Proton and cadmium adsorption by the archaeon Thermococcus zilligii: Generalising the contrast between thermophiles and mesophiles as sorbents

Adsorption by microorganisms can play a significant role in the fate and transport of metals in natural systems. Surface complexation models (SCMs) have been applied extensively to describe metal adsorption by mesophilic bacteria, and several recent studies have extended this framework to thermophilic bacteria. We conduct acid-base titrations and batch experiments to characterise proton and Cd adsorption onto the thermophilic archaeon Thermococcus zilligii. The experimental data and the derived SCMs indicate that the archaeon displays significantly lower overall sorption site density compared to previously studied thermophilic bacteria such Anoxybacillus flavithermus, Geobacillus stearothermophilus, G. thermocatenulatus, and Thermus thermophilus. The thermophilic bacteria and archaea display lower sorption site densities than the mesophilic microorganisms that have been studied to date, which points to a general pattern of total concentration of cell wall adsorption sites per unit biomass being inversely correlated to growth temperature

Selected geochemical reactions in heavy metal-chlorophenol systems

The subsurface mobility of metals and chlorophenols occurring together in contaminated groundwaters is controlled by the extent of aqueous complexation between them and by their adsorption onto aquifer materials. However, no previous experimental studies have quantified these interactions within a thermodynamic framework. This study has investigated the aqueous complexation of Cd, Pb and Cu by 2,4,6-trichlorophenolate (TCP--) and pentachlorophenolate (PCP--) at 25°C, using ion selective electrode potentiometry and ultraviolet spectrophotometry. A single 1:1 aqueous complex forms in each system, with log stability constants and is errors calculated to be: Cd(TCP) +, 2.5 +/- 0.2; Pb(TCP)+, 3.0 +/- 0.3; Cu(TCP) +, 4.9 +/- 0.2; Cd(PCP)+, 2.9 +/- 0.2; Pb(PCP) +, 2.8 +/- 0.3; and Cu(PCP)+, 4.2 +/- 0.2. The adsorption of Cd, Pb, Cu and Al by the common soil bacterium Bacillus licheniformis has been examined at 25°C in electrolytes of varying ionic strength. The cell walls display carboxyl, phosphate and hydroxyl surface functional groups, with pKa values and 1 s errors of 5.2 +/- 0.3, 7.5 +/- 0.4 and 10.2 +/- 0.5, respectively. The average log K values for Cd, Pb, Cu and Al adsorption onto the B. licheniformis surface carboxyl sites, with 1s errors, are 3.9 +/- 0.5, 4.6 +/- 0.3, 4.9 +/- 0.4 and 5.8 +/- 0.3, respectively. The Constant Capacitance double layer model provides the best description of ionic strength-dependent adsorption behaviour, although the model parameters vary between independently grown bacterial cultures, possibly due to cell wall variation arising from genetic variation during reproduction. There is insignificant adsorption of TCP to alpha-A12O3, but a strong affinity between TCP and B. subtilis. The TCP-B. subtilis adsorption data are best described by a site-specific model in which the negative form of TCP forms a 1:1 surface complex with the neutral hydroxyl functional groups of the bacteria (log K = 2.3 +/- 0.3) and the neutral form of T

Cadmium Ion Biosorption by the Thermophilic Bacteria Geobacillus stearothermophilus and G. thermocatenulatus

This study reports surface complexation models (SCMs) for quantifying metal ion adsorption by thermophilic microorganisms. In initial cadmium ion toxicity tests, members of the genus Geobacillus displayed the highest tolerance to CdCl(2) (as high as 400 to 3,200 μM). The thermophilic, gram-positive bacteria Geobacillus stearothermophilus and G. thermocatenulatus were selected for further electrophoretic mobility, potentiometric titration, and Cd(2+) adsorption experiments to characterize Cd(2+) complexation by functional groups within and on the cell wall. Distinct one-site SCMs described the extent of cadmium ion adsorption by both studied Geobacillus sp. strains over a range of pH values and metal/bacteria concentration ratios. The results indicate that a functional group with a deprotonation constant pK value of approximately 3.8 accounts for 66% and 80% of all titratable sites for G. thermocatenulatus and G. stearothermophilus, respectively, and is dominant in Cd(2+) adsorption reactions. The results suggest a different type of functional group may be involved in cadmium biosorption for both thermophilic strains investigated here, compared to previous reports for mesophilic bacteria

Metamodelling of Naturalised Groundwater Levels at a Regional Level in New Zealand

Groundwater is under pressure from increasing demands for agriculture, industry, domestic uses and support of ecosystems. Understanding the natural state of a groundwater system helps policy makers manage groundwater sustainably. Here we developed a metamodelling approach based on stepwise linear regression that emulates the functionality of physically-based models in the three primary aquifers of the Greater Wellington region of New Zealand. The inputs for the metamodels included local weather data, and nearby river flow data. The metamodels were calibrated and validated against the available simulations of naturalised groundwater level time series from physically-based models for 47 selected wells. For 36 of these wells, the metamodels had Nash-Sutcliffe Efficiency and coefficient of determination over 0.5, showing that they could adequately mimic naturalised groundwater level dynamics as simulated by the physically-based groundwater models. The remaining 11 wells had unsatisfactory performance and were typically located far away from rivers or along the coast. The results also showed that modelled groundwater levels in the aquifer’s recharge zone were more sensitive to short-term (less than 2 weeks lag) than long-term river flow (above 4 weeks to 1 year lag), whereas the converse pattern was observed for the aquifer’s discharge zone. Although some special considerations are needed, this metamodelling framework can be generally applied to other aquifers to support groundwater resource management at a lower cost than updating physically-based models

Arsenate–Ferrihydrite Systems from Minutes to Months: A Macroscopic and IR Spectroscopic Study of an Elusive Equilibrium

Sorption by ferrihydrite is an important control on As­(V) concentrations in many oxic aquatic systems. There are significant discrepancies in reported sorption constants (log­(<i>K</i>_As)), which presents a problem for quantifying and understanding this important system. A review of reported ferrihydrite–As­(V) sorption studies indicated a positive correlation between reaction time used in the experiments and the log­(<i>K</i>_As) values derived from the data. In this paper, we study the kinetics of As­(V) sorption over ≈3000 h in nine systems with varying pH and As­(V)/Fe. Ferrihydrite was stable in all systems containing As­(V), and the [As­(V)] in solution decreased linearly as a function of log­(<i>t</i>) (termed Elovich kinetics) over the full 3000 h in most systems. A stable [As­(V)] was only observed in systems with low As­(V)/Fe and low pH. Apparent As­(V) sorption constants were derived from the data at specific time intervals using the diffuse layer model and equations describing log­(<i>K</i>_As) values as a function of time provide a way to describe this elusive equilibrium. IR spectra support the hypothesis that slow interparticle diffusion is responsible for the slow approach to equilibrium. This work resolves discrepancies in previous studies of As­(V)–ferrihydrite and provides equations to allow for system appropriate log­(<i>K</i>_As) values to be used

Reference conditions and threshold values for nitrate-nitrogen in New Zealand groundwaters

Management of groundwater quality is assisted by an understanding of reference conditions, which describe the concentration ranges expected for key substances in the absence of human impact. This study evaluates reference conditions for NO3–N in New Zealand groundwater based on three complementary methods: hierarchical cluster analysis, relationships to groundwater age, and regression against a measure of land-use impact. The three methods result in very similar national-scale estimates of reference conditions for NO3–N concentration in oxic, minimally impacted groundwater, with the 80th, 90th and 95th percentiles found to be 1.65 ± 0.12, 1.97 ± 0.14 and 2.32 ± 0.14 mg/l, respectively (weighted average ± 95% confidence level), in good general agreement with previous studies from New Zealand and overseas. Anoxic groundwaters were treated separately for definition of reference conditions, with the 80th and 90th percentiles of NO3–N found to be 0.04 ± 0.01 and 0.16 ± 0.01, respectively (the 95th percentile could not be estimated reliably). For both oxic and anoxic groundwater, where a site-specific investigation has not been conducted to estimate reference conditions at a local scale, we suggest that the 80th percentile is an appropriate national-scale default threshold, to match the thresholds used for surface waters under the Australian and New Zealand Guidelines for Fresh and Marine Water Quality.</p

Assessing the representativeness of a groundwater quality monitoring network

The monitoring sites comprising a state of the environment (SOE) network must be carefully selected to ensure that they will be representative of the broader resource. Hierarchical cluster analysis (HCA) is a data-driven technique that can potentially be employed to assess the representativeness of a SOE monitoring network. The objective of this paper is to explore the use of HCA as an approach for assessing the representativeness of the New Zealand National Groundwater Monitoring Programme (NGMP), which is comprised of 110 monitoring sites across the country

Use of hierarchical cluster analysis to assess the representativeness of a baseline groundwater quality monitoring network: comparison of New Zealand's national and regional groundwater monitoring programs

Baseline monitoring of groundwater quality aims to characterize the ambient condition of the resource and identify spatial or temporal trends. Sites comprising any baseline monitoring network must be selected to provide a representative perspective of groundwater quality across the aquifer(s) of interest. Hierarchical cluster analysis (HCA) has been used as a means of assessing the representativeness of a groundwater quality monitoring network, using example datasets from New Zealand. HCA allows New Zealand's national and regional monitoring networks to be compared in terms of the number of water-quality categories identified in each network, the hydrochemistry at the centroids of these water-quality categories, the proportions of monitoring sites assigned to each water-quality category, and the range of concentrations for each analyte within each water-quality category. Through the HCA approach, the National Groundwater Monitoring Programme (117 sites) is shown to provide a highly representative perspective of groundwater quality across New Zealand, relative to the amalgamated regional monitoring networks operated by 15 different regional authorities (680 sites have sufficient data for inclusion in HCA). This methodology can be applied to evaluate the representativeness of any subset of monitoring sites taken from a larger network