Tannin-based coagulants: Current development and prospects on synthesis and uses.

Population growth, industrialization, urbanization, and agriculture lead to a decrease in the availability of clean water. Coagulation/flocculation is one of the most common operations in water, urban wastewater, and industrial effluents treatment systems. Usually, this process is achieved using conventional coagulants that have their performance affected by pH, are poorly biodegradable, produce a huge volume of sludge, and are associated with degenerative diseases. As a substitute for these chemicals, natural coagulants have been highly researched for the last ten/fifteen years, especially the tannin-based (TB) ones. This review paper highlights the advantages of using these greener products to treat different types of water, wastewater, and effluents, especially from dairy, cosmetics, laundries, textile, and other industries. TB coagulants can successfully remove turbidity, color, suspended solids, soluble organic (chemical/biochemical oxygen demand) and inorganic matter (total phosphate, and heavy metals), and microorganisms. TB coagulants are compatible with other treatment technologies and can be used as coagulant-aid to reduce the consumption of chemicals. TB coagulants can reduce operating costs of water treatment due to less alkalinity consumption, as pH adjustment is sometimes unnecessary, and the production of a smaller volume of biodegradable sludge. TB coagulants can be synthesized by valorizing wastes/by-products, from the bark of some specific trees and skins/pomace of different fruits and vegetables. The strengths, weaknesses, opportunities, and threats (SWOT) on TB coagulants are discussed. The progress of TB coagulants is promising, but some threats should be overcome, especially on tannin extraction and cationization. The market competition with conventional coagulants, the feasibility of application in real waters, and the reluctance of the industries to adapt to new technologies are other weaknesses to be surpassed.

Antimony removal from water by pine bark tannin resin: Batch and fixed-bed adsorption.

Antimony is present in water by natural causes but is also mobilized in the environment by anthropogenic activities, particularly mining. Considering its toxicological behavior, antimony removal from contaminated groundwater and mine effluents is necessary. In this work, Sb(III) and Sb(V) removal from aqueous solution was studied using a resin prepared from pine bark tannins. Subsequent iron loading of the tannin resin was tested, but this chemical modification was shown not to improve adsorptive properties. Tannin resin (unmodified form) presented a good ability to uptake antimony, with maximum adsorption capacities, evaluated in batch mode, of 30-33 mg g-1 (Sb(III), pH 6) and 16-47 mg g-1 (Sb(V), pH 2), depending on the particle size. The performance of the adsorbent was not affected by high levels of sulfate, which characterize most mining-impacted waters, but depending on Sb-load of the water it could be moderately affected by metal cations coexisting in solution. The applicability of the tannin resin on Sb(III) uptake was confirmed in continuous fixed-bed experiments. Breakthrough curves were obtained for different inlet adsorbate concentrations, bed heights, flow rates and aqueous media (distilled water and a simulated mine effluent). The adsorptive capacity of the tannin resin was practically maintained and adsorbent usage rates as low as 0.11 kg m-3 were determined to treat efficiently (90% removal) 1 mg-Sb(III) L-1 contaminated water. Overall, tannin resin is a bio-derived sorbent that shows affinity for antimony in both redox states, being stable in pH conditions commonly found in Sb-contaminated waters.

Tannin-Adsorbents for Water Decontamination and for the Recovery of Critical Metals: Current State and Future Perspectives.

Biosorption is known as an effective way to clean-up water from organic and inorganic contaminants and has also emerged as a promising technology to recover critical substances. Tannins are renewable materials, coming from multiple vegetable sources. A variety of biosorbents have been developed from tannins, including tannin resins, rigid foams, composites with mesoporous silica, cellulose, collagen, and magnetic adsorbents. These materials have shown an excellent ability to uptake heavy-metal cations (Cd(II), Cu(II), Pb(II), Ni(II), Cr(III)), owning to the chelating ability provided by the plentiful adjacent hydroxyl groups. In addition, tannin-adsorbents have shown exceptional ability to remove Cr(VI), and to uptake Au(III) and Pd(II) from strong acidic solutions, which has evident application in the recovery of precious metals from e-wastes leaching. The fact that tannin-adsorbents can reduce the oxidation state of these adsorbates to Cr(III) and to elemental species of Au and Pd is interesting. Adsorption of dyes, surfactants, pharmaceuticals and antimony is also feasible, but the removal of certain metalloid species, such as arsenic and phosphate, seems to be limited even after applying chemical modifications. This article presents a systematic review on the preparation of tannin-adsorbents and their application in water decontamination and in the recovery of critical metals.

Bioadsorptive removal of Pb(II) from aqueous solution by the biorefinery waste of Fucus spiralis.

In the context of developing the circular economy that enables a more sustainable use of the available resources and minimum waste generation, marine macroalgae have attracted the attention of researchers and industry due to its potential as a renewable resource. The current work aims to contribute to the design of a complete biorefinery processing, using Fucus spiralis seaweed (brown division) as starting material, and to determine the potential of the derived waste as biosorbent of heavy metals in aqueous solution. The macroalgae waste was obtained after the sequential separation of polyphenols, fucoidan and alginate extracts from F. spiralis. The capacity of F. spiralis waste for Pb(II) removal was successfully tested through biosorption tests. The uptake of Pb(II) was found to be very fast (few hours to achieve equilibrium). Tests performed with an initial metal concentration of 20 mg/L established the best adsorbent dosage (0.50 g/L) and an optimum pH of 4.5. In these conditions, lead was almost completely removed from the aqueous solution. Maximum adsorption capacity predicted by Langmuir model was 132 ±â€¯14 mg/g (pH 4.5 ±â€¯0.5, 20 °C). Desorption studies were conducted with different possible eluents. The best results were obtained with EDTA 0.1 mol/L, generating a 95 ±â€¯4% desorption. F. spiralis biomass can therefore be submitted to a complete biorefinery processing and design in the attempt to fulfil the "zero-waste" concept.

Arsenate and arsenite adsorption onto iron-coated cork granulates.

There is a growing demand for low-cost, effective adsorbents for arsenic removal from water intended for human consumption in affected rural areas. This work presents a novel adsorbent based on the coating of cork granulates with iron (oxy)hydroxides for the removal of As(III) and As(V) from aqueous matrices. A 26-3 fractional factorial design was used to determine the optimal conditions for the iron coating procedure. The optimal adsorbent was produced by coating low-density cork granulates with iron (oxy)hydroxides precipitated from a 0.05 mol L-1 FeCl3 solution at pH 7, 20 °C temperature and 20 g L-1 S/L ratio, in a single coating cycle. Arsenic adsorption was found to be dependent on pH, with inverse trends for As(III) and As(V). The iron leaching from the adsorbent was also taken into account to select the optimum pH, which was pH 9 for As(III) and pH 3 for As(V). Adsorption kinetics were better described by the pseudo-second-order model for As(III) and the Elovich model for As(V). Equilibrium was reached in 16 h for As(III) at pH 9 and 48 h for As(V) at pH 3. The isotherm models indicated different adsorption behaviours for As(III) and As(V), with better fits by Langmuir and Freundlich models, respectively. The Langmuir maximum adsorption capacity of iron-coated cork adsorbent for As(III) at pH 9 was 4.9 ±â€¯0.3 mg g-1. However, at low equilibrium concentrations, As(V) adsorption was higher than As(III) (e.g. 2.1 ±â€¯0.2 mg g-1 in equilibrium with 0.16 ±â€¯0.03 mg L-1). Speciation studies and XPS analyses indicated that no substantial oxidation of As(III) to As(V) occurred during the adsorption process. The study shows that iron coating can enhance both arsenate and arsenite adsorption capacity of cork materials, leading to an innovative natural adsorbent with high resilience and stability, with possible application in arsenic remediation.

Arsenic removal from water using iron-coated seaweeds.

Arsenic is a semi-metal element that can enter in water bodies and drinking water supplies from natural deposits and from mining, industrial and agricultural practices. The aim of the present work was to propose an alternative process for removing As from water, based on adsorption on a brown seaweed (Sargassum muticum), after a simple and inexpensive treatment: coating with iron-oxy (hydroxides). Adsorption equilibrium and kinetics were studied and modeled in terms of As oxidation state (III and V), pH and initial adsorbate concentration. Maximum adsorption capacities of 4.2 mg/g and 7.3 mg/g were obtained at pH 7 and 20 °C for arsenite and arsenate, respectively. When arsenite was used as adsorbate, experimental evidences pointed to the occurrence of redox reactions involving As(III) oxidation to As(V) and Fe(III) reduction to Fe(II), with As(V) uptake by the adsorbent. The proposed adsorption mechanism was then based on the assumption that arsenate was the adsorbed arsenic species. The most relevant drawback found in the present work was the considerable leaching of iron to the solution. Arsenite removal from a mining-influenced water by adsorption plus precipitation was studied and compared to a traditional process of coagulation/flocculation. Both kinds of treatment provided practically 100% of arsenite removal from the contaminated water, leading at best in 12.9 µg/L As after the adsorption and precipitation assays and 14.2 µg/L after the coagulation/flocculation process.

Treatment of a simulated textile wastewater in a sequencing batch reactor (SBR) with addition of a low-cost adsorbent.

Color removal from textile wastewaters, at a low-cost and consistent technology, is even today a challenge. Simultaneous biological treatment and adsorption is a known alternative to the treatment of wastewaters containing biodegradable and non-biodegradable contaminants. The present work aims at evaluating the treatability of a simulated textile wastewater by simultaneously combining biological treatment and adsorption in a SBR (sequencing batch reactor), but using a low-cost adsorbent, instead of a commercial one. The selected adsorbent was a metal hydroxide sludge (WS) from an electroplating industry. Direct Blue 85 dye (DB) was used in the preparation of the synthetic wastewater. Firstly, adsorption kinetics and equilibrium were studied, in respect to many factors (temperature, pH, WS dosage and presence of salts and dyeing auxiliary chemicals in the aqueous media). At 25 °C and pH 4, 7 and 10, maximum DB adsorption capacities in aqueous solution were 600, 339 and 98.7 mg/g, respectively. These values are quite considerable, compared to other reported in literature, but proved to be significantly reduced by the presence of dyeing auxiliary chemicals in the wastewater. The simulated textile wastewater treatment in SBR led to BOD5 removals of 53-79%, but color removal was rather limited (10-18%). The performance was significantly enhanced by the addition of WS, with BOD5 removals above 91% and average color removals of 60-69%.

Waste metal hydroxide sludge as adsorbent for a reactive dye.

An industrial waste sludge mainly composed by metal hydroxides was used as a low-cost adsorbent for removing a reactive textile dye (Remazol Brilliant Blue) in solution. Characterization of this waste material included chemical composition, pH(ZPC) determination, particle size distribution, physical textural properties and metals mobility under different pH conditions. Dye adsorption equilibrium isotherms were determined at 25 and 35 degrees C and pH of 4, 7 and 10 revealing reasonably fits to Langmuir and Freundlich models. At 25 degrees C and pH 7, Langmuir fit indicates a maximum adsorption capacity of 91.0mg/g. An adsorptive ion-exchange mechanism was identified from desorption studies. Batch kinetic experiments were also conducted at different initial dye concentration, temperature, adsorbent dosage and pH. A pseudo-second-order model showed good agreement with experimental data. LDF approximation model was used to estimate homogeneous solid diffusion coefficients and the effective pore diffusivities. Additionally, a simulated real effluent containing the selected dye, salts and dyeing auxiliary chemicals, was also used in equilibrium and kinetic experiments and the adsorption performance was compared with aqueous dye solutions.