Ferrous iron phosphorus in sediments: development of a quantification method through 2,2'-bipyridine extraction

Improving nutrients ratio in class A biosolids through vivianite recovery: Insights from a wastewater resource recovery facility

Class A biosolids from water resource recovery facilities (WRRFs) are increasingly used as sustainable alternatives to synthetic fertilizers. However, the high phosphorus to nitrogen ratio in biosolids leads to a potential accumulation of phosphorus after repeated land applications. Extracting vivianite, an FeP mineral, prior to the final dewatering step in the biosolids treatment can reduce the P content in the resulting class A biosolids and achieve a P:N ratio closer to the 1:2 of synthetic fertilizers. Using ICP-MS, IC, UV-Vis colorimetric methods, Mössbauer spectroscopy, and SEM-EDX, a full-scale characterization of vivianite at the Blue Plains Advanced Wastewater Treatment Plant (AWTTP) was surveyed throughout the biosolids treatment train. Results showed that the vivianite-bound phosphorus in primary sludge thickening, before pre-dewatering, after thermal hydrolysis, and after anaerobic digestion corresponded to 8 %, 52 %, 40 %, and 49 % of the total phosphorus in the treatment influent. Similarly, the vivianite-bound iron concentration also corresponded to 8 %, 52 %, 40 %, and 49 % of the total iron present (from FeCl3 dosing), because the molar ratio between total iron and total incoming phosphorus was 1.5:1, which is the same stoichiometry of vivianite. Based on current P:N levels in the Class A biosolids at Blue Plains, a vivianite recovery target of 40 % to ideally 70 % is required in locations with high vivianite content to reach a P:N ratio in the resulting class A biosolid that matches synthetic fertilizers of 1:1.3 to 1:2, respectively. A financial analysis on recycling iron from the recovered vivianite had estimated that 14-25 % of Blue Plain's annual FeCl3 demand can potentially be met. Additionally, model simulations with Visual Minteq were used to evaluate the pre-treatment options that maximize vivianite recovery at different solids treatment train locations.

Distribution characteristics of reactive silicon in six water bodies in the Yangtze River Basin in China

Terrestrial silicon (Si) from biogeochemically weathered rocks and soils into oceans must pass through several water bodies, resulting in some Si immobilized. Hence, the knowledge on Si distribution characteristics in different water bodies at a basin scale is helpful to understand Si immobilization. A total of 65 surface sediments and corresponding overlying water samples were sampled from six water bodies (Dianchi Lake, DL; Dadu River, DR; Tuojiang River, TR; Honghu Lake, HL; Donghu Lake, DhL; Taihu Lake, TL) in the Yangtze River Basin of China, total dissolved Si (TDSi) in overlying water and exchangeable Si (Ex-Si), active non-biogenic Si (NBSi), and total acid dissolved Si (TADSi) in sediments were analyzed. Water chemical parameters (pH, EC, and TDP) and sediment components (LOI, TN, TP, and TADFe) showed that the water environment characteristics of six water bodies differed. TDSi differed among regions and between lakes and rivers, significantly higher in water bodies in the upper reaches and rivers than the middle or lower reaches and lakes (p < 0.05), respectively. Ex-Si in sediments in the upper reaches was significantly higher than in the middle or lower reaches (p < 0.05), except for DhL, whose Ex-Si was the highest. Mean TADSi and active NBSi were significantly higher in lakes than rivers (p < 0.05). Oxidation of sediments significantly increased TDSi in overlying water and active NBSi in sediments (p < 0.01). Si forms in six water bodies significantly depended on components of the sediments (e.g. active Ca2+, Mg2+, Fe, and Al3+) and water chemical parameters (p < 0.05). Our results suggest that immobilization of Si in water bodies in the Yangtze River Basin depends on the types of water bodies and sediments, lakes and Fe-Al dominated sediments have a high potential to immobilize Si, but anthropogenic interference should not be ignored.

Correlation between phosphorus removal technologies and phosphorus speciation in sewage sludge: focus on iron-based P removal technologies

Phosphorus recovery from sewage sludge as secondary raw materials or as a direct P-rich fertiliser is one of the top frontrunner solutions to tackle Phosphorus (P) scarcity and depletion. However, the efficiency of this P recovery process greatly depends on its phosphorus dissolution potential, which in return relies on the phosphorus speciation in the sewage sludge. This article investigates the potential correlation between P speciation in sewage sludge and the iron-based P removal technologies used in sewage treatment plants (STP) through an innovative sequential extraction method based on the SEDEX method that distinguishes quantitatively between ferrous bound phosphate and ferric bound phosphate. XRD and SEM-EDX were also used to characterise P and Fe species in the studied sludge qualitatively. Principal component analysis showed that the sludge characterised by P bound to ferric iron (as the dominant P fraction) are mostly correlated with sludge produced from the CPR process (chemical phosphorus removal) and primary sludge. Moreover, sludge with a non-negligible amount of P bound to ferrous iron were correlated with sludge from the mixed EBPR-CPR process (Enhanced Biological P Removal assisted with CPR). However, Vivianite was only found in CPR sludge with Fe/P molar ratio higher than 0.6.

Evaluation of the Potential Release Risk of Internal N and P from Sediments—A Preliminary Study in Two Freshwater Reservoirs in South China

Growing evidence has demonstrated the influence of internal nitrogen (N) and phosphorus (P) on harmful algae blooms in eutrophic freshwater ecosystems. However, the main controlling factors for internal N and P release risks, and whether these factors vary as environmental conditions change, remains poorly understood. We evaluated potential release risks of N and P from sediments in two freshwater reservoirs in Beihai City, southern China, by evaluating apparent nutrient fluxes during simulated static incubation experiments at two temperatures (15 °C and 25 °C). Sediments were analyzed to determine their basic properties as well as N and P fractions. Results showed that the main controlling factors of the apparent fluxes in dissolved total P, soluble reactive P, total N, and ammonium were related to sediment adsorption properties, redox properties, and microbial-mediated properties (e.g., water-extractable P, total inorganic N, redox-sensitive P, total organic carbon, organic P). The primary controlling factors for apparent N and P fluxes were dependent on the form of N and P and changed with temperature. The results suggest that care should be taken when simply using total N and P contents in sediments to evaluate their internal nutrient release risks.

Effects of consecutive culture of Penaeus vannamei on phosphorus transformation and microbial community in sediment

Phosphorus (P) is highly related to water quality during shrimp culture. Recognizing P transformation in pond-based cultures is crucial for sustainable and healthy aquaculture. However, P transformation remains unclear in the sediment of Penaeus vannamei cultures, although commercial species have been pervasive worldwide. To determine P transformation, samples with different culture years were collected from Zhejiang province, China. Sequential chemical extraction was applied to reveal the composition of inorganic P, while phosphatase activity was used to evaluate the biomineralization of organic P. The results indicated that the consecutive culture of Penaeus vannamei promoted the dissolution potential of sedimentary P. This was attributed to anoxic iron reduction that increased the formation of loosely bound P and Fe (II)-P. However, this phenomenon was dominated by biomineralization, which transformed the organic P to inorganic P. The results suggested that consecutive culture changed the microbial community structure in the sediment as well as the gene functions. The Shannon Wiener index showed that increasing the culture duration significantly decreased the stability of the microbial community. Overall, this study suggests that long-term consecutive culture of Penaeus vannamei may increase the P release potential of the sediment, which increases the risk of pond eutrophication.

Available acid consumption capacity of sediments in six water bodies in the Yangtze River Basin in China

Acid-base reactivity is a fundamental property of sediments and is responsible for sediments' multiple roles in aquatic ecosystems. However, little information currently exists about the composition, magnitude, and change of the available acid consumption capacity (AACC) of sediments. To optimize reaction conditions, we developed operational procedures to determine AACC using base titration to recover surplus acid in suspensions. We characterized the sediment AACC of Dianchi Lake (DL), Daduhe River (DR), Tuojiang River (TR), Honghu Lake (HL), Wuhan Donghu Lake (DhL), and Taihu Lake (TL) in the Yangtze River Basin, China. The procedure demonstrated that reacting 40 mL 0.1 M HCl with fresh sediments equivalent to 1.0 g dry weight for 4 h and recovering surplus acid in the suspension by NaOH titration to an endpoint pH of 3.0 could determine sediment AACC. Sediment AACC in the Yangtze River Basin had high regional variability. The mean magnitude of AACC among sites was ranked DL > DR > DhL > TR > HL > TL, which is extremely similar to their geographical location from the upper to lower reaches of the Yangtze River Basin. Qualitative results from acid titration curves showed that more components contributed to AACC in DL, DR, TR, and DhL sediments than to those in HL and TL sediments. The correlation between AACC and the total amount of multivalent cations released indicated that AACC depended significantly on labile acid-soluble minerals that contain multivalent cations (Fe³⁺, Fe²⁺, Ca²⁺, Al³⁺, Mg²⁺, and Mn²⁺) (p < 0.01). Based on the contribution percentages of multivalent cations to AACC, sediment AACC of six water bodies were divided into two types: Ca-Mg dominated (DL, DR, and TR) and Fe-Al dominated (HL, DhL, and TL). We suggest that sediment AACC complexing with pH can contribute to a better description of the acid-base characteristics of sediments.

Thin-layer fine-sand capping of polluted sediments decreases nutrients in overlying water of Wuhan Donghu Lake in China

Capping water body sediments with a thin layer of sand is an effective technique to decrease nutrient concentrations in the water column and accelerate ecological restoration of eutrophic water bodies. However, long-term effects of thin-layer sand capping in shallow lakes are reported less often. Using clean fine sand and geotextile mats as capping materials for sediments collected from Wuhan Donghu Lake in China, we designed a 290-day tank experiment with 3 cm of sand capping at four percentages of sediment coverage from 25 to 100% and a control (no capping). We monitored total nitrogen (TN), total phosphorus (TP), nitrate (NO₃^-), ammonia (NH₄⁺), and soluble reactive phosphorus (SRP) in the overlying water every 7 days. Mean TN and NO₃^- concentrations were significantly the lowest (P < 0.05) at 50% coverage. Further increase in coverage kept them slightly fluctuating. NH₄⁺ concentration was significantly lowest (P < 0.05) at 75% coverage. The relation between coverage and mean TP and SRP concentrations indicated that 75% coverage significantly decreased (P < 0.05) them, and increasing coverage to 100% decreased them even more. The fluxes of TN and TP estimated between sediments and overlying water showed that the thin fine-sand layer significantly increased the function of sediments as a sink of TN from overlying water and the potential of a sand layer to block release of TP from sediments (P < 0.05). Our results suggested that if thin-layer sand capping were applied to Wuhan Donghu Lake, more than 50% coverage is required to decrease nutrients in the lake's water.

An innovative approach for sequential extraction of phosphorus in sediments: Ferrous iron P as an independent P fraction

Accurate identification of phosphorus (P) forms is crucially important for understanding the geochemical cycle of P; however, until now the role of ferrous iron P (Fe(II)-P) buried in sediments has been completely ignored in nearly all sequential extraction procedures developed. Using sediment cores sampled from Donghu Lake in Wuhan, China, this study explored a modified version of widely used sequential P extraction method (SEDEX; Ruttenberg, 1992) in which Fe(II)-P was identified as an independent fraction. Based on the high selectivity of the extractant (0.2% 2,2'-bipyridine+0.1 M KCl) and the dissolution equilibrium of P, procedures for extracting Fe(II)-P were optimized using a 1:100 solid:liquid ratio and extraction at 50 ± 1 °C for 24 h. The sedimentary P extracted was divided into five fractions: loosely-bound P, Fe(II)-P, CDB-P, Ca-P and O-P. Fe(II)-P was the predominant fraction in fresh sediments in Donghu Lake, accounting for 15.7-49.9% of TP, with a mean of 31.6%. The mean values of Ca-P, O-P, CDB-P and loosely-bound P were 28.4%, 22.7%, 17.1% and 4.3%, respectively. Combined with component analysis of extracts and recovery experiments of standard reference minerals (vivianite, Fe3(PO4)2·8H2O) in natural sediments, extraction of Fe(II)-P with 0.2% 2,2-bipridine and 0.1 M KCl was robust, with a good recovery rate (88.7-100.6%) and little of the Ca-P dissolved. It is possible to use this innovative SEDEX not only to distinguish the contribution of different P matrices in fresh sediments, but also to investigate the transformation of sedimentary P under different redox conditions. Therefore, greater focus on Fe(II)-P is necessary, because it is a major sink for the geochemical process of sedimentary P.

Collismycin A biosynthesis in Streptomyces sp. CS40 is regulated by iron levels through two pathway-specific regulators

Two putative pathway-specific regulators have been identified in the collismycin A gene cluster: ClmR1, belonging to the TetR-family, and the LuxR-family transcriptional regulator ClmR2. Inactivation of clmR1 led to a moderate increase of collismycin A yields along with an early onset of its production, suggesting an inhibitory role for the product of this gene. Inactivation of clmR2 abolished collismycin A biosynthesis, whereas overexpression of ClmR2 led to a fourfold increase in production yields, indicating that ClmR2 is an activator of collismycin A biosynthesis. Expression analyses of the collismycin gene cluster in the wild-type strain and in ΔclmR1 and ΔclmR2 mutants confirmed the role proposed for both regulatory genes, revealing that ClmR2 positively controls the expression of most of the genes in the cluster and ClmR1 negatively regulates both its own expression and that of clmR2. Additionally, production assays and further transcription analyses confirmed the existence of a higher regulatory level modulating collismycin A biosynthesis in response to iron concentrations in the culture medium. Thus, high iron levels inhibit collismycin A biosynthesis through the repression of clmR2 transcription. These results have allowed us to propose a regulatory model that integrates the effect of iron as the main environmental stimulus controlling collismycin A biosynthesis.