Mangroves are woody plants that grow at the interface between land and sea in tropical and sub-tropical latitudes where they exist in conditions of high salinity, extreme tides, strong winds, high temperatures and muddy, anaerobic soils. There may be no other group of plants with such highly developed morphological and physiological adaptations to extreme conditions.

Because of their environment, mangroves are necessarily tolerant of high salt levels and have mechanisms to take up water despite strong osmotic potentials. Some also take up salts, but excrete them through specialized glands in the leaves. Others transfer salts into senescent leaves or store them in the bark or the wood. Still others simply become increasingly conservative in their water use as water salinity increases Morphological specializations include profuse lateral roots that anchor the trees in the loose sediments, exposed aerial roots for gas exchange and viviparous waterdispersed propagules.

Mangroves create unique ecological environments that host rich assemblages of species. The muddy or sandy sediments of the mangal are home to a variety of epibenthic, infaunal, and meiofaunal invertebrates Channels within the mangal support communities of phytoplankton, zooplankton and fish. The mangal may play a special role as nursery habitat for juveniles of fish whose adults occupy other habitats (e.g. coral reefs and seagrass beds).

Because they are surrounded by loose sediments, the submerged mangroves' roots, trunks and branches are islands of habitat that may attract rich epifaunal communities including bacteria, fungi, macroalgae and invertebrates. The aerial roots, trunks, leaves and branches host other groups of organisms. A number of crab species live among the roots, on the trunks or even forage in the canopy. Insects, reptiles, amphibians, birds and mammals thrive in the habitat and contribute to its unique character.

Living at the interface between land and sea, mangroves are well adapted to deal with natural stressors (e.g. temperature, salinity, anoxia, UV). However, because they live close to their tolerance limits, they may be particularly sensitive to disturbances like those created by human activities. Because of their proximity to population centers, mangals have historically been favored sites for sewage disposal. Industrial effluents have contributed to heavy metal contamination in the sediments. Oil from spills and from petroleum production has flowed into many mangals. These insults have had significant negative effects on the mangroves.

Habitat destruction through human encroachment has been the primary cause of mangrove loss. Diversion of freshwater for irrigation and land reclamation has destroyed extensive mangrove forests. In the past several decades, numerous tracts of mangrove have been converted for aquaculture, fundamentally altering the nature of the habitat. Measurements reveal alarming levels of mangrove destruction. Some estimates put global loss rates at one million ha y−1, with mangroves in some regions in danger of complete collapse. Heavy historical exploitation of mangroves has left many remaining habitats severely damaged.

These impacts are likely to continue, and worsen, as human populations expand further into the mangals. In regions where mangrove removal has produced significant environmental problems, efforts are underway to launch mangrove agroforestry and agriculture projects. Mangrove systems require intensive care to save threatened areas. So far, conservation and management efforts lag behind the destruction; there is still much to learn about proper management and sustainable harvesting of mangrove forests.

Mangroves have enormous ecological value. They protect and stabilize coastlines, enrich coastal waters, yield commercial forest products and support coastal fisheries. Mangrove forests are among the world's most productive ecosystems, producing organic carbon well in excess of the ecosystem requirements and contributing significantly to the global carbon cycle. Extracts from mangroves and mangrove-dependent species have proven activity against human, animal and plant pathogens. Mangroves may be further developed as sources of high-value commercial products and fishery resources and as sites for a burgeoning ecotourism industry. Their unique features also make them ideal sites for experimental studies of biodiversity and ecosystem function. Where degraded areas are being revegetated, continued monitoring and thorough assessment must be done to help understand the recovery process. This knowledge will help develop strategies to promote better rehabilitation of degraded mangrove habitats the world over and ensure that these unique ecosystems survive and flourish.

Biology of mangroves and mangrove Ecosystems

The mobility of As in soils depends on several factors including redox potential, soil mineralogy, pH, and the presence of other oxyanions that compete with As for soil retention sites. We investigated the effects of pH and competing anions on the adsorption of arsenate [As(V)] on a-FeOOH (goethite) and y-AI(OHb (gibbsite). Batch equilibrium As(V) adsorption experiments were conducted with P and MO as competing anions in order to produce single-anion [As(V), P, and MO] and binary-anion [As(V/P and As(V)/Mo] adsorption envelopes (adsorption vs. solution pH). Arsenate and P single-anion adsorption envelopes were similar with substantial adsorption occurring across a wide pH range, including pH values above the points of zero charge of the oxides. Maximum MO adsorption occurred across a narrower pH range (pH 4-6). On both oxides, equimolar P concentrations decreased As(V) adsorption within the pH range 2 to 11, whereas MO decreased As(V) adsorption only below pH 6. The constant capacitance model was used to predict competitive surface complexation behavior between As(V)/P and As(V)/Mo using intrinsic equilibrium constants [Kd, tint)] optimized from single-anion data. In addition, the model was applied using one-site (monodentate) and two-site (monodentate + bidentate) conceptualizations of the oxide surface. The two approaches gave comparable fits to experimental adsorption data and were consistent with competitive adsorption observed in binary adsorption envelopes. and MO has been described in whole soils with a competitive Freundlich-type isotherm equation (Roy et al., 1986), though the applicability o f the model was limited to cases where the As/P and As/Mo equilibrium concentration ratios were >20. Barrow (1974) investigated As(V) and P competitive adsorption in soil and found that, though As(V) desorbed some previously adsorbed P, a substantial portion o f the bound P was not displaced by As(V). Based on competitive adsorption between As(V) and P on goethite, Hingston et al. (1971) postulated that the goethite surface contains adsorption sites common to both As(V) and P anions, as well as sites that adsorb either one anion or the other. The ability to predict As(V) adsorption in complicated systems such as whole soils will require quantitative information on the adsorption of As(V) by individual soil minerals in the presence of competing anions.

Modeling competitive adsorption of arsenate with phosphate and molybdate on oxide minerals

Adsorption and oxidation reactions of arsenite (As(III)) at the mineral−water interface are two important factors affecting the fate and transport of arsenic in the environ ment. Numerous studies have concluded that As(III) is more soluble and mobile than arsenate (As(V)) in soils, though very little experimental work has demonstrated the differences in reactivity and stability of As(III) and As(V) at the mineral−water interface. In this investigation, As(III) adsorption on kaolinite, illite, montmorillonite, and amorphous aluminum hydroxide (am-Al(OH)3) was studied as a function of pH and ionic strength and was compared with As(V) adsorption. High-performance liquid chromatog raphy−hydride generation atomic absorption spectrophotometry (HPLC−HGAAS) was employed for direct determination of As(III) and As(V). In addition, surface complexation modeling was used to describe As(III) and As(V) adsorption on the four minerals. It was revealed that alkaline solutions (pH > 9) without mineral solids caused homoge...

Adsorption and stability of arsenic(III) at the clay mineral-water interface

Rice husk as a potentially low-cost biosorbent for heavy metal and dye removal : an overview

Arsenite [As(III)] and arsenate [As(V)] are highly toxic inorganic arsenic species that represent a potential threat to the environment and human health. Iron oxides including poorly crystalline oxides, e.g., ferrihydrite, play a significant role in controlling dissolved As concentration and limit the mobility and bioavailability of As(III) and As(V). Adsorption occurs by ligand exchange of the As species for OH2 and OH- in the coordination spheres of surface structural Fe atoms. The objective of this study was to evaluate H+/OH- release stoichiometry and changes in surface charge properties of the adsorbent during the adsorption of arsenite and arsenate on ferrihydrite in the pH range of 4−10. This information, which is not directly accessible through spectroscopic studies, provides important clues to bonding mechanism. While arsenate adsorption resulted in the net release of OH- at pH 4.6 and 9.2, arsenite adsorption resulted in net OH- release at pH 9.2 and net H+ release at pH 4.6. The amount of H+ or...

Arsenite and arsenate adsorption on ferrihydrite : Surface charge reduction and net OH- release stoichiometry

Thermodynamics data of As species were used to develop the solubility isotherms of As minerals and solution species. The effect of redox on the stability of As minerals and the distribution of As solution species was also simulated. In oxygenated and alkaline systems, Ca3(AsO4)2 was the most stable As mineral followed by Mn3(AsO4)2. It was postulated that in such environments Ca3(AsO4)2 and Mn3(AsO4)2 might precipitate. Arsenic(V) oxide was too soluble to become an important solid phase in the aqueous environment. Under reduced and acidic concitions (pe + pH 8), arsenate species were important whereas in reduced systems (pe + pH < 8), As(III) species were in abundance. Arsine gas can only form if the system was very reduced and acidic. Below pH 2.2, H3AsO4° was in abundance. As pH increased, H2AsO4− species was important, followed by AsO43− above pH 12. The results of thermodynamic stability and solubility models have been discussed in reference to soil environment.

Environmental behavior of arsenic in soils: Theoretical

Sediment composition and metal concentrations in mangrove leaves from the Saudi coast of the Arabian Gulf

In Saudi Arabia, trace metal enrichment of marine environment may result from a number of anthropogenic activities, especially from petroleum and petrochemical industry wastes. The objective of this study was to determine metal concentrations in the sediments from the Arabian Gulf coast of Saudi Arabia. This study may serve as baseline information for future evaluation of metal pollution in this region.

Metal concentrations in the sediments from the Arabian Gulf coast of Saudi Arabia.

The concern about mercury (Hg) pollution of the marine environment started with the well publicized case of Minimata (Japan) where in the 1950s several persons died or became seriously ill after consuming fish or shellfish containing high levels of methylmercury. It is now accepted that Hg contaminated seafoods constitute a hazard to human health. To safeguard humans, accurate determination of Hg in marine biota is, therefore, very important. Two steps are involved in the determination of total Hg in biological materials: (a) decomposition of organic matrix (sample preparation), and (b) determination of Hg in aliquot samples. Although the procedures for determining Hg using the cold vapor technique are well established, sample preparation procedures have not been standardized. In general, samples of marine biota have been prepared by digesting different weights at different temperatures, by using mixtures of different chemicals and of varying quantities, and by digesting for variable durations. The objectives of the present paper were to evaluate the effects of sample weights and digestion temperatures on Hg determination in fish.

Sample weight and digestion temperature as critical factors in mercury determination in fish.

The concentrations of cadmium (Cd), Chromium (Cr), cobalt (Co), copper (Cu), manganese (Mn), lead (Pb), nickel (Ni), and zinc (Zn) were determined in the acid digest of sediment, shrimp and crab from a heavily polluted area using Perkin-Elmer Atomic Absorption Spectrophotometer Model 560 and Graphite Furnace Model HGA 500. Results are presented in a table.

T. H. Zaidi

Papers

Environmental behavior of arsenic in soils: Theoretical

Sediment composition and metal concentrations in mangrove leaves from the Saudi coast of the Arabian Gulf

Metal concentrations in the sediments from the Arabian Gulf coast of Saudi Arabia.

Sample weight and digestion temperature as critical factors in mercury determination in fish.

Heavy metal concentrations in shrimp, crab, and sediment obtained from AD-Dammam sewage outfall area.