Adsorption is one of the most widely applied techniques for environmental remediation. Its kinetics are of great significance to evaluate the performance of a given adsorbent and gain insight into the underlying mechanisms. There are lots of references available concerning adsorption kinetics, and several mathematic models have been developed to describe adsorption reaction and diffusion processes. However, these models were frequently employed to fit the kinetic data in an unsuitable or improper manner. This is mainly because the boundary conditions of the associated models were, to a considerable extent, ignored for data modeling. Here we reviewed several widely-used adsorption kinetic models and paid more attention to their boundary conditions. We believe that the review is of certain significance and improvement for adsorption kinetic modeling.

Phosphate originated from industrial effluents is one of the key factors responsible for eutrophication of the receiving waterways especially in the developing countries such as China. In the current study we proposed a novel process to immobilize nanoparticulate hydrated ferric oxide (HFO) within a macroporous anion exchange resin D-201, and obtained a hybrid adsorbent (HFO-201) for enhanced phosphate removal from aqueous system. The resulting HFO-201 possesses two types of adsorption sites for phosphate removal, the ammonium groups bound to the D-201 matrix and the loaded HFO nanoparticles. The coexisting sulfate anion strongly competes for ammonium groups, which bind phosphate through electrostatic interaction. However, it does not pose any noticeable effect on phosphate adsorption by the loaded HFO nanoparticles, which is driven by the formation of the inner-sphere complexes. Batch adsorption experiments also indicated that HFO-201 exhibits a little higher capacity for phosphate than the commercially available phosphate-specific adsorbent ArsenXnp, which possesses similar structure of HFO-201 and is produced by another patented technique. Fixed-bed column tests indicate that phosphate retention by HFO-201 from the synthetic waters results in the significant decrease of P from 2mg/L to less than 0.01mg/L, with the treatment capacity of w700 bed volume (BV) per run, while that for D-201 was less than 200 BV under otherwise identical conditions. Such satisfactory performance of the hybrid adsorbent is mainly attributed to the specific affinity of HFO toward phosphate as well as the Donnan membrane effect exerted by the anion exchanger support D-201. Moreover, the exhausted HFO-201 was amenable to efficient in situ regeneration with a binary NaOH-NaCl solution for repeated use without any significant capacity loss. Similar satisfactory results were also observed by using a phosphate-containing industrial effluent as the feeding solution.

The adsorption equilibria of dimethyl phthalate (DMP) and diethyl phthalate (DEP) on two hyper-cross-linked polymer resins (NDA-99 and NDA-150) in aqueous solution were investigated at 298 K. And a coal-based granular activated carbon (AC-750) was chosen for comparison. All the adsorption equilibrium data of DMP were well fitted by the Polanyi-based isotherm modeling (Polanyi-Manes (PM) equation), and the characteristic curves of the three adsorbents were obtained. It is noteworthy that a reasonably good agreement was obtained between the combined micropore and mesopore volume of adsorbents and the corresponding adsorption volume capacity for phthalates. Compared to the granular activated carbon (AC-750), the greater adsorption performances of the two resins (NDA-99 and NDA-150) were assumed to result from their more abundant micro- and mesopore structure, where phthalates can be intensively adsorbed by pore-filling mechanism. According to the exponent b value of the PM equation, NDA-99 and NDA-150 show the more micro-and mesopore heterogeneity than AC-750. On the other hand, the functional groups on the adsorbent surfaces did not take a notable effect on the adsorption equilibria of phthalates. The theory equilibrium adsorption amounts of DEP, predicted by the specific characteristic curve of each adsorbent, agree well with the experimental ones, respectively. The characteristic curve of hyper-cross-linked polymer resins and its prediction of phthalates adsorption calculated by Polanyi-based isotherm modeling have a potential applicability for field applications.

Removal and recovery of aromatic pollutants from water by solid adsorbents have been of considerable concern recently. Relatively limited information is available on desorption kinetics and isotherms aspects. In the present study, batch desorption experiments of loaded 4-nitrophenol on a hyper-cross-linked polymer resin NDA-701, a polymeric adsorbent Amberlite XAD-4 and a granular activated carbon GAC-1 were carried out to study the effects of reaction temperature and pore-size distribution of adsorbents. 4-Nitrophenol desorbed rapidly in the early stages, followed by a much slower release, whichwas described well by a first-order two-component four-parameter model. The adsorption and desorption equilibrium isotherms were well described by Freundlich equation and the apparent adsorption-desorption hysteresis (hysteresis index) for 4-nitrophenol followed an order as: NDA-701≈XAD-4 GAC-1. Analysis of adsorption-desorption process suggested that higher reaction temperature and larger proportion of macropore of adsorbents favoured desorption kinetic and reduced adsorption-desorption hysteresis. All the results indicated that NDA-701, with a specific bimodal property in macro-and micropore region, had excellent potential as an adsorption material for achieving high removal and recovery of 4-nitrophenol from the contaminated water. These results will also advance the understanding of the adsorption and desorption behavior of hyper-cross-linked polymer resin in the wastewater treatment systems.

In the current study, a new hybrid adsorbent HMO-001 was fabricated by impregnating nanosized hydrous manganese dioxide (HMO) onto a porous polystyrene cation exchanger resin (D-001) for enhanced lead removal from aqueous media. D-001 was selected as a support material mainly because of the potential Donnan membrane effect exerted by the immobilized negatively charged sulfonic acid groups bound to the polymeric matrix, whichwould result in preconcentration and permeation enhancement of lead ions prior to their effective sequestration by the impregnated HMO. HMO-001 was characterized by scanning electron micrograph (SEM), transmission electron micrograph (TEM), and X-ray diffraction (XRD). Lead adsorption onto HMO-001 was dependent upon solution pH due to the ion-exchange nature, and it can be represented by the Freundlich isotherm model and pseudo-first order kinetic model well. The maximum capacity of HMO-001 toward lead ion was about 395mg/g. As compared to D-001,HMO-001 exhibited highly selective lead retention fromwaters in the presence of competing Ca2+,Mg2+, andNa+atmuch greater levels than the target toxic metal. Fixed-bed column adsorption of a simulated water indicated that lead retention on HMO-001 resulted in a conspicuous decrease of this toxic metal from 1 mg/L to below 0.01 mg/L (the drinking water standard recommended by WHO). The exhausted adsorbent particles are amenable to efficient regeneration by the binary NaAc-HAc solution for repeated use without any significant capacity loss. All the results validated the feasibility of HMO-001 for highly effective removal of lead from contaminated waters.