Modeling micropollutant removal by nanofiltration and reverse osmosis membranes: considerations and challenges.

Organic micropollutants (OMPs) in drinking water constitute a potential risk to human health; therefore, effective removal of these pollutants is required. Nanofiltration (NF) and reverse osmosis (RO) are promising membrane-based technologies to remove OMPs. In NF and RO, the rejection of OMPs depends on the properties and characteristics of the membrane, the solute, and the solution. In this review, we discuss how these properties can be included in models to study and predict the rejection of OMPs. Initially, an OMP classification is proposed to capture the relevant properties of 58 OMPs. Following the methodology described in this study, more and new OMPs can be easily included in this classification. The classification aims to increase the comprehension and mechanistic understanding of OMP removal. Based on the physicochemical principles used to classify the 58 OMPs, it is expected that other OMPs in the same groups will be similarly rejected. From this classification, we present an overview of the rejection mechanisms involved in the removal of specific OMP groups. For instance, we discuss the removal of OMPs classified as perfluoroalkyl substances (e.g., perfluorooctanoic acid, PFOA). These substances are highly relevant due to their human toxicity at extremely low concentration as well as their persistence and omnipresence in the environment. Finally, we discuss how the rejection of OMPs can be predicted by describing both the membrane-solution interface and calculating the transport of solutes inside the membrane. We illustrate the importance and impact of different rejection mechanisms and interfacial phenomena on OMP removal and propose an extended Nernst-Plank equation to calculate the transport of solutes across the membrane due to convection, diffusion, and electromigration. Finally, we show how the theory discussed in this review leads to improved predictions of OMP rejection by the membranes.

Sedimentation dynamics and equilibrium profiles in multicomponent mixtures of colloidal particles.

In this paper we give a general theoretical framework that describes the sedimentation of multicomponent mixtures of particles with sizes ranging from molecules to macroscopic bodies. Both equilibrium sedimentation profiles and the dynamic process of settling, or its converse, creaming, are modeled. Equilibrium profiles are found to be in perfect agreement with experiments. Our model reconciles two apparently contradicting points of view about buoyancy, thereby resolving a long-lived paradox about the correct choice of the buoyant density. On the one hand, the buoyancy force follows necessarily from the suspension density, as it relates to the hydrostatic pressure gradient. On the other hand, sedimentation profiles of colloidal suspensions can be calculated directly using the fluid density as apparent buoyant density in colloidal systems in sedimentation-diffusion equilibrium (SDE) as a result of balancing gravitational and thermodynamic forces. Surprisingly, this balance also holds in multicomponent mixtures. This analysis resolves the ongoing debate of the correct choice of buoyant density (fluid or suspension): both approaches can be used in their own domain. We present calculations of equilibrium sedimentation profiles and dynamic sedimentation that show the consequences of these insights. In bidisperse mixtures of colloids, particles with a lower mass density than the homogeneous suspension will first cream and then settle, whereas particles with a suspension-matched mass density form transient, bimodal particle distributions during sedimentation, which disappear when equilibrium is reached. In all these cases, the centers of the distributions of the particles with the lowest mass density of the two, regardless of their actual mass, will be located in equilibrium above the so-called isopycnic point, a natural consequence of their hard-sphere interactions. We include these interactions using the Boublik-Mansoori-Carnahan-Starling-Leland (BMCSL) equation of state. Finally, we demonstrate that our model is not limited to hard spheres, by extending it to charged spherical particles, and to dumbbells, trimers and short chains of connected beads.

On the stability of the polymer brushes formed by adsorption of ionomer complexes on hydrophilic and hydrophobic surfaces.

We have studied the effect of normal forces and shear forces on the stability and functionality of a polymer brush layer formed upon adsorption of polymeric micelles on hydrophilic and hydrophobic surfaces. The micelles consist of oppositely charged polyelectrolyte blocks (poly(acrylic acid) and poly(N-methyl 2-vinyl pyridinium iodide), and a neutral block (poly(vinyl alcohol)) or neutral grafts (poly(ethylene oxide)). The strength of the attachment of the micellar layers to various substrates was evaluated with Atomic Force Microscopy. Flow cell experiments allowed for the evaluation of long-term stability of coatings in lateral flow. Fixed angle optical reflectometry was used to quantify protein (BSA) adsorption on the micellar layers after their exposure to flow. The results show that adsorbed micellar layers are relatively weakly attached to hydrophobic surfaces and much stronger to hydrophilic surfaces, which has a significant impact on their stability. Adsorbed layers maintain their ability to suppress protein adsorption on hydrophilic surfaces but not on hydrophobic surfaces. Due to the relatively weak attachment to hydrophobic surfaces the structure of adsorbed layers may easily be disrupted by lateral forces, such that the complex coacervate-brush structure no longer exists.

Shear banding and rheochaos in associative polymer networks.

We present experimental evidence of an instability in the shear flow of transient networks formed by telechelic associative polymers. Velocimetry experiments show the formation of shear bands, following a complex pattern upon increasing the overall shear rate. The chaotic nature of the stress response in transient flow is indicative of spatiotemporal fluctuations of the banded structure. This is supported by time-resolved velocimetry measurements.

The well-being of youngsters coming from six different family types.

Families, or rather parents are in a state of flux: sometimes they row a lot, they divorce, they go and live together again and they die. This is why more and more children grow up in family forms other than the nuclear families with their biological fathers and mothers. What, in the long run, is the effect of this on the well-being of adolescents and young adults between 12-30? We have examined intact families that function well, mediocre or badly, one-parent families and stepfamilies after a divorce and one-parent families after being widowed. The data derive from the Dutch longitudinal panel study "Utrecht Study of Adolescent Development" (USAD). The results of 1772 respondents between 12 and 30 years old are presented from the third wave by the end of 1997. Compared to youngsters of well functioning nuclear families, the youngsters of discordant nuclear families show a worse physical and mental health, their parental fixation is not so strong, they tend to drink more alcohol, smoke more cigarettes and use more soft drugs. Children of divorced families are notable for their relational behaviour: they enter into relations at an early age, usually they are more sexually experienced and they report more relational problems. Children of widowed one-parent families do well. In other family types girls suffer a little more from the burden of life than boys. It appears that the effects are hardly different when the children leave home.

Family problems and youth unemployment.

This study attempted to determine the extent to which family and personal characteristics relate to the employment situation of adolescents. Data were drawn from the Utrecht Study of Adolescent Development (USAD), which investigated, longitudinally, a national sample of Dutch youths aged 12 to 24 years in 1991. Specifically, two waves of a sample of 955 non-school-going respondents between 18 and 27 years old were analyzed. Parental divorce, parental unemployment (only for males), low parental affective involvement, and adolescent relationship problems were related to youth unemployment, but educational career and work commitment were not. For males, parental unemployment demonstrated the strongest correlation with youth unemployment. For females, only variables in the relational domain played a role in explaining unemployment; relationship variables were also important predictors of male unemployment. The results suggest that the family factors included in this study are better predictors of youth unemployment than are the classic individual (personal) variables.

How do vocational and relationship stressors and identity formation affect adolescent mental health?

PURPOSE: This article examines the effects of stressors in both the vocational and relationship career of youngsters in the formation of their identity; the effects of identity formation on adolescent mental health; the influence of career stressors on mental health, directly or via identity, and differences in these effects on boys and girls. METHODS: Data were used from the Dutch national panel study, Utrecht Study of Adolescent Development, a study of developmental processes as they occur in the life course of young people during the 1990s. Using LISREL, we tested hypotheses on two waves of a sample of 1222 respondents between 15 and 24 years of age in Wave 1 (1991). RESULTS: The correlation between relationship stressors and relationship identity can be neglected, while vocational stressors lead to a less achieved vocational identity, particularly in boys. Occupational and relationship identity have similar effects on mental health (i.e., the more achieved the identity, the better the person's mental health). Vocational and especially relationship stressors lead to poorer mental health, but did not affect the mental health of boys and girls differently. The same goes for the influence of relationship and vocational identity formation on mental health. CONCLUSIONS: Career stressors, especially stressors in the relationship domain, appear to have significant long-term effects on adolescent mental health. Vocational and relationship identity formation are also significant predictors for adolescent mental health.

Transitions in family structure and adolescent well-being.

This article examines the effects of transitions in family structure on physical health, thoughts of suicide, mental health, relational well-being, and employment situation of adolescents from four family structures: stable intact families, conflict intact families, single-parent families, and stepfamilies. Data were used from the Utrecht Study of Adolescent Development (USAD), a study of developments in the life course of young people during the 1990s. Results are presented from over 2,500 respondents between 15 and 24 years of age. Their parents were also interviewed on a number of topics. Transition in family structure after marital problems, divorce, and remarriage does appear to have significant long-term effects on a number of adolescent well-being variables. Young people from single-parent families have the lowest scores on the different indicators; they are more likely to have relational problems and experience unemployment as compared with youngsters from stable intact families. Adolescents from conflict intact families and stepfamilies have moderate scores. These effects remain after controlling for family income, gender, age, and educational level.

Effects of parental divorce and youth unemployment on adolescent health.

The aim of this study is to gain more insight into the effects of two important events on adolescent health in the life course of youngsters, namely parental divorce and being unemployed. We made use of the dataset of the Utrecht Study of Adolescent Development, a longitudinal panel study, based on a representative sample of young people in the Netherlands (12-24 years old). We used the data of non-school-going youngsters who are at least 18 years old. Parental divorce does indeed have negative effects on adolescent health: however, this applies only for girls. Being unemployed or having been unemployed has negative effects on psychological health, especially for boys. But this experience does not have negative effects on physical health, either for boys or girls. Any conclusions on the consequences of parental divorce and youth unemployment for adolescent health have to be specified, at least for sex.