclear how the properties of biochar control its ability to sorb metals. In this work, physicochemical properties of a variety of biochars, made from four types of feedstock at three pyrolysis temperatures (300, 450 and 600°C) were compared to their ability to sorb arsenic (As) and lead (Pb) in aqueous solutions. Experimental results showed that both feedstock types and pyrolysis temperature affected biochar's production rate, i.e., ratio of mass of biochar and biomass, thermal stability, elemental composition, non-combustible component (NCC) content, pH values, surface areas and thus their sorption ability to the two metals in aqueous solution. In general, the high temperature biochars had low O/C and H/C ratios, were more carbonized with larger surface area, and were more concentrated with alkaline cations. In addition, biochars made from woody feedstocks had larger surface area, but lower NCC contents than that made from grasses under the same conditions. Although all the tested biochars removed both As and Pb from aqueous solutions, they showed different sorption abilities because of the variations in properties. Statistical analyses suggested that feedstock type affected the sorption ability of the biochars to both As and Pb significantly (p<0.001). Pyrolysis temperature, however, showed little influence on biochar sorption of Pb in aqueous solutions. Statistical analyses also showed that electrostatic interaction played an important role in controlling the sorption of both As(V) and Pb(II) onto the biochar. Other mechanisms, such as precipitation and surface complexation, could also control the sorption of Pb(II) onto the biochars.

Manganese oxide-modified biochars: preparation, characterization, and sorption of arsenate and lead.

This work explored two modification methods to improve biochar's ability to sorb arsenic (As) and lead (Pb). In one, pine wood feedstock was pyrolyzed in the presence of MnCl2·4H2O (MPB) and in the other it was impregnated with birnessite via precipitation following pyrolysis (BPB). The resulting biochars were characterized using thermogravimetry, X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, and energy-dispersive X-ray analyses. The dominant crystalline forms of Mn oxides in the MPB and BPB were manganosite and birnessite, respectively. Batch sorption studies were carried out to determine the kinetics and magnitude of As(V) and Pb(II) onto the biochars. As(V) and Pb(II) sorption capacities of MPB (0.59 and 4.91 g/kg) and BPB (0.91 and 47.05 g/kg) were significantly higher than that of the unmodified biochar (0.20 and 2.35 g/kg). BPB showed the highest sorption enhancement because of the strong As(V) and Pb(II) affinity of its birnessite particles.

Removal of arsenic by magnetic biochar prepared from pinewood and natural hematite.

There is a need for the development of low-cost adsorbents to removal arsenic (As) from aqueous solutions. In this work, a magnetic biochar was synthesized by pyrolyzing a mixture of naturally-occurring hematite mineral and pinewood biomass. The resulting biochar composite was characterized with X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and energy-dispersive X-ray analysis (EDS). In comparison to the unmodified biochar, the hematite modified biochar not only had stronger magnetic property but also showed much greater ability to remove As from aqueous solution, likely because the γ-Fe2O3 particles on the carbon surface served as sorption sites through electrostatic interactions. Because the magnetized biochar can be easily isolated and removed with external magnets, it can be used in various As contaminant removal applications.

Interaction effects of climate and land use/land cover change on soil organic carbon sequestration.

Historically, Florida soils stored the largest amount of soil organic carbon (SOC) among the conterminous U.S. states (2.26 Pg). This region experienced rapid land use/land cover (LULC) shifts and climate change in the past decades. The effects of these changes on SOC sequestration are unknown. The objectives of this study were to 1) investigate the change in SOC stocks in Florida to determine if soils have acted as a net sink or net source for carbon (C) over the past four decades and 2) identify the concomitant effects of LULC, LULC change, and climate on the SOC change. A total of 1080 sites were sampled in the topsoil (0-20 cm) between 2008 and 2009 representing the current SOC stocks, 194 of which were selected to collocate with historical sites (n = 1251) from the Florida Soil Characterization Database (1965-1996) for direct comparison. Results show that SOC stocks significantly differed among LULC classes--sugarcane and wetland contained the highest SOC, followed by improved pasture, urban, mesic upland forest, rangeland, and pineland while crop, citrus and xeric upland forest remained the lowest. The surface 20 cm soils acted as a net sink for C with the median SOC significantly increasing from 2.69 to 3.40 kg m(-2) over the past decades. The SOC sequestration rate was LULC dependent and controlled by climate factors interacting with LULC. Higher temperature tended to accelerate SOC accumulation, while higher precipitation reduced the SOC sequestration rate. Land use/land cover change observed over the past four decades also favored the C sequestration in soils due to the increase in the C-rich wetland area by ~140% and decrease in the C-poor agricultural area by ~20%. Soils are likely to provide a substantial soil C sink considering the climate and LULC projections for this region.

Ionic strength reduction and flow interruption enhanced colloid-facilitated Hg transport in contaminated soils.

The effects of ionic strength (IS) reduction (5-0.05mM) and flow interruption (FI, flow stopped for 7d) on colloid and Hg release in the leachate were examined in column experiment. Two Hg contaminated soils (13.9 and 146mg/kg) were used, with Hg concentrations in colloids being 2-4 times greater than bulk soils. Based on sequential extraction, Hg concentrations in organic matter (OM) fraction were the most abundant in soils (31-48%). Column leaching after IS reduction and FI released large amounts of colloidal Hg, accounting for 44-48% of released Hg. The highest colloidal Hg concentrations at 27.8 and 360µg/L were observed at ∼1 pore volume after FI. Concentration distribution of colloidal OM and colloidal Fe was similar to colloidal Hg in the leachate, showing peak concentrations after IS reduction and FI. Most of the released colloidal Hg was in OM fraction (37-53%), with some in Fe/Mn oxide fraction (11-19%). Based on composition of released colloids and Hg fractionation in soils and colloids, colloidal OM could serve as an important carrier for Hg transport in soils.

Biochar amendment affects leaching potential of copper and nutrient release behavior in contaminated sandy soils.

Copper (Cu) contamination to soil and water is a worldwide concern. Biochar has been suggested to remediate degraded soils. In this study, column leaching and chemical characterization were conducted to assess effects of biochar amendment on Cu immobilization and subsequent nutrient release in Cu-contaminated Alfisol and Spodosol. The results indicate that biochar is effective in binding Cu (30 and 41%, respectively, for Alfisol with and without spiked Cu; 36 and 43% for Spodosol) and reducing Cu leaching loss (from ∼47 to 10% for the Cu-spiked Alfisol and from 48 to 9% for the Cu-spiked Spodosol). Copper was likely retained on biochar surfaces through complexation, as suggested by Fourier-transform infrared spectra. Biochar amendment converts a portion of Cu from available pool to more stable forms, thus resulting in decreased activities of free Cu and increased activity of organic Cu complexes in leachate. Reduction of >0.45-µm solids and nanoparticles concentrations in leachate was also observed. In addition, biochar application rate was correlated negatively with P, Ca, Mg, Zn, Mn, and NH-N concentration ( < 0.05) but positively with K and Na concentration ( < 0.05) in leachates. These results documented the potential of biochar as an effective amendment for Cu immobilization and mitigation of leaching risk for some nutrients.

Particulate copper in soils and surface runoff from contaminated sandy soils under citrus production.

Soil contamination by copper (Cu) is a worldwide concern. Laboratory incubation and soil Cu characterization were conducted to examine the effects of external Cu loading and liming on Cu speciation in both bulk soil and particulates of an Alfisol and Spodosol under citrus production. Also, drainage water from the sites was evaluated for dissolved and particulate forms of Cu. Soil available Cu estimated by CaCl2, NH4OAc, or Mehlich-3 extraction significantly increased with external Cu loads and decreased with soil pH. Most increases in soil Cu occurred in the exchangeable and oxide-bound fractions. Organically bound Cu was the dominant fraction in both bulk soil and particulates, but more in particulates than bulk soil (P ≤ 0.001). Organically bound Cu was highly correlated with total recoverable Cu (P ≤ 0.01), increased significantly with external Cu loads (P ≤ 0.001), and decreased with soil pH (P ≤ 0.05). Lime addition converted part of Cu from available pools to more stable forms. Organically bound Cu complexes were found to dominate in soil solution or surface runoff. These results indicate that most Cu accumulated in the contaminated soils is highly mobile, and thus may impact citrus production and the environment.

Orthophosphate Leaching in St. Augustinegrass and Zoysiagrass Grown in Sandy Soil under Field Conditions.

Phosphorus (P) is required to maintain healthy, high-quality, warm-season turf. However, excessive P applications to soils with poor P retention capabilities may lead to leaching losses to groundwater. This field study was conducted to determine the maximum P fertilizer application rate to (Walt.) [Kuntze] 'Floratam' St. Augustinegrass (St. Augustinegrass) and 'Empire' zoysiagrass (zoysiagrass) below which P leaching is minimized. Five P levels ranging from 0 to 5.0 g P m yr were surface applied as triple superphosphate. Turf was established on an uncoated, low-P sand with negligible P retention capacity. Leaf and root growth, tissue P concentration, soil P concentration, soil P saturation, leachate volume, and orthophosphate (P) concentration in leachates were measured. Mehlich 1-extractable soil P (M1-P) and soil P saturation ratio (PSR) increased with time as the P rate increased. Lower M1-P and PSR values were measured with St. Augustinegrass, which absorbed more P than did zoysiagrass. The root system of St. Augustinegrass was larger and deeper compared with zoysiagrass, promoting greater P uptake and less P leaching. If tissue analysis indicates that P fertilization is required and the soil has the capacity to retain additional P, application of 0.8 g P m yr to zoysiagrass and 1.07 g P m yr to St. Augustinegrass is appropriate and does not result in increased P leaching.

Nutrient removal using biosorption activated media: preliminary biogeochemical assessment of an innovative stormwater infiltration basin.

Soil beneath a stormwater infiltration basin receiving runoff from a 23 ha predominantly residential watershed in north-central Florida, USA, was amended using biosorption activated media (BAM) to study the effectiveness of this technology in reducing inputs of nitrogen and phosphorus to groundwater. The functionalized soil amendment BAM consists of a 1.0:1.9:4.1 mixture (by volume) of tire crumb (to increase sorption capacity), silt and clay (to increase soil moisture retention), and sand (to promote sufficient infiltration), which was applied to develop an innovative stormwater infiltration basin utilizing nutrient reduction and flood control sub-basins. Comparison of nitrate/chloride (NO(3)(-)/Cl(-)) ratios for the shallow groundwater indicates that prior to using BAM, NO(3)(-) concentrations were substantially influenced by nitrification or variations in NO(3)(-) input. In contrast, for the new basin utilizing BAM, NO(3)(-)/Cl(-) ratios indicate minor nitrification and NO(3)(-) losses with the exception of one summer sample that indicated a 45% loss. Biogeochemical indicators (denitrifier activity derived from real-time polymerase chain reaction and variations in major ions, nutrients, dissolved and soil gases, and stable isotopes) suggest that NO(3)(-) losses are primarily attributable to denitrification, whereas dissimilatory nitrate reduction to ammonium is a minor process. Denitrification was likely occurring intermittently in anoxic microsites in the unsaturated zone, which was enhanced by the increased soil moisture within the BAM layer and resultant reductions in surface/subsurface oxygen exchange that produced conditions conducive to increased denitrifier activity. Concentrations of total dissolved phosphorus and orthophosphate (PO(4)(3-)) were reduced by more than 70% in unsaturated zone soil water, with the largest decreases in the BAM layer where sorption was the most likely mechanism for removal. Post-BAM PO(4)(3-)/Cl(-) ratios for shallow groundwater indicate predominantly minor increases and decreases in PO(4)(3-) with the exception of one summer sample that indicated a 50% loss. Differences in nutrient variations between the unsaturated zone and shallow groundwater may be the result of the intensity and duration of nutrient removal processes and mixing ratios with water that had undergone little biogeochemical transformation. Observed nitrogen and phosphorus losses demonstrate the potential, as well as the future research needs to improve performance, of the innovative stormwater infiltration basin using BAM for providing passive, economical, stormwater nutrient-treatment technology to support green infrastructure.

Soil property control of biogeochemical processes beneath two subtropical stormwater infiltration basins.

Substantially different biogeochemical processes affecting nitrogen fate and transport were observed beneath two stormwater infiltration basins in north-central Florida. Differences are related to soil textural properties that deeply link hydroclimatic conditions with soil moisture variations in a humid, subtropical climate. During 2008, shallow groundwater beneath the basin with predominantly clayey soils (median, 41% silt+clay) exhibited decreases in dissolved oxygen from 3.8 to 0.1 mg L and decreases in nitrate nitrogen (NO-N) from 2.7 mg L to <0.016 mg L, followed by manganese and iron reduction, sulfate reduction, and methanogenesis. In contrast, beneath the basin with predominantly sandy soils (median, 2% silt+clay), aerobic conditions persisted from 2007 through 2009 (dissolved oxygen, 5.0-7.8 mg L), resulting in NO-N of 1.3 to 3.3 mg L in shallow groundwater. Enrichment of δN and δO of NO combined with water chemistry data indicates denitrification beneath the clayey basin and relatively conservative NO transport beneath the sandy basin. Soil-extractable NO-N was significantly lower and the copper-containing nitrite reductase gene density was significantly higher beneath the clayey basin. Differences in moisture retention capacity between fine- and coarse-textured soils resulted in median volumetric gas-phase contents of 0.04 beneath the clayey basin and 0.19 beneath the sandy basin, inhibiting surface/subsurface oxygen exchange beneath the clayey basin. Results can inform development of soil amendments to maintain elevated moisture content in shallow soils of stormwater infiltration basins, which can be incorporated in improved best management practices to mitigate NO impacts.

Spectroscopic models of soil organic carbon in Florida, USA.

Soil organic carbon (SOC) is an indicator of ecosystem quality and plays a major role in the biogeochemical cycles of major nutrients and water. Shortcomings exist to estimate SOC across large regions using rapid and cheap soil sensing approaches. Our objective was to estimate SOC in 7120 mineral and organic soil horizons in Florida using visible/near-infrared diffuse reflectance spectroscopy (VNIRS) calibrated by committee trees and partial least squares regression (PLSR). The derived VNIRS models were validated using independent datasets and explained up to 71 and 38% of the variance of SOC in mineral and organic horizons, respectively. We stratified the mineral horizons into seven soil orders and derived PLSR models for each order, which explained from 32% (Histosols) to 75% (Ultisols) of the variance of SOC concentration in validation mode. Estimates of SOC from all models were highly scattered along the regression lines, especially for high SOC values, and the slopes of the regression lines were generally <1 because VNIRS models tended to underestimate high SOC values and overestimate low SOC. Despite the great scatter of estimates in the prediction plots, VNIRS models had reasonable explanatory power for mineral horizons, given the heterogeneity of soils and environmental conditions in Florida, and have potential for the rapid assessment of SOC, with implications for regional SOC assessments, modeling, and monitoring. However, VNIRS models for organic horizons were hampered by small sample size and had very limited explanatory power.

Bench-scale recovery of phosphorus from flushed dairy manure wastewater.

Recovery of phosphorus (P) from flushed dairy manure in an easily-dewatered form would enable farmers to manage P as a resource rather than land-apply it in excess at environmental risk. The purpose of this study was to evaluate (i) the feasibility of P recovery and (ii) the form of recovered P from flushed dairy manure wastewater using crystallization in a fluidized-bed reactor. Wastewater was pumped directly from a dairy farm reservoir and continuously fed in parallel through four bench-scale fluidized-bed reactors deployed on-site. Chemical additives (NaOH and MgSO4) required for recovery were injected directly into the zone of fluidization. Recovered P forms were assessed by X-ray diffraction, scanning electron microscopy, and micro-elemental analysis. Recovery of P as poorly-crystalline hydroxylapatite (HAP) was documented in coatings ultrasonically removed from quartz seed grains following fluidization at elevated pH in conjunction with MgSO4 injection. Addition of MgSO4 was required to prevent CaCO3 precipitation upon pH elevation and hence enable calcium phosphate precipitation. It is likely that MgSO4 inhibited CaCO3 via formation of MgCO3 (aq). Periclase (MgO), which also served as an effective seed material, generated sufficient alkalinity at grain surfaces to precipitate abundant CaCO3 and in some cases detectable Ca phosphate even without NaOH addition to elevate pH of bulk solution.

Inhibition of calcium phosphate precipitation under environmentally-relevant conditions.

Precipitation of Ca phosphates plays an important role in controlling P activity and availability in environmental systems. The purpose of this study was to determine inhibitory effects on Ca phosphate precipitation by Mg(2+), SO(4)(2-), CO(3)(2-), humic acid, oxalic acid, biogenic Si, and Si-rich soil clay commonly found in soils, sediments, and waste streams. Precipitation rates were determined by measuring decrease of P concentration in solutions during the first 60 min; and precipitated solid phases identified using X-ray diffraction and electron microscopy. Poorly-crystalline hydroxyapatite (HAP: Ca(5)(PO(4))(3)OH) formed in control solutions over the experiment period of 24 h, following a second-order dependence on P concentration. Humic acid and Mg(2+) significantly inhibited formation of HAP, allowing formation of a more soluble amorphous Ca phosphate phase (ACP), and thus reducing the precipitation rate constants by 94-96%. Inhibition caused by Mg(2+) results from its incorporation into Ca phosphate precipitates, preventing formation of a well-crystalline phase. Humic acid likely suppressed Ca phosphate precipitation by adsorbing onto the newly-formed nuclei. Presence of oxalic acid resulted in almost complete inhibition of HAP precipitation due to preemptive Ca-oxalate formation. Carbonate substituted for phosphate, decreasing the crystallinity of HAP and thus reducing precipitation rate constant by 44%. Sulfate and Si-rich solids had less impact on formation of HAP; while they reduced precipitation in the early stage, they did not differ from the control after 24 h. Results indicate that components (e.g., Mg(2+), humic acid) producing relatively soluble ACP are more likely to reduce P stability and precipitation rate of Ca phosphate in soils and sediments than are components (e.g., SO(4)(2-), Si) that have less effect on the crystallinity.

Time dependency and irreversibility of water desorption by drinking-water treatment residuals: implications for sorption mechanisms.

Drinking-water treatment residuals (WTRs) are being evaluated as cost-effective sorption media for use in environmental remediation. Data from previous work have suggested that intraparticle phosphorus (P) diffusion into micropores is the rate-limiting mechanism of P sorption by WTRs. We used isothermal thermogravimetric analysis (TG) to study water desorption/resorption dynamics as they relate to steric diffusion rate limitations for prospective sorbates. Results showed that air-dried WTR particles contain significant amounts of water. Only about 40% of water desorbed isothermally (70 degrees C) for 10 h was readsorbed when particles were reexposed to ambient temperature and moisture conditions. This hysteresis related closely with time dependency of water loss, suggesting steric diffusional hindrance of water re-adsorption by meso- and micropores. The irreversibly desorbed water may be the component requiring increased kinetic energy to overcome diffusional resistance. Another possible factor in irreversibility could be pore shrinkage. Samples incubated for 12 months at 70 degrees C prior to TG analysis showed no hysteresis at 70 degrees C. Isothermal water losses with time fit well (r2 = 0.95) the diffusion model of Kabai. These results are consistent with an aqueous pore network that would account for high phosphorus sorption capacity and hysteresis that has been recently documented for WTRs.

Long-term phosphorus effects on evolving physicochemical properties of iron and aluminum hydroxides.

Iron (Fe) and aluminum (Al) hydroxides are highly reactive components in environmental processes, such as contaminant fate and transport. Phosphorus (P) sorption by these components can decrease environmental problems associated with excess accumulation of P in soils. The long-term stability of P sorbed by Fe/Al hydroxides is of major concern. Synthetic Fe and Al hydroxides coprecipitated with P (1:1 metal:P molar ratio) were incubated at 70 degrees C for 24 months to simulate natural long-term weathering processes that could influence the stability of sorbed P. Heat incubation (70 degrees C) of the untreated (no P) Al hydroxides resulted in drastic decreases (within the first month of incubation) in oxalate-Al extractability, specific surface area (SSA), and micropore volume with time. These changes were consistent with the formation of pseudoboehmite. Untreated Fe hydroxides showed no formation of crystalline components following heating (70 degrees C) for 24 months. Much smaller changes in oxalate-Al, P extractability, and SSA values were observed in the P-treated Al particles when compared with the untreated. Phosphorus treatment of both Fe and Al hydroxides stabilized the particle surfaces and prevented structural arrangements toward a long-range ordered phase. Slight reduction in SSA of the P-treated particles was related to dehydration phenomena during heating at 70 degrees C. Monitoring of physicochemical properties of the solids after heating at 70 degrees C for 2 years showed that sorbed P may be stable in the long-term. Understanding long term physicochemical properties may help engineers to optimize the Fe/Al hydroxides performance in several environmental/industrial applications.

Physicochemical properties related to long-term phosphorus retention by drinking-water treatment residuals.

Drinking-water treatment residuals (WTRs) are nonhazardous materials that can be obtained free-of-charge from drinking-water treatment plants to reduce soluble phosphorus (P) concentrations in poorly P sorbing soils. Phosphorus sorption capacities of WTRs can vary 1-2 orders of magnitude, on the basis of short-term equilibration times (up to 7 d), but studies dealing with long-term (weeks to months) P retention by WTRs are lacking. Properties that most affect long-term P sorption capacities are pertinent to the efficacy of WTRs as amendments to stabilize P in soils. This research addressed the long-term (up to 80 d) P sorption/desorption characteristics and kinetics for seven WTRs, including the influence of specific surface area (SSA), porosity, and total C content on the overall magnitude of P sorption by seven WTRs. The data confirm a strong but variable affinity for P by WTRs. Aluminum-based WTRs tended to have higher P sorption capacity than Fe-based WTRs. Phosphorus sorption with time was biphasic in nature for most samples and best fit to a second-order rate model. The P sorption rate dependency was strongly correlated with a hysteretic P desorption, consistent with kinetic limitations on P desorption from micropores. Oxalate-extractable Al + Fe concentrations of the WTRs did not effectively explain long-term (80 d) P sorption capacities of the WTRs. Micropore (CO2-based) SSAs were greater than BET-N2 SSAs for most WTRs, except those with the lowest (<80 g kg(-1)) total C content. There was a significant negative linear correlation between the total C content and the CO2/N2 SSA ratio. The data suggest that C in WTRs increases microporosity, but reduces P sorption per unit pore volume or surface area. Hence, variability in C content confounds direct relations among SSA, porosity, and P sorption. Total C, N2-based SSA, and CO2-based SSAs explained 82% of the variability in the long-term P sorption capacities of the WTRs. Prediction of long-term P sorption capacities for different WTRs may be achieved by taking into account the three proposed variables.

Intraparticle phosphorus diffusion in a drinking water treatment residual at room temperature.

Phosphorus (P) has been recognized as one of the major limiting nutrients that are responsible for eutrophication of surface waters, worldwide. Efforts have been concentrated on reducing P loads reaching water bodies, via surface runoff and/or leaching through a soil profile. Use of drinking water treatment residuals (WTRs) is an emerging cost-effective practice to reduce soluble P in poorly P-sorbing soils or systems high in P. Literature suggests that WTRs have huge P sorption capacities. We hypothesized that P sorption would be limited by diffusional constraints imposed by the WTR particles. Selected chemical and physical (specific surface area, particle size distribution) characteristics of an iron-based WTR were measured. Sorption P isotherms at room temperature were constructed, and sorption kinetics were monitored. An intraparticle diffusion model was utilized to fit the kinetic data. Results showed that the WTR dramatically reduced soluble P, showing nonequilibrium characteristics, even after 80 d of reaction. Specific surface area (SSA) measured with CO2 gas was significantly greater than the traditional BET-N2 value (28 versus 3.5 m2 g(-1)), suggesting that a large amount of internal surfaces might be present in the WTR. The intraparticle P diffusion model was modified to include the wide particle size distribution of the WTR. The intraparticle diffusion model fitted the data well (r2 = 0.83). We calculated a maximum apparent P diffusion coefficient value of 4 x 10(-15) cm2 s(-1), which agrees with published values for intraparticle diffusion in microporous sorbents. This work may be useful for predicting long-term sorption characteristics of WTRs, since WTRs have been suggested as potential long-term immobilizers of sorbed P in P-sensitive ecosystems.