Separation phenomena in UF and NF in the recovery of organic acids from kraft black liquor

Analysis of aliphatic and phenolic compounds present in industrial black liquors using HPLC-DAD and IC-MS/MS methods

Carboxylic acids and aromatic compounds are essential building blocks and starting materials for the production of a wide range of fine chemicals and materials. Their recovery from kraft black liquor, an industrial effluent from pulp and paper mills, is a promising way to produce alternative bio-based chemicals. Reliable methods are needed to identify and quantify the molecules of interest in complex mixtures such as black liquors. First, an HPLC-DAD-based method was developed for the determination of aliphatic acids and phenolic compounds. It allowed the separation of 31 aliphatic and phenolic compounds. The method was applied to the identification of aliphatic and phenolic compounds in industrial black liquors. Then, an IC-MS/MS method was developed to confirm the identification and quantification of organic compounds in black liquor samples. 22 compounds were detected and identified by MS/MS detection. According to both methods, the major aliphatic acids in softwood kraft black liquor are formic acid (9.8 g/L), acetic acid (7.1 g/L), lactic acid (5.2 g/L), glycolic acid (4.7 g/L), 2-hydroxybutyric acid (2.3 g/L), and oxalic acid (1.3 g/L). Phenolic compounds were detected at very low levels (total concentration 1.4 g/L). This study demonstrates the value of a multi-technique strategy for the identification and quantification of organic compounds in complex matrices such as black liquor.

Microbial valorization of kraft black liquor for production of platform chemicals, biofuels, and value-added products: A critical review

The proper treatment and utilization of kraft black liquor, generated from the pulp and paper industry through the kraft pulping method, is required to reduce environmental impacts prior to the final disposal. It also improves the economic performance through the utilization of waste. Microbial valorization appears to demonstrates the dual benefits of waste management and resource recovery by providing an innovative solution to convert kraft black liquor into resource for reuse. A comprehensive review on the microbial valorization of kraft black liquor, describing the role in valorization and management, is still lacking in the literature, forming the rationale of this article. Thus, the present study reviews and systematically discusses the potential of utilizing microorganisms to valorize kraft black liquor as a sustainable feedstock to develop a numerous portfolio of platform chemicals, bioenergy, and other value-added products. This work contributes to sustainability and resource efficiency within the pulp and paper industry. The recent developments in utilization of synthetic biology tools and molecular techniques, including omics approaches for engineering novel microbial strains, for enhancing kraft black liquor valorization has been presented. This review explores how the better utilization of kraft black liquor in the pulp and paper industry contributes to achieving UN Sustainable Development Goals (SDGs), particularly clean water and sanitation (SDG 6) as well as the affordable and clean energy goal (SDG 7). The current review also addresses challenges related to toxicity, impurities, low productivity, and downstream processing that serve as obstacles to the progress of developing highly efficient bioproducts. The new directions for future research efforts to fill the critical knowledge gaps are proposed. This study concludes that by implementing microbial valorization techniques, the pulp and paper industry can transition from a linear to a circular bioeconomy and eco-friendly manage the kraft black liuor. This approach showed to be effective towards resource recovery, while simultaneously minimizing the environmental burden.

Isolation of Carboxylic Acids and NaOH from Kraft Black Liquor with a Membrane-Based Process Sequence

In kraft pulping, large quantities of biomass degradation products dissolved in the black liquor are incinerated for power generation and chemical recovery. The black liquor is, however, a promising feedstock for carboxylic acids and lignin. Efficient fractionation of black liquor can be used to isolate these compounds and recycle the pulping chemicals. The present work discusses the fractionation of industrial black liquor by a sequence of nanofiltration and bipolar membrane electrodialysis units. Nanofiltration led to retention of the majority of lignin in the retentate and to a significant concentration increase in low-molecular-weight carboxylic acids, such as formic, acetic, glycolic and lactic acids, in the permeate. Subsequent treatment with bipolar membrane electrodialysis showed the potential for simultaneous recovery of acids in the acid compartment and the pulping chemical NaOH in the base compartment. The residual lignin was completely retained by the used membranes. Diffusion of acids to the base compartment and the low current density, however, limited the yield of acids and the current efficiency. In experiments with a black liquor model solution under optimized conditions, NaOH and acid recoveries of 68-72% were achieved.

Influence of various shapes of alumina nanoparticle in integrated polysulfone membrane for separation of lignin from woody biomass and salt rejection

Lignin valorization is essential in proposing an economic perspective as a raw material for valuable compounds. The bio-refineries require adequate processing to improve the high purity of lignin. Meanwhile, nanofiltration is fascinated attention to obtain high purity value-added products. The effect of alumina nanoparticles on the fabrication of mixed matrix membranes (MMM) has contributed to improvising filtration performance. However, incorporating nanoparticles is a significant issue regarding appropriate size and shape integrated into membrane for better filtration efficiency. The influence of shapes of alumina nanoparticles has been investigated into polysulfone (PSf) membranes for salt and lignin separation. The morphology of alumina was tailored with spindle, cubic, and spherical shapes synthesized at a different calcination temperature of 250, 500, 700 and 900 °C, respectively. The phase transitions were confirmed in X-ray diffraction (XRD) analysis, and the shape of the nanoparticles was observed in a high-resolution transmission electron microscope (HRTEM). The separation efficiency of membranes was tested with salt rejection using sodium sulfate, calcium chloride, potassium sulfate, and sodium chloride. The lignin was extracted from prehydrolysed sawdust, and the synthetic lignosulfonic acid sodium salt solution was separated. The higher lignin rejection of 98.6% and 97.9% were obtained for cubic shaped gamma phase alumina mixed matrix membrane. The high rejection of lignin occurred due to narrow pores channels that could resist the transfer of lignin through the membrane. The results proved that the controllable organization of PSf/alumina mixed matrix membranes could apply for lignocellulose compounds with good efficiency.

Membrane Filtration Opportunities for the Treatment of Black Liquor in the Paper and Pulp Industry

Black liquor is a highly alkaline liquid by-product of the kraft pulping process, rich in organic molecules (hemicelluloses, lignin, and organic acids) and inorganic pulping chemicals such as sodium salts and sulphur-containing compounds. The release of this wastewater without further treatment could have serious environmental and financial implications. Therefore, a costly treatment process is used nowadays. Nanofiltration has been studied in the last few years as a promising alternative to recycle the cooking chemicals required for the separation of lignin and cellulose, but the development of pH-stable membranes with the potential to operate at industrial scales is fundamental in order to make this possible. In this study, the filtration performance of two in-house made membranes is evaluated and compared with a commercial NF membrane to determine the viability of their use for the treatment of black liquor. For this purpose, filtration experiments with simulated black liquor were performed. We identified that Membrane A has the higher potential for this application due to its competitive permeate flux (ca. 24 L m−2 h−1 at a trans-membrane pressure of 21.5 bar), and high rejection of organic components and salts from the cooking liquor (on average, 92.50% for the TOC, 84.10% for the CO32−, 88.70% for the sulphates, 73.21% for the Na+, and 99.99% for the Mg2+).

New insights into effect of alkaline cleaning on fouling behavior of polyamide nanofiltration membrane for wastewater treatment

Membrane fouling is an intractable issue in wastewater treatment by nanofiltration (NF) membrane, and alkaline cleaning is the most effective approach to remove organic fouling on NF membrane. However, it was found that pore swelling of NF membrane induced by alkaline cleaning might reduce cleaning efficiency, and it is never quantified and its effect on membrane fouling behavior is still mysterious. In this work, membrane pore swelling effect (~9.7%, increment of effective pore size) induced by alkaline cleaning (pH 11) is confirmed and its effect on fouling behavior of the polyamide NF membrane is investigated based on experimental and modelling results. It is found that the alkali-induced pore swelling phenomenon would disappear after water filtration at neutral pH for 30 min, and if such cleaned membrane is faced by the small foulants during this pore shrinkage period, the concentration polarization and membrane fouling would be severer, and the subsequent alkaline cleaning is less effective because more foulants enter the enlarged pores and are tightly embedded in the membrane. Thus, the irreversible fouling of the NF membrane increases from 20% to 40% while its permeability recovery declines from 100% to 67% after six fouling/cleaning cycles. When an anionic surfactant sodium dodecyl sulfate (SDS, 10 mM) is added in the alkaline cleaning solution, the adsorption of SDS in/on the membrane can not only improve its hydrophilicity and negative charge, but also quickly eliminate the alkali-induced pore swelling effect and avoid the accumulation of foulants in the pores, thereby enhancing the antifouling performance of the NF membrane. Using the alkaline SDS cleaning, the irreversible fouling of the NF membrane maintains below 10% while its permeability recovery keeps above 100% in six continuous fouling/cleaning cycles.

Fractionation of polyphenols from thermomechanical pulp mill process water by flotation and membrane integrated process

Fractionation of phenolic compounds in thermomechanical pulp mills was performed with a coupling of a prior treatment realized by flotation and a ceramic membrane process. Two lines of membranes filtration were tested. After a common 150 kDa clarification, 1 kDa filtration was performed with or without previous 5 kDa filtration. Flotation was shown to be inevitable to retain lipophilic compounds which cause severe membrane fouling. 150 kDa permeate flux was 20% higher when process water was firstly floated and was around 260 L h^-1 m^-2. 1 kDa membrane was fouled with 31% of irreversible fouling without previous 5 kDa filtration and phenolic compounds purity reached only 26% in this 1 kDa permeate. Phenolic compounds as lignin-like substances which might be attached to hemicelluloses were recovered in 5 kDa retentate. Retentate of 1 kDa might contain a major fraction of lignin derivatives with molecular weights around 1 kDa free or linked with phenolic acids. Permeate of 1 kDa contained 14% of phenolic compounds such as lignans and free phenolic acids purified at 50%.

Production of carboxylic acids from the non-lignin residue of black liquor by hydrothermal treatments

The present study assesses, for the first time, the use of the non-lignin residue from Kraft black liquor as a renewable source of carboxylic acids. For this purpose, the liquid fraction obtained after separating the lignin from the black liquor by acid precipitation was subjected to different hydrothermal treatments. It was found that the formation of carboxylic acids can be maximized at 190 °C, 70 bar and under an inert atmosphere, with concentrations after 2 h of 29.0 g/l of oxalic acid, 1.8 g/L of malic acid, 10.0 g/L of lactic acid, 4.1 g/L of formic acid, 11.8 g/L of acetic acid and 3.4 g/L of propionic acid. The presence of an oxidizing atmosphere generated a less concentrated, but more purified, stream of acids than that obtained by thermal hydrolysis, simplifying the subsequent downstream processing.

Membrane Technologies for Lactic Acid Separation from Fermentation Broths Derived from Renewable Resources

Lactic acid (LA) was produced on a pilot scale using a defined medium with glucose, acid whey, sugar bread and crust bread. The fermentation broths were then subjected to micro- and nanofiltration. Microfiltration efficiently separated the microbial cells. The highest average permeate flow flux was achieved for the defined medium (263.3 L/m²/h) and the lowest for the crust bread-based medium (103.8 L/m²/h). No LA losses were observed during microfiltration of the acid whey, whilst the highest retention of LA was 21.5% for crust bread. Nanofiltration led to high rejections of residual sugars, proteins and ions (sulphate, magnesium, calcium), with a low retention of LA. Unconverted sugar rejections were 100% and 63% for crust bread and sugar bread media respectively, with corresponding LA losses of 22.4% and 2.5%. The membrane retained more than 50% of the ions and proteins present in all media and more than 60% of phosphorus. The average flux was highly affected by the nature of the medium as well as by the final concentration of LA and sugars. The results of this study indicate that micro- and nanofiltration could be industrially employed as primary separation steps for the biotechnologically produced LA.