Adsorption kinetics is a key-issue for a successful activated carbon selection and design of the treatment system. Crucial predictive aspects are the determination of the diffusion coefficients and the establishment of the controlling adsorption step. Several kinetic models have been developed and two of the most frequently used, the homogeneous surface diffusion model (HSDM) and the Boyd's model, were applied to microcystins, and particularly MC-LR adsorption. Different initial MC-LR concentrations and carbons (particle diameter, porosity), yielding diverse Biot numbers (Bi), were tested. The model outcomes were compared, namely the Boyd's effective intraparticle diffusion coefficient (D i ) with the HSDM surface diffusion coefficient (D s ), as well as the Bi and Boyd's criteria to establish the controlling adsorption step, which constitute a novel approach. Very good HSDM fittings were achieved with a constant diffusion model (D s independent of MC-LR surface concentration). D i was similar to D s whenever Boyd plots intercepted the origin. The Biot limit above which it may be considered that intraparticle diffusion is the rate limiting step depended on the carbon. A lower limit was observed for smaller carbons.

This paper investigates the powdered activated carbon (PAC) contribution to the fouling control by natural organic matter (NOM) in PAC/UF hybrid process, as well as the foulant behaviour of the PAC itself. Solutions of NOM surrogates (humic acids, AHA, and tannic acid, TA) and AOM/EOM (algogenic organic matter/extracellular organic matter) fractions from a Microcystis aeruginosa culture were permeated through an ultrafiltration (UF) hollow-fibre cellulose acetate membrane (100kDa cut-off). The greatest impairment on flux and the poorest rejection were associated with polysaccharide-like EOM substances combined with mono and multivalent ions. PAC, either in the absence or in the presence of NOM, did not affect the permeate flux nor the reversible membrane fouling, regardless of the NOM characteristics (hydrophobicity and protein content) and water inorganics. However, PAC controlled the irreversible membrane fouling, minimising the chemical cleaning frequency. Furthermore, PAC enhanced AHA and TA rejections and the overall removal of AOM, although it was apparently ineffective for the highly hydrophilic EOM compounds.

This work aims to contribute to a better understanding of the ionic strength effect on microcystin and natural organic matter (NOM) surrogate adsorption by analyzing the importance of adsorbate molecular size, and surface concentration. Adsorption kinetics and/or isotherms were performed on PAC Norit SA-UF for four microcystin variants (MC-LR, MC-LY, MC-LW, MC-LF), and three NOM surrogates (salicylic acid (SA), tannic acid (TA), Aldrich humic acid (AHA)) at different solution ionic strengths. Results showed that the ionic strength effect depends upon the adsorbate surface concentration, cation charge (mono or divalent), and adsorbate molecular size. Potassium seemed not to affect the MC-LR adsorption, while calcium enhanced MC-LR kinetics and adsorption capacity. K+ and, particularly, Ca2+ improved the adsorption kinetics of the other microcystin variants. For identical surface concentration and ionic strength, the impact of K+ and Ca2+ on NOM surrogates depended on the adsorbate molecular size: K+ effect was only observed for AHA, whereas Ca2+ caused no effect on SA adsorption, slightly enhanced TA adsorption, and greatly enhanced AHA adsorption. MC-LR isotherms with two salt concentrations (KCl or CaCl2) indicated that, for the studied range of equilibrium surface concentration (5.3-18.7 mg/g), an enhanced adsorption regime prevails, and no transition regime was observed.

The aim of this study was to evaluate the ultrafiltration (UF) performance for removing Microcystis aeruginosa cells under different growth ages (1, 2, 3 and 4 months old). Special attention was given to cell damaging and subsequent release of microcystins to permeate. Experiments were performed with a hollow-fibre cellulose acetate membrane (100 kDa). UF achieved an absolute removal of M. aeruginosa single cells, producing chlorophyll-a free water and with a turbidity below 0.1 NTU. Cell lysis occurred at all cell growth phases although greater damage was observed for older cultures. However, the permeate quality was never deteriorated and its microcystin concentration was always identical or lower than the dissolved concentration in the feed water. The hydrophilic UF membrane presented low adsorption of microcystins but the microcystin rejection increased in the presence of algogenic organic matter (AOM). The type rather than the overall concentration of salts and organics ruled the membrane fouling, the 1-month-old suspension (polysaccharide-rich AOM with scaling multivalent ions) presenting higher fouling potential than the 3-month-old suspension (protein-rich AOM with much lower content of multivalent ions).