Trace metals in river bed sediments: an assessment of their partitioning and bioavailability by using multivariate exploratory analysis

Concentration of heavy metals in soil and leaves of Conocarpus Erectus Tree: A Biomonitoring Study, Ahvaz, Iran

Heavy metals biomonitoring was performed using the Conocarpus erectus tree in Ahvaz city. Composite leaf and soil samples were collected from 23 selected stations. The concentrations of heavy metals (Pb, Zn, Cd, Cu, Fe and Mn) in leaf and soil samples, and bioavailability of metals in soil samples were determined. Examination of soil physicochemical parameters proved neutral to slightly alkaline nature, and low organic matter content in the soil samples. The mean concentration of heavy metals in soil was: Fe > Mn > Zn > Pb > Cu. Ecological risk assessment of heavy metals was in the range of safe to low risk (RI < 150). Although the concentration of metals in the more polluted areas was higher in both leaf and soil samples, there was no significant relationship between the concentrations of metals in the leaf and soil samples. This relationship is even lower between the bioavailable component of metals in the soil and the concentration of metals in the leaves. Transfer factor values based on total contents of metals in soil samples indicated that Conocarpus erectus is mostly contaminated with Zn and Cu. The results of Accumulation factor of plant revealed that Pb, Zn, and Fe were mostly enriched in the plant by anthropogenic activities. MAI values in heavy and light traffic, and industrial areas were 11.88, 8.01 and 8.15, respectively. In general, it is evident that the Conocarpus erectus leaves accumulate heavy metals in polluted areas, so it can be used as a bioindicator of air pollution with heavy metals in regions with similar conditions.

Cadmium immobilization in lake sediment using different crystallographic manganese oxides: Performance and mechanism

Cd pollution in sediments poses severe threats to environmental safety and human health. Mn oxides have potential merit for the remediation of Cd pollution in sediment but have not received enough attention. Although Mn oxides have proven effective as adsorbents for removing heavy metals from water/wastewater, the performance and the underlying mechanism of Cd immobilization in sediments by Mn oxides remain unclear. Here, three crystallographic Mn oxides δ-MnO₂, γ-MnOOH, and Mn₃O₄ were used as amendments to investigate their potential for the in situ immobilization of Cd in lake sediment. Experimental data showed that when the sediment samples were treated with synthesized Mn oxides at dosages of 2% and 6% (w/w) for 56 days, the TCLP (toxicity characteristic leaching procedure) leachable Cd in the sediment decreased by 43.9-66.81%, and the PBET (physiologically based extraction test) extractable Cd decreased by 45.16-99.40%. Additionally, the acid-soluble fraction of Cd was partially transformed to a residual fraction, resulting in a 27.55-35.49% decrease in acid-soluble Cd and a 25.16-30.36% increase in the residual Cd fraction. Sediment pH and oxidation-reduction potential were important factors affecting the bioavailability of Cd in the remediation process. Furthermore, scanning electron microscopy, X-ray diffractometer, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy analysis illustrated that the interaction between the amendment and Cd mainly involved complexation with O-containing groups, ion-exchange as > OCd⁺, and precipitation with carbonate. The efficient remediation capacity and associated mechanism for Mn oxides provide insights for the improved restoration of heavy metal-contaminated sediment.

Current Options in the Valorisation of Vine Pruning Residue for the Production of Biofuels, Biopolymers, Antioxidants, and Bio-Composites following the Concept of Biorefinery: A Review

Europe is considered the largest producer of wine worldwide, showing a high market potential. Several wastes are generated at the different stages of the wine production process, namely, vine pruning, stalks, and grape marc. Typically, these residues are not used and are commonly discarded. Portugal generates annually approximately 178 thousand metric tons of wine production waste. In this context, the interest in redirecting the use of these residues has increased due to overproduction, great availability, and low costs. The utilization of these lignocellulosic biomasses derived from the wine industry would economically benefit the producers, while mitigating impacts on the environment. These by-products can be submitted to pre-treatments (physical, chemical, and biological) for the separation of different compounds with high industrial interest, reducing the waste of agro-industrial activities and increasing industrial profitability. Particularly, vine-pruning residue, besides being a source of sugar, has high nutritional value and may serve as a source of phenolic compounds. These compounds can be obtained by bioconversion, following a concept of biorefinery. In this framework, the current routes of the valorisation of the pruning residues will be addressed and put into a circular economy context.

Contamination and ecological risk assessment of trace elements in sediments of the Anzali Wetland, Northern Iran

In this paper, concentrations of some heavy metals in surficial sediments of the International Anzali Wetland were measured, this wetland is located in the northern part of Iran. Sediment pollution levels were examined and analyzed using reliable pollution indices including Pollution Load Index (PLI), Geoaccumulation Index (Igeo) and Enrichment Factor (CF), and finally it was revealed that heavy metal pollution ranged from low to moderate loads in the wetland. According to Sediment Quality Guidelines (SQGs) and Ecological Risk Index (ERI), it was concluded that As and Ni may have significant toxic impacts on aquatic organisms and also according to Effect Range Median (ERM), the toxicity probability of sediments in the Anzali wetland was estimated at 21%.

Selection of priority management of rivers by assessing heavy metal pollution and ecological risk of surface sediments

This study aimed to select rivers of priority management through the assessment of heavy metal pollution of sediments. We investigated the distribution characteristics of heavy metals in surface sediments of the Nakdong River in South Korea and used various pollution indices to assess pollution risk and identify factors influencing pollution. The kriging method was used to determine heavy metal distribution. The pollution load index, potential ecological risk index, mean PEL quotient, and the Canada Council of Ministers of the Environment sediment quality index were used as sediment pollution assessment methods. The toxicity evaluation was performed on sites that appeared to be contaminated, by applying existing methods for assessing sediment pollution level and the national standards for evaluating the pollution level. The toxicity test was performed on Hyalella azteca, and a methodology for assessing sediment pollution level was proposed. Ecotoxicity was assessed at seven sites that were found to have heavy metal contaminants. The results showed that sites N1, N8, T28, and T29 were not toxic, while T8, T19, and T21 were. Thus, this study shows that high heavy metal pollution does not necessarily lead to a toxic environment. To assess sediment pollution, an additional assessment of toxicity should be made, along with assessments of existing sediment pollution. Our results demonstrate that streams showing high sediment pollution levels should be granted priority in management. The efforts should particularly focus on Cu at T8, Cr at T19, and Hg at T21.

Multivariate statistical analyses for water and sediment quality index development: A study of susceptibility in an urban river

In this study, multivariate statistical analyses were performed to develop water and sediment quality indexes, allowing us (i) to select with reliability the most appropriate chemical variables for the evaluation of river quality susceptibility; (ii) to weight the influence of each variable based on monitored data; (iii) to consider possible synergism or antagonism derived from the combined effect of several pollutants; and (iv) to express the quality as a deviation from selected site-specific reference conditions. For the establishment of these threshold/maximum values, combining two biological indicators related to denitrifying bacteria in sediments turned out to be applicable to ensure compliance with the European water quality standard. The joint implementation of water and sediment quality indexes assisted us in the rapid detection of the deleterious effect of different anthropogenic contamination sources, as well as the influence of hydrological regime seasonality on river quality. In addition, metal-dependent water quality appeared to be coupled to sediment dynamics, since they were preferentially adsorbed onto sediments during low flow seasons, whereas there was potential for metal mobilization to water during sediment resuspension in high flow seasons. Therefore, an annual determination of sediment quality index was also recommended as suitable tool for prospective monitoring water quality, identifying those sites which could deserve special attention during certain periods, and planning future strategies for river quality improvement. However, two limitations were found: (1) sediment was not appropriate for water physicochemical quality early monitoring due to organic matter and nutrient continuous transformation; and (2) a multimetric index did not provide a concise and definitive quality information, thus a new tool for combining with quality index was proposed for specifically evaluate the water and sediment quality by identifying pollutant/s of concern at each location.

Analysis of Biomaterials as Green Coagulants to Control Suspended Solids for Surface Water Treatment

This study explores the use of natural, ecological coagulant-flocculants to reduce suspended particles in water. Three compounds were tested, namely: diatomaceous earth, calcium lactate and lactic acid. For this purpose, experiments in jar tests were carried out and the best compound was submitted to an optimization in order to evaluate the most significant parameters affecting its use as coagulant-flocculant. First results evidenced that lactic acid remove 71% of the suspended particles during the first five minutes, and up to 83% during the first 15 min. To optimize its use, the range of suspended particles concentration, lactic acid dose and salinity gradient was tested by means of an incomplete 3³ factorial design. This technique allows reducing the number of experiments to be carried out through a response surface methodology, which enables to infer the values of the dependent variables in not studied situations, by means of predictive equations. As a result of the experiments carried out, optimal conditions to remove suspended particles were set at a lactic acid concentration of 1.75 g·L^-1. As lactic acid may be obtained biotechnologically from organic wastes, this use supposes a promising area by keeping products and materials in use and contributing to a circular economy.

Evaluating the role of particle size on urban environmental geochemistry of metals in surface sediments

In this study, non-destructive techniques (X-ray Diffraction, Infrared and Scanning Electron Microscopy with Energy Dispersive spectroscopies) and invasive procedures (pseudo-total and sequential metal extraction methodologies) were used to highlight the significance of evaluating different particle sizes of sediments for assessing the potential environmental and health implications of metal geochemistry in an urban ecosystem. The variability in composition and properties between bulk (<2 mm) and fine (<63 μm) fractions influenced the availability, and by extension, the toxicity of metals. Indeed, the fine fraction presented not only higher metal pseudo-contents, but also greater available metal percentages. Besides the larger surface area per unit of mass and the high content of clay minerals, it was observed that it was principally Fe/Mn oxyhydroxides that favour adsorption of metals on the fine surface sediments. However, although we demonstrated that the origin of metals in the bulk surface sediments was predominantly lithogenic, use of the <2 mm fraction proved to be a useful tool for identifying different sources of available metals throughout the Deba River catchment. Specifically, discharges of untreated industrial and urban wastewaters, and even effluents from wastewater treatment plants were considered to greatly increase the health risk associated with metal availability. Finally, an evaluation of sediment dynamics in different hydrological conditions has highlighted the role played by each particle size as a vector of metal transport towards the coastal area. While resuspension of fine surface sediments notably induced significantly higher particulate metal concentrations in water during the dry season, resuspension of bulk surface sediments and, fundamentally, downstream transport of suspended particulate matter became more relevant and lowered the ecological risk during the wet season. Greater attention therefore needs to be paid to the new hydrological scenarios forecast to result from climate change, in which longer seasons with low river discharges are forecast.

Holistic approach for quantification and identification of pollutant sources of a river basin by analyzing the open drains using an advanced multivariate clustering

Global scarcity of freshwater has been gearing towards an unsustainable river basin management and corresponding services to the humans. It needs a holistic approach, which exclusively focuses on effective river water quality monitoring and quantification and identification of pollutant sources, in order to address the issue of sustainability. These days, rivers are heavily contaminated due to the presence of organic and metallic pollutants released from several anthropogenic sources, such as industrial effluents, domestic sewage, and agricultural runoff. It is astonishing to note that even in many developing countries, most of these contaminants are carried through open drains, which enter river premises without proper treatment. Such practice not only devastates riverine ecosystem but also gives rise to deadly diseases, such as minimata and cancer in humans. Considering these issues, the present study develops a novel approach towards simultaneous identification of major sources of pollution in the rivers, along with critical pollutants and locations using an advanced hierarchical cluster and multivariate statistical analysis. A systematic approach has been developed by agglomerating both R-mode and Q-mode analysis, which develops monoplots, two-dimensional biplots, rotated component matrices, and dendrograms (using "SPSS" and "Analyse It" software) to reveal relationships among various quality parameters to identify the pollutant sources along with clustering of critical sampling sites and pollutants. A case study of the Ganges River Basin of India has been considered to demonstrate the efficacy and usefulness of the model by analyzing 85 open drains. Both organic and metallic pollutants are analyzed simultaneously as well as separately to get a holistic understanding of all the relationships and to broaden the perspective of water characterization. Results provide a comprehensive guidance to the policy makers and water managers to optimize corrective efforts, minimize further damage, and improve the water quality condition to ensure sustainable development of the river basin.

Evaluation of temporary seasonal variation of heavy metals and their potential ecological risk in Nzhelele River, South Africa

Surface water is often used as alternative source of drinking water in many regions of the world where the potable water supply is erratic or not present. The concentration of heavy metals was assessed using an Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES). The impact of temporary seasonal variation on the contamination level was also assessed. Contamination factor (CF) and Hakanson’s potential ecological risk (Er and RI) indices were used to evaluate the toxicity levels of the metals in the sediments of the river. Higher concentrations of Fe, Mn, Pb, Cu and Zn were determined in the dry season in the river water whereas in the sediments higher levels were recorded for Al, Fe, Cd, Cr, Cu and Zn in the wet season. The average CF values for all the metals showed a low contamination level in the sediment, except for Cu and Cd which had moderate and considerable contamination levels, respectively. Similarly, Cu showed a moderate ecological risk level (Er = 62.90) only in March 2014, for other months Cu and the other metals investigated had Er values < 40 which implies low ecological risk. The mean relative abundance of heavy metals in the sediments follows the trend Al>Fe>Mn>u>Zn>Cr>Pb>Cd.

Estimation and influence of physicochemical properties and chemical fractions of surface sediment on the bioaccessibility of Cd and Hg contaminant in Langat River, Malaysia

This study applied the use of sequential extraction technique and simple bioaccessibility extraction test to quantify the bioavailable fractions and the human bioaccessible concentration of metals collected from nine stations in surface sediment of the Langat River. The concentrations of total and bioaccessible metals from different stations were in the range of 0.49-1.04, 0.10-0.32 μg g^-1 for T-Cd, Bio-Cd, respectively, and 12.9-128.03, 2.06-8.53 μg kg^-1 for T-Hg, Bio-Hg, respectively. The results revealed highest R-Bio-Cd in Banting station (55.3 %), while the highest R-Bio-Hg was in Kajang station (49.61 %). The chemical speciation of Cd in most sampling stations was in the order of oxidisable-organic > residual > exchangeable > acid-reducible, while speciation of Hg was in the order of exchangeable > residual > oxidisable-organic > acid-reducible. The correlation matric of mean content showed that the TOM, particle size and Mg⁺⁺ in polluted surface sediments was highly correlated with total mercury. The PCA showed that the main factors influencing the bioaccessibility of Hg in surface sediments were the sediment TOM, F1 (EFLE) and F3 (oxidation-organic), while the factor influencing the bioaccessibility of Cd was the F3 (oxidation-organic) and T-Cd.

Investigating geochemical factors affecting heavy metal bioaccessibility in surface sediment from Bernam River, Malaysia

The present study applied the use of sequential extraction technique and simple bioaccessibility extraction test to investigate the bioavailable fractions and the human bioaccessible concentration of metals collected from nine stations in surface sediment of the Bernam River. The concentrations of total and bioaccessible metals from different stations were in the range of 0.30-1.43 μg g^-1 and 0.04-0.14 μg g^-1 for total cadmium and bioaccessibility of cadmium, respectively, 6.20-288 μg kg^-1 and 2.06-8.53 μg kg^-1 for total mercury and bioaccessibility of mercury, respectively, and 9.2-106.59 μg g^-1 and 0.4-2.75 μg kg^-1 for total tin and bioaccessibility of tin, respectively. The chemical speciation of Cd in most sampling stations was in the order of oxidisable-organic > acid-reducible > residual > exchangeable, while the chemical speciation of Hg was in the order of exchangeable > residual > oxidisable-organic > acid-reducible and the chemical speciation of Hg was in the order of residual > oxidisable-organic > acid-reducible > exchangeable. The principal component analysis showed that the main factors influencing the bioaccessibility of mercury in surface sediments were the sediment total organic matter, cation exchange capacity, and easily, freely, or leachable and exchangeable fraction, and the factors influencing the bioaccessibility of tin were the total tin and cation exchange capacity, while the bioaccessibility of Cd in surface sediments was influenced by the only factor which is the easily, freely, or leachable and exchangeable fraction.

Chemical and physiological metal bioaccessibility assessment in surface bottom sediments from the Deba River urban catchment: Harmonization of PBET, TCLP and BCR sequential extraction methods

In the present study, the physiologically based extraction test PBET (gastric and intestinal phases) and two chemical based extraction methods, the toxicity characteristic leaching procedure (TCLP) and the sequential extraction procedure BCR 701 (Community Bureau of Reference of the European Commission) have been used to estimate and evaluate the bioaccessibility of metals (Fe, Mn, Zn, Cu, Ni, Cr and Pb) in sediments from the Deba River urban catchment. The statistical analysis of data and comparison among physiological and chemical methods have highlighted the relevance of simulate the gastrointestinal tract environment since metal bioaccessibility seems to depend on water and sediment properties such as pH, redox potential and organic matter content, and, primordially, on the form in which metals are present in the sediment. Indeed, metals distributed among all fractions (Mn, Ni, Zn) were the most bioaccessible, followed by those predominantly bound to oxidizable fraction (Cu, Cr and Pb), especially near major urban areas. Finally, a toxicological risk assessment was also performed by determining the hazard quotient (HQ), which demonstrated that, although sediments from mid- and downstream sampling points presented the highest metal bioaccessibilities, were not enough to have adverse effects on human health, Cr being the most potentially toxic element.

Sediment metal bioavailability in Lake Taihu, China: evaluation of sequential extraction, DGT, and PBET techniques

The European "Community Bureau of Reference" (BCR) sequential extraction procedure, diffusive gradient in thin-films technique (DGT), and physiologically based extraction test were applied to assess metal bioavailability in sediments of Lake Taihu (n = 13). Findings from the three methods showed that Cd was a significant problem in the western lake whereas Cu, Zn, and Ni pollution was most severe in the northern lake. Results from the sequential extraction revealed that more than 50 % of the Cu and Zn were highly mobile and defined within the extractable fraction (AS1 + FM2 + OS3) in the majority of the sediments, in contrast extractable fractions of Ni and Cd were lower than 50 % in most of the sampling sites. Average Cu, Zn, Ni, and Cd bioaccessibilities were <50 % in the gastric phase. Zn and Cd bioaccessibility in the intestinal phase was ∼50 % lower than the gastric phase while bioaccessibilities of Cu and Ni were 47-57 % greater than the gastric phase. Linear regression analysis between DGT and BCR measurements indicated that the extractable fractions (AS1 + FM2 + OS3) in the reducing environment were the main source of DGT uptake, suggesting that DGT is a good in situ evaluation tool for metal bioavailability in sediments.

Influence of anthropogenic inputs and a high-magnitude flood event on metal contamination pattern in surface bottom sediments from the Deba River urban catchment

The purpose of this study was to assess the influence of anthropogenic factors (infrastructure construction and industrial and wastewater inputs) and hydrological factors (high-magnitude flood events) on metal and organic contamination and on the source variability of sediments taken from the Deba River and its tributaries. The pollution status was evaluated using a sequential extraction procedure (BCR 701), enrichment factor, individual and global contamination factors and a number of statistical analysis methods. Zn, Cu and Cr were found to have significant input from anthropogenic sources, with moderately severe enrichment, together with an extremely high potential risk of contamination. The principal scavenger of Cu and Cr was organic matter, whereas Zn was uniformly distributed among all non-residual fractions. For Fe, the anthropogenic contribution was more obviously detected in bulk sediments (<2 mm) than in fine fractions (<63 μm). Finally, the recent construction of a rail tunnel traversing Wealden Facies evaporites, together with intense rainfalls, was the main reason for the change in the source variability of bottom sediments and metal distribution in headwaters. The occurrence of a high-magnitude flood event resulted in a washout of the river bed and led to a general decrease in fine-grained sediment and metal concentrations in labile fractions of channel-bottom sediments, and a consequent downstream transfer of the pollution.

Revisiting methods for the determination of bioavailable metals in coastal sediments

A simple methodology for the determination of bioavailability of fourteen metals in coastal sediments has been developed by simulating the conditions of digestive process of marine fishes. With this aim, a representative sediment composite sample was treated with hydrochloric acid solutions at different pH values, temperatures and contact times, in the presence and absence of Pepsin and Trypsin. The addition of Pepsin and Trypsin did not affect the extraction of most elements. As a result of the present study, the digestion with a hydrochloric acid solution at pH 1, 40°C and 12h is proposed. Adjustments of the temperature and time reaction could be made according to the specific ecosystem under study. The amount of metal extracted by other methods based on acetic acid was lower than that extracted by HCl treatment proposed.

Risk assessment of heavy metals in surface sediments from the Yanghe River, China

The magnitude and ecological relevance of metal pollution from the upstream of water sources after emergency pollution events was investigated by applying a set of complementary sediment quality assessment methods: (1) geochemical assessment based on background value (the geoaccumulation index); (2) comparisons with sediment quality guidelines (SQGs); (3) an evaluation of the combined pollution according to the risk index (RI); and (4) investigation of the chemical patterns of target heavy metals (Cd, Zn, Cr, Pb, Ni). The geoaccumulation indices (I_geo) suggested that the magnitude of heavy metal pollution of the sediment of Yanghe River decreased in the order of Cd > Zn > Pb > Cr > Ni. Risk analysis also suggested that Cd and Zn concentrations were sufficiently elevated as to cause adverse biological effects in this study area. According to the RI values, 27% of total sampling sites showed considerable ecological risk for the water body, and 53% of total sampling sites showed very high ecological risk for the waterbody. Sediment-bound Cd was found to be predominantly associated with the exchangeable phase of the sediment (25%–68%), while Cr, Ni, Zn and Pb showed the strongest association with the residual fractions (60%–92%, 53%–67%, 24%–85% and 35%–67%, respectively).

The Pseudomonas community in metal-contaminated sediments as revealed by quantitative PCR: a link with metal bioavailability

Pseudomonas bacteria are ubiquitous Gram-negative and aerobic microorganisms that are known to harbor metal resistance mechanisms such as efflux pumps and intracellular redox enzymes. Specific Pseudomonas bacteria have been quantified in some metal-contaminated environments, but the entire Pseudomonas population has been poorly investigated under these conditions, and the link with metal bioavailability was not previously examined. In the present study, quantitative PCR and cell cultivation were used to monitor and characterize the Pseudomonas population at 4 different sediment sites contaminated with various levels of metals. At the same time, total metals and metal bioavailability (as estimated using an HCl 1 m extraction) were measured. It was found that the total level of Pseudomonas, as determined by qPCR using two different genes (oprI and the 16S rRNA gene), was positively and significantly correlated with total and HCl-extractable Cu, Co, Ni, Pb and Zn, with high correlation coefficients (>0.8). Metal-contaminated sediments featured isolates of the Pseudomonas putida, Pseudomonas fluorescens, Pseudomonas lutea and Pseudomonas aeruginosa groups, with other bacterial genera such as Mycobacterium, Klebsiella and Methylobacterium. It is concluded that Pseudomonas bacteria do proliferate in metal-contaminated sediments, but are still part of a complex community.

Mercury distribution, speciation and bioavailability in sediments from the Pearl River Estuary, Southern China

Surface sediments and sediment cores collected from the Pearl River Estuary (PRE) were analyzed for total mercury (THg) concentrations and speciation using a sequential extraction method. The mobility of Hg in sediments was also assessed using a series of single extraction methods. The surface sediments from the PRE showed slightly elevated levels of Hg, with concentrations ranging from 109 to 453 ng/g. The vertical profile of THg in sediment cores indicated an accelerated input of Hg over the past decades. The organo-chelated and strong-complexed Hg species were the dominant Hg species in the sediments, while the more mobile phases of Hg made up less than 0.5% of THg. Less than 10% of the Hg in the sediments was extracted by single extraction, depending on the extractant employed. Significant relationships were found between the total organic carbon and THg, geochemical speciation, and extractability, indicating the important role of organic matter in controlling the distribution, mobility, and bioavailability of Hg in sediments.

Potential adverse effects of applying phosphate amendments to immobilize soil contaminants

Seven-day batch equilibrium experiments were conducted to measure the efficacy of four phosphate amendments (trisodium trimetaphosphate [TP3], dodecasodium phytate [Na-IP6], precipitated calcium phytate [Ca-IP6], and hydroxyapatite [HA]) for immobilizing Ni and U in organic-rich sediment. Using the eight-step modified Miller's sequential extraction procedure and the USEPA's Toxicity Characteristic Leaching Procedure, the effect of these amendments on the distribution of Ni and U was assessed. Relative to unamended controls, equilibrium aqueous-phase U concentrations were lower following HA and Ca-IP6 additions but higher following TP3 and Na-IP6 amendments, whereas aqueous Ni concentrations were not decreased only in the Na-IP6 amended treatment relative to the control. The poor rates of contaminant immobilization following TP3 and Na-IP6 amendments correlate with the dispersion of organic matter and organo-mineral colloids, which probably contain sorbed U and Ni. While all amendments shifted the U distribution toward more recalcitrant soil fractions, Ni was redistributed to more labile soil fractions. This study cautions that the addition of orthophosphates and organophosphates as contaminant immobilizing amendments may in fact have adverse effects, especially in high-organic soils. Particular attention is warranted at sites with mixed contaminants with varying geochemical behaviors.